Introduction
Clarification of core pathways and discovery of new central molecules in the
pathophysiology of rare bone disorders of impaired mineral metabolism have triggered
the development of novel, targeted therapies. The recent approval of new drugs is
promising for the outcome of severely affected patients. The phosphaturic hormone
Fibroblast Growth Factor 23 (FGF23) was found to have critical role in the
pathogenesis of various rare hypophosphatemic disorders, including the genetic
disorder of X-linked Hypophosphatemia (XLH) and the acquired syndrome of
Tumor-Induced Osteomalacia (TIO). Burosumab, a monoclonal antibody against FGF23,
was approved as the first specific treatment for children and adult patients with
XLH, and recently for TIO patients.
Asfotase alfa is another groundbreaking drug, and the first enzymatic replacement
therapy. It substitutes, in a bone-specific manner, the decreased activity of Tissue
Nonspecific Alkaline Phosphatase (TNSALP), that is responsible for the multisystemic
disorder of hypophosphatasia. Its favorable effect on the course and complications
of the disease has led to the approval of the drug for the treatment of
childhood-onset HPP. However, more data are needed to ensure long-term safety and
cost-efficacy of these agents. Herein, we concisely present the pathophysiology and
the therapeutic options of the two rare disorders of mineral metabolism, focusing
on
the new FDA-approved drugs that add important medical value for their treatment.
FGF23 and its role in bone metabolism
Phosphate is, following calcium, the second more abundant mineral in the human
body. Approximately 85% of phosphate is stored in bone (hydroxyapatite
crystals), 15% in the intracellular space (phospholipids, nucleic acids,
high-energy nucleotides, signaling) and less than 1% of phosphate is
found in the extracellular fluid as inorganic phosphate (Pi) [1 ]. Extracellular phosphate levels are
regulated by hormones, such as parathyroid hormone (PTH), calcitriol
[1,25(OH)2 D], and FGF23, which balance intestinal absorption and
urinal excretion of phosphate according to dietary intake ([Fig. 1 ]).
Fig. 1 Interactions of FGF-23, PTH, and calcitriol in the
regulation of extracellular phosphate levels. PTH, produced by
parathyroid cells, and FGF23, produced by osteocytes, suppress proximal
tubular phosphate reabsorption by reducing the expression of type IIa
and IIc sodium/phosphate cotransporters (NaPi-IIa, NaPi-IIc). PTH
increases and FGF23 decreases 1,25(OH)2D levels. 1,25(OH)2D stimulates
sodium-dependent intestinal phosphate absorption by enhancing the
expression of type IIb and III sodium/phosphate cotransporters
(NaPi-IIb, III). Dietary phosphate intake regulates the expression of
these cotransporters as well: low dietary intake enhances renal and
intestinal phosphate reabsorption, whereas high dietary intake inhibits
renal phosphate reabsorption.
Sodium/phosphate cotransporters (NaPi cotransporters) stimulate phosphate
reabsorption, both in renal and small intestinal cells. They are responsible for
reabsorption of about 80–90% of the phosphate filtered from
glomeruli [2 ]. Type IIa and IIc are found
in the brush border membrane of renal proximal tubular cells, and type IIb and
III (pit-1, pit-2) are located in the brush border membrane of intestinal cells.
Their expression is regulated by dietary phosphate intake and hormones, such as
FGF23, PTH and calcitriol. Rise in extracellular phosphate, PTH and FGF23 reduce
the expression of renal cotransporters, promoting phosphate excretion, whereas
calcitriol and decrease in extracellular phosphate enhance the expression of
intestinal cotransporters, increasing phosphate reabsorption [1 ]
[2 ].
FGF23 is a phosphaturic hormone produced in bone, and mainly in osteocytes. It is
transcribed and translated as an inactive, peptide of 251 amino acids. The
cleavage of the first 24 amino acids (signal peptide) gives rise to the active
form (intact FGF23). This peptide can be further cleaved into inactive C- and
N-terminal fragments [3 ]. For its action,
it binds to a receptor complex, which consists of Fibroblast Growth Factor
Receptor 1 (FGFR1) and the co-receptor Klotho, thus activating intracellular
signaling pathways like Extracellular signal-regulated kinase pathway (ERK)
[2 ]. By downregulating the expression
of CYP27B1, which encodes 25-hydroxyvitamin D-1α-hydroxylase, and
upregulating the expression of CYP24A1, which encodes 25- hydroxyvitamin
D-24-hydroxylase, FGF23 suppresses the production of 1,25(OH)2 D in
renal proximal tubules. In conclusion, FGF23 can cause hypophosphatemia directly
via renal phosphate wasting and indirectly through decreasing circulating levels
of calcitriol [1 ]
[4 ].
FGF23-related diseases and FGF23 inhibition as a new therapeutic
option
Rickets is a metabolic disease affecting the growing bone in children and
adolescents. The underlying defective bone mineralization may be induced by
calcium, phosphate and/or vitamin D deficiency. Nutritional vitamin D
deficiency is the leading cause of rickets, whereas hereditary forms are rare,
accounting for about 13% of all cases [5 ].
Hereditary hypophosphatemic rickets is a group of rare disorders, associated with
renal phosphate wasting and subsequent hypophosphatemia and bone mineralization
defects, such as rickets and osteomalacia. A variety of genetic disorders
affecting phosphatonins or phosphate co-transporters are responsible for
decreased renal tubular phosphate reabsorption or transport, resulting to
hypophosphatemia with normal serum calcium. Serum FGF23 levels may be elevated
in some of these genetic defects (FGF23-related or -dependent Hypophosphatemic
rickets), while others do not affect serum FGF23 levels (FGF23-independent
Hypophosphatemic rickets). Among genetic FGF23-related disorders, X-linked
hypophosphatemia results from inactivating mutations in the PHEX gene and is the
most prevalent form [1 ]. On the other
hand, Tumor-induced osteomalacia and hypophosphatemia following intravenous iron
infusion account for the majority of acquired cases of FGF23-related
hypophosphatemia [1 ]. The spectrum of
genetic hypophosphatemic disorders, the affected gene and protein encoded, along
with the laboratory characteristics of each disorder are summarized in [Table 1 ].
Table 1 Genetically-induced syndromes of Hypophosphatemic
Rickets.
FGF23-related hypophosphatemic disorders
Disorder
Gene/protein
Laboratory findings
FGF23
TmP/GFR/UrinaryCa/creatinine
Serum Ca/P
ALP/PTH
1,25(OH)
2
D
X-linked dominant hypophosphatemic rickets
PHEX/Phosphate regulating endopeptidase
↑, n.c.
↓/n.c.
n.c./↓
↑/↑, n.c.
n.c., ↓
Autosomal dominant hypophosphatemic rickets
FGF23/Fibroblast growth factor 23
↑, n.c.
↓/n.c.
n.c./↓
↑/↑, n.c.
n.c., ↓
Autosomal recessive hypophosphatemic rickets Type 1
DMP1/Dentin matrix acidic phosphoprotein 1
↑, n.c.
↓/n.c.
n.c./↓
↑/↑, n.c.
n.c., ↓
Autosomal recessive hypophosphatemic rickets Type 2
ENPP1/Ectonucleotide
pyrophosphatase/phosphodiesterase 1
↑, n.c.
↓/n.c.
n.c./↓
↑/↑, n.c.
n.c., ↓
Hypophosphatemic rickets with hyperparathyroidism
KL/a-Klotho
↑
↓/n.c.
n.c.,↓/↑
↑/↑
n.c.
Osteoglophonic dysplasia
FGFR1/Fibrolast growth factor receptor 1
↑, n.c.
↓, n.c./n.c.
n.c./↓, n.c.
↑, n.c./↑, n.c.
n.c., ↓
McCune-Albright Syndrome
GNAS/Guanine nucleotide binding protein alpha
↑, n.c.
↓, n.c./n.c.
n.c./↓, n.c.
↑, n.c./↑, n.c.
n.c., ↓
Raine Syndrome
FAM20C/Family with sequence similarity 20, member
c
↑, n.c.
↓, n.c./n.c.
n.c./↓, n.c.
↑, n.c./↑, n.c.
n.c., ↓
Opsismodysplasia
INPPL1/Inositol polyphosphate phosphatase-like 1
↑, n.c.
↓, n.c./n.c.
n.c./↓, n.c.
↑, n.c./↑, n.c.
n.c., ↓
FGF23-independent hypophosphatemic disorders
Hereditary hypophosphatemic rickets with hypercalciuria
SLC34A3/Sodium-dependent phosphate transport protein
2 C
↓, n.c.
↓/↑
n.c./↓
↑, n.c./n.c.
↑
Hypophosphatemic rickets with nephrolithiasis and
osteoporosis type 1
SLC34A1/Sodium-dependent phosphatase transport
protein 2 A
↓, n.c.
↓/↑
n.c.,↓/↑
↑, n.c./↓, n.c.
↑
Hypophosphatemic rickets with nephrolithiasis and
osteoporosis type 2
SLC9A3R1/Sodium-hydrogen exchanger regulatory factor
1
↓, n.c.
↓/↑
n.c./↓
↑/↓, n.c.
↑
Infantile hypercalcemia type 2
SLC34A1/Sodium-dependent phosphatase transport
protein 2 A
↓, n.c.
↓/↑
n.c./↓
↑/↓, n.c.
↑
Fanconi renotubular syndrome type 2
SLC34A1/Sodium-dependent phosphatase transport
protein 2 A
↓, n.c.
↓/↑
n.c./↓
↑/↓, n.c.
↑
Dent disease 1
CLCN5/Chloride voltage-gated channel 5
↓, n.c.
↓/↑
n.c./↓
↑/↓, n.c.
↑
Dent disease 2 (Lowe syndrome)
OCRL1/Inositol polyphosphate-5-phosphatase
↓, n.c.
↓/↑
n.c./↓
↑/↓, n.c.
↑
FGF23: Fibroblast Growth Factor 23; TmP/GFR: Tubular maximum
transport of Phosphate per Glomerular Filtration Rate; Ca: Calcium; P:
Phosphate; ALP; Alkaline Phospshatase; PTH: Parathormone.;
↑ increase, ↓ decrease, n.c. no change.
FGF23 levels seem to be a useful tool for the discrimination of hypophosphatemic
rickets disorders. The diagnosis of FGF23-related hypophosphatemia may be
suspected if a patient with rickets appears with chronic hypophosphatemia and
inappropriately elevated serum FGF23 levels. In contrast to what is observed in
FGF23-related hypophosphatemic diseases, chronic hypophosphatemia of other
causes suppresses FGF23 production ([Table
1 ]).
X-linked hypophosphatemia (XLH)
XLH is a rare metabolic disorder, with an incidence of approximately 1:20 000 in
USA [1 ], and is the most common cause of
genetic hypophosphatemic rickets [2 ]. The
responsible genetic disorder is a mutated, inactivated PHEX gene
(phosphate-regulating gene with Homology to Endopeptidases that maps to the X
chromosome), which is expressed mainly in bone and teeth [2 ], and results in FGF23 excess. How
exactly PHEX regulates FGF23 levels remains unclear [4 ]. There have been reported over 300 PHEX
mutations causing the disease, the majority of which are inherited, but some of
them can arise de novo [1 ]. FGF23 excess
leads to lifelong phosphate wasting, by inhibiting phosphate renal and
intestinal reabsorption, due to blockage of renal NaPi-II cotransporters and
suppression of 1,25-dihydroxyvitamin D levels, respectively. PHEX inactivation
also appears to interfere with the metabolism of some bone extracellular matrix
proteins (e. g., osteopontin, bone sialoprotein, dentin matrix protein
1). Abnormal metabolism of these proteins gives rise to fragments called ASARM
(acidic serine aspartate-rich MEPE associated motifs) peptides, which impede
bone mineralization and cause phosphate wasting [4 ]
[6 ]
[7 ]. The consequent rickets or osteomalacia
are the most common clinical findings of XLH, and they are usually present early
in childhood. Especially in children, common clinical features include lower
extremities deformities, disproportion, short stature, motor delay,
musculoskeletal pain and stiffness. Enthesopathies, arthropathies, fractures and
pseudofractures cause chronic pain in adult patients and reduce their quality of
life. Other symptoms may include dental complications, such as recurrent root
abscesses, and hearing loss [1 ]
[8 ]
[9 ]. A patient with rickets/osteomalacia, chronic hypophosphatemia
and elevated serum FGF23 levels raises the suspicion of XLH, even without
positive family history or genetic testing [1 ].
According to the pathophysiology of the hypophosphatemic FGF23-related diseases,
hypophosphatemia, hyperphosphaturia, high ALP, high or high normal serum levels
of FGF23 and low or low normal levels of 1,25(OH)2 D are recognized as
the most common laboratory findings [1 ]
[2 ]. Normally, chronic
hypophosphatemia should suppress FGF23 expression. Therefore, high or
inappropriately normal serum FGF23 levels provides a useful serological tool for
discriminating FGF23-related hypophosphatemic rickets from hypophosphatemia of
other causes.
There are numerous methods to measure serum FGF23, including intact enzyme-linked
immunosorbent assay (intact ELISA), ELISA C-terminal assay, and automated
chemiluminescent enzyme immunoassay (CLEIA). The intact assay measures the
biologically active FGF23, using two monoclonal antibodies, one directed to the
N-terminal domain and the other one to the C-terminal domain. The C-terminal
assay detects both intact and fragmented FGF23, using two kinds of antibodies
that bind to different epitopes of C-terminal portion and is considered to
reflect the amount of FGF23 transcription or translation [3 ]. CLEIA assay measures full-length FGF23.
The disadvantage of C-terminal and CLEIA assay is that the antibodies detect
C-terminus of the molecule and the whole molecule, respectively, therefore they
both do not discriminate between the biologically inactive and the intact FGF23
molecule. In contrast, the intact FGF23 assay measures only the biologically
active intact molecule [10 ]. However, both
intact and C-terminal assay may be useful, and they generally correlate well,
except from cases where the processing of FGF23 is accelerated.
An intact serum FGF23 with cut-off value of 30 pg/ml has been
proposed to rise the suspicion of FGF23-related disease [2 ], and values greater than
45 pg/ml are considered pathological [11 ]. Regarding the evaluation of phosphate
excretion, tubular reabsorption of phosphate must be estimated, by calculating
the maximum threshold of phosphate tubular reabsorption, corrected for
glomerular filtration rate (TmP/GFR). Values<0.82 are considered
to indicate hyperphosphaturia [11 ].
Tumor-induced osteomalacia (TIO)
Tumor-induced osteomalacia is a paraneoplastic syndrome derived from FGF23
overexpression by tumor cells, and thus, shares common pathogenetic mechanisms
to all FGF23-related hypophosphatemic disorders, including XLH. It is also known
as Oncogenic Osteomalacia and is mainly observed in cases of bone and soft
tissue malignancy [2 ]
[12 ]. These mostly benign, slowly growing
tumors are localized in various sites, among which in legs, in craniofacial
regions, but also in colon, ovaries, and prostate [1 ]. Nevertheless, there have been reported
a few malignant and metastatic cases of these Phosphaturic Mesenchymal Tumors
(PMTs) [10 ]. The exact pathophysiology is
not quite understood, although some tumors causing TIO have been identified as
Klotho-expressing [2 ] and others have been
shown to carry fusions between Fibronectin gene (FN) and either Fibroblast
Growth Factor 1 gene (FGF1) or Fibroblast Growth Factor Receptor 1 gene (FGFR1).
These processes mediate an overactivation of FGFR1 signaling and excess of FGF23
actions [12 ].
Clinical features of TIO consist of progressive musculoskeletal pain, fatigue,
proximal muscle weakness, and multiple fractures, leading to long-term
disability and high morbidity. Profound weakness is the most characteristic
symptom, which may distinguish TIO from other phosphate wasting disorders [4 ]. Because of its nonspecific symptoms,
high awareness of the disease is required to avoid misdiagnosis, delayed
diagnosis, or inappropriate treatment. TIO has common laboratory findings with
other hypophosphatemic FGF23-related diseases (low serum phosphate and
1,25(OH)2 D levels, elevated serum intact FGF23 and ALP,
hyperphosphaturia). Additional assistance in localizing the tumor may be given
by FDG-PET/CT ([18 F]-2-fluoro-2-deoxy-d -glucose
(FDG) – positron emission tomography/computed tomography) or
imaging studies that target somatostatin receptors, like 68Ga-DOTA-based
PET/CT (gallium-68 Dotatate-based PET/CT) [11 ], or even selective venous sampling of
FGF23 in challenging cases [1 ]. Biopsies
or aspirations of PMTs are not suggested since they increase the concern for
tumor seeding [11 ]. The gold standard for
diagnosis of osteomalacia is bone biopsy and it helps to assess the severity of
bone disease in TIO [12 ].
Conventional therapy
Conventional therapy for XLH includes the substitution of phosphate and active
vitamin D to all symptomatic patients and those with osteomalacia. Positive
effect on bone metabolism, mineralization and architecture was observed in
recent case series [2 ]. Patients treated
with phosphate and vitamin D supplements showed less hypomineralized
periosteocytic lesions in bone biopsy [13 ]. Although supportive therapy helps reducing pain, improves fracture
healing and dental disease, no benefit was observed regarding enthesopathies
[1 ]
[8 ]. In another study, conventional therapy failed to achieve a normal
height in patients with XLH, when compared to healthy control group [2 ]. Human growth hormone (hGH) has been
introduced as an additional therapy to increase height in children, but it seems
to have poor efficacy [14 ]. Lastly, apart
from pharmacological approach, there is often the need for orthopedic
intervention.
In patients with TIO, first choice treatment, yet not always appliable, has
always been the complete surgical removal of the responsible tumor. Resection of
the phosphaturic tumor normalizes the biochemical markers of the disease and
improves osteomalacia [12 ]. However, a
minority of cases may recur after primary surgery, as showed in a retrospective
study of 230 patients with confirmed TIO. Indeed, Xiang Li et al. demonstrated
that 7% of cases recurred with a median time of recurrence of 33 months
[15 ]. When resection is not an option,
either due to patient’s comorbidities or due to the difficulty in
localizing the tumor or due to the location of the malignancy, less invasive
methods may be used. Radiotherapy and CT-guided radiofrequency may be effective
and well tolerated in patients with TIO and may help in recurrence and
metastases prevention [12 ]
[16 ]][17 ]. In cases of unresectable tumors, phosphate and active vitamin D
supplements improve osteomalacia and symptoms [2 ].
Nevertheless, the narrow therapeutic window requires careful monitoring to avoid
vitamin D overdose, hypercalciuria with or without hypercalcemia, and the
potential consequence of nephrocalcinosis, nephrolithiasis and renal impairment
[1 ]
[2 ]. Patient’s adherence to conventional treatment may be
compromised due to gastrointestinal adverse symptoms (diarrhea and abdominal
pain) and the multiple daily dosing regimen required [1 ]
[2 ]. Phosphate and calcitriol dosage are
20–40 mg/kg and 20–30 ng/kg
daily respectively for children, and 1–3 g/d and
0.75–3 μg/d respectively for adults [12 ]. It is shown that administering
phosphorus 5–7 times daily per os diminishes the risk of secondary
hyperparathyroidism, by retaining an adequate phosphorus storage [2 ]
[12 ]
[18 ]. Cinacalcet has been
suggested as an adjuvant treatment when phosphate and calcitriol supplementation
is inadequate or contraindicated. The activation of calcium-sensing receptor
(CaSR) and the consequent hypoparathyroidism are associated with a reduction of
the FGF23-related phosphaturia [19 ].
However, its use is not spread due to limited data [19 ]. In order to prevent complications such
as secondary/tertiary hyperparathyroidism, nephrolithiasis and renal
impairment, patients should be monitored every 3 to 6 months for serum levels of
calcium, phosphate, ALP and PTH, 24-hour renal calcium excretion, as well as for
their renal function [20 ].
Burosumab
The risks of mineral supplementation, patient’s noncompliance or
non-adherence and the failure of the conventional approaches to deal with the
underlying defect of phosphate metabolism, brought out the need for a better
treatment. Identification of FGF23 as the central molecule in the
pathophysiology of these hypophosphatemic diseases, directed the attention of
scientific community towards finding an agent that could possibly block FGF23
action. Preclinical studies in hypophosphatemic mice, showed that inhibition of
FGF23, FGF Receptor or ERK signaling suppressed FGF23 and its action, improving
hypophosphatemia [21 ]
[22 ]
[23 ]. Moreover, when anti-FGF23 antibodies were administered in
hypophosphatemic mice, phosphate levels, rickets, bone mineralization and muscle
weakness were ameliorated [24 ]
[25 ].
Thereafter, numerous clinical trials have been conducted that support the
efficacy and favorable safety profile of the drug. The first phase I study of a
single administration of anti-FGF23-antibody KRN23 in adult patients with XLH
demonstrated increase in serum phosphate, 1,25(OH)2 D and
TmP/GFR, in a dose-dependent mode [26 ]. Of note, subcutaneous (sc.) administration of the drug led to a
prolonged effect, in contrast to intravenous (iv.) injections [27 ]. In a subsequent phase II trial, a
monoclonal antibody that targets FGF23, burosumab, was administered sc. to 52
child patients with XLH rickets either every 2 weeks or every 4 weeks [28 ]. The results confirmed the benefits,
and additionally, showed a radiographic improvement of rickets according to
Thacker Rickets Severity Score (RSS) and Radiographic Global Impression of
Change (RGI-C) [28 ]; maintenance of the
results after 64 weeks of follow-up was observed [28 ]. Moreover, the researchers recorded
that twice-monthly administration is more effective than monthly-injections in
children [28 ]. On the other hand, in
adults, once-monthly regimen exhibited long-term favorable effects with
prolonged serological improvements for more than a year [27 ]. A phase III study including children
aged 1–12 years demonstrated that burosumab was superior to conventional
therapy in improving, among many parameters, serum phosphate and
1,25(OH)2 D, radiographic rickets findings and
length/height Z-score at week 64 [29 ]. In addition, a controlled, multicenter trial including children
indicated that burosumab improves long bone mineralization and growth compared
to conventional treatment with phosphate and vitamin D supplements [30 ]. The majority of adverse events related
to treatment were linked to injections site reactions (ISRs) (erythema,
pruritus, rash, swelling, discomfort, hypersensitivity, inflammation), were mild
to moderate and lasted only a few days, with no discontinuation of the treatment
or study. Approximately 10% of each group (conventional therapy and
burosumab) had a more serious adverse event that was considered unrelated to
treatment [29 ]. However, it is important
to clarify whether the increased rate of dental abscesses with burosumab
treatment versus conventional therapy were drug-related or related to patient
variability or even a direct dental favorable effect of conventional treatment
[29 ]. A recent trial including 5
children with genetically confirmed XLH, proved that when administrating
burosumab sc. at 0.8 mg/kg every 2 weeks, the positive effects
on phosphate metabolism, ALP levels and growth were continued for a year [31 ]. No serious adverse events, like those
related to conventional therapy, were reported with burosumab, however mild
transient adverse events related to injection site as well as headache were
noted [31 ].
After the first clinical trial of Carpenter et al. [26 ], numerous trials investigating the
efficacy and safety of burosumab in XLH included adult patients. A randomized,
double-blind, placebo-controlled phase III trial with 134 adults, with confirmed
PHEX mutation [32 ], indicated that
burosumab improved serum phosphate, fracture healing, stiffness and physical
function scores. Patients in the burosumab group had a 16.8-fold greater chance
in fully fracture healing than those in the placebo group at week 24. Bone
metabolism markers were also improved significantly [32 ]. Continuation of burosumab treatment
assured a sustained corrected phosphorus state, and ongoing fracture healing and
musculoskeletal improvement [33 ]. Another
phase III trial, focused on assessing the effect of burosumab on
histomorphometric measures of osteomalacia in adult patients with XLH.
Transiliac bone biopsies at week 48 proved enhancement of all
osteomalacia-histomorphometric parameters, including osteoid volume/bone
volume, osteoid thickness, osteoid surface/bone surface and
mineralization lag time. Additional improvement of serum phosphorus levels, bone
markers and fracture healing were observed [34 ]. These trials confirmed the good safety profile of the drug for
adult patients. There were no serious adverse events related to burosumab,
regarding serum or urine calcium, phosphate, PTH or nephrocalcinosis [32 ]
[34 ]. Nevertheless, there are a few worrisome data regarding some
potentially serious adverse events that require further research. Imel et al.
observed that a patient had a deterioration of coronary artery calcification
upon dose escalation of burosumab [27 ].
Burosumab was also shown to improve osteomalacia and its related biochemical
features in patients with TIO. Jan De Bur et al., in a phase II study, reported
increased and maintained serum phosphorus levels at week 144 in 14 American
adult patients with TIO under burosumab therapy [35 ]. Moreover, transiliac bone biopsies at week 48 suggested
improvement of osteomalacia parameters: osteoid volume/bone, osteoid
thickness and mineralization lag time. Patients demonstrated less pain and
weakness, improved fracture healing and had fewer new fractures at week 144. It
is important that no serious adverse events were related to treatment, and 9
burosumab-related events were all mild to moderate in severity, proving the
safety of the drug [35 ]. Burosumab
exhibited similar safety profile, improved phosphate metabolism and osteomalacia
in a phase II open-label study including 13 Japanese and Korean TIO patients
[36 ]. Imanishi et al. reported that
monthly administration of burosumab led to elevation of fasting serum phosphate
level, along to an increase followed by decrease in bone biomarkers, and
improved motor function and fracture healing. No serious related adverse events
were observed, as previously [36 ].
These results triggered the approval of burosumab (Crysvita) for treatment of
child and adult patients with XLH by FDA (Food and Drug Administration) and EMA
(European Medicines Agency) in 2018, and since then in numerous countries (Asia,
South America, Middle East). The approval of FDA regarding TIO treatment
followed in June 2020. To date, the published consensus statement from Haffner
et al. [37 ], provides key recommendations
for burosumab clinical use [8 ]
[18 ]
[37 ]. Regarding children and adolescents, burosumab treatment should
be considered in XLH patients aged older than 6 months, who are not candidates
or do not respond for any reason to conventional treatment or have complications
related to conventional therapy. Radiographic evidence of overt bone disease is
another reason to consider starting burosumab in those patients [37 ]. Regarding adults, treatment with
burosumab should be considered in symptomatic patients with persistent bone
and/or joint pain that limits daily activities, as well as in patients
with pseudofractures, fractures or inadequate response to conventional therapy
(grade B, moderate recommendation). Moreover, initiation of burosumab should be
considered in all XLH patients who experience complications of the conventional
therapy (grade D, weak recommendation) [37 ].
During burosumab therapy, supplementation with phosphate or 1,25-dihydroxyvitamin
D is contraindicated and must be stopped at least two weeks before beginning
burosumab [4 ]. It should not be started in
patients with significant renal insufficiency [4 ], as defined by estimated glomerular filtration rate (eGFR) below
29 ml/min/1.73 m2 . In children, it is
administered sc. in a dosage of 0.8 mg/kg every two weeks, and
1 mg/kg every four weeks in adults, with a starting dose of
0.5 mg/kg. Before initiating therapy, patients must have a
fasting serum phosphate of<2.5 mg/dl, while titration
and dose adjustments are made based on fasting serum phosphate [4 ]. Regular follow up with blood tests
should be performed, including phosphate, calcium, PTH and ALP [30 ]. The pathophysiology of XLH and TIO
along with the new therapeutic approach of burosumab are demonstrated in [Fig. 2 ] and [Table 2 ].
Fig. 2 Burosumab cancels the effect of FGF23 in renal phosphate
regulation. a : NaPi II cotransporters mediate renal phosphate
reabsorption. b : FGF23 excess inhibits the transporter resulting
in impaired reabsorption of phosphate and phosphaturia. c :
Burosumab is an antibody against FGF23, prevents FGF23-mediated
inhibition of the phosphate transporter and allows the reabsorption of
phosphate (NaPi II; Sodium/Phosphate cotransporters type II, Pi;
inorganic phosphate, FGF23; Fibroblast Growth Factor 23).
Table 2 Summary of new approved treatment agents for XLH
and HPP.
Burosumab
Asfotase alfa
Commercial name
Crysvita
Strensiq
Mechanism of action
Anti-FGF23 antibody
Enzyme replacement therapy
Indications
X-linked hypophosphatemia (XLH)-Tumor induced osteomalacia
(TIO)
Hypophosphatasia (HPP)
Contraindications
Hyperphosphatemia, phosphate and calcitriol supplementation,
Renal insufficiency
Allergy
Approval (FDA)
2018 (XLH)/2020(TIO)
2015
Effects
Rickets ↓
Survival rate ↑
Bone mineralization ↑
Bone mineralization ↑
Growth ↑
Respiratory status ↑
Fracture healing ↑
Fracture healing ↑
Physical function ↑
Physical function ↑
Pain ↓
Pain ↓
Bone formation markers ↑
Bone formation markers ↑
Cognitive development ↑
Common side effects
Injection site reactions
Injection site reactions
Headache
Ectopic calcifications
Bone metabolism parameters
Calcium
n.c.
n.c. , ↓
Phosphate
↑
n.c. , ↑↓
Calcitriol
↑
–
ALP
↓
↑
TmP/GFR
↑
–
PLP
-
↓
Dose regimen
Pediatric patients
(>6 months) 0.8 mg/kg q 2 weeks
sc.
1 mg/kg q 4 weeks sc.
2 mg/kg/d three days a week sc. or
1 mg/kg/d six days a week sc.
Adult patients
↑ increase, ↓ decrease, n.c. no change, s.c.
subcutaneously.
Pan-fibroblast growth factor receptor (FGFR) kinase inhibitor
After the identification of the FN1-FGFR1 translocation as the molecular defect
in the pathogenesis of some PMTs, FGFR1 inhibition emerged as an appealing new
therapeutic target. Wöhrle et al. investigated the impact of a novel
selective, pan-specific FGFR inhibitor, infigratinib (BGJ398), in
hypophosphatemic mouse model of XLH [38 ].
They showed that infigratinib suppresses FGF23 renal signaling leading to
increased 1,25(OH)2 D biosynthesis and correction of hypophosphatemia
and hypocalcemia in Hyp mice. Furthermore, long-term treatment of Hyp mice with
infigratinib led to a complete normalization of hypophosphatemia and
hypocalcemia, enhanced bone mineralization, normalized bone turnover, restored
growth plate organization and increased longitudinal growth of the long bones
[38 ].
Hartley et al. described the effect of infigratinib in the treatment of a
66-year-old man with metastatic PMT [39 ].
The pan-FGFR tyrosine kinase inhibitor induced an immediate and dramatic
response regarding FGF23 levels and metastases regression. Infigratinib received
FDA approval in 2021 for the treatment of cholangiocarcinomas and thus might
hold promise for clinical use in FGF23-related hypophosphatemic disorders in the
future, such as XLH and TIO. The efficacy and safety profile of FGFR tyrosine
kinase inhibitors in TIO is under ongoing research.
Alkaline Phosphatase and its role in bone mineralization
Human alkaline phosphatase (ALP) is a glycoprotein composed of four isoenzymes:
the intestinal (IAP), the placental (PLAP), the placental-like or germ cell
(GCAP), and the liver/bone/kidney (tissue non-specific; TNSALP)
isoenzyme. Coding genes for IAP, GCAP and PLAP are located in chromosome 2 and
the derived isoforms are 90 to 98% identical in their amino acid
sequences, while TNSALP gene is mapped to chromosome 1 and the derived TNSALP
has less than 50% homology with the three aforementioned isoenzymes
[40 ]
. The cells genetically
determined to express TNSALP are found mainly in bone (hypertrophic chondrocytes
and osteoblasts), liver and kidney. TNSALP is also found in teeth
(odontoblasts), central nervous system, fibroblasts, and other cell types [41 ]
. It is anchored to the plasma
membrane, in a way that follows membrane’s fluidity [40 ].
The metalloenzyme ALP possesses an important role in bone mineralization, by
hydrolyzing inorganic pyrophosphate (PPi) to inorganic phosphate (Pi). PPi is
provided by nucleotide pyrophosphatase phosphodiesterase-1 (NPP1) from
nucleotide triphosphates and by ankyrin in membranes of hypertrophic
chondrocytes and osteoblasts. Of note, PPi is a strong inhibitor of
hydroxyapatite formation. This balance between the activity of TNSALP, that
regulates the ratio of Pi to PPi, and the activities of NPP1 and ankyrin, that
regulate PPi load, is considered to be critical for bone mineralization [42 ]
[43 ]. In vitro, inhibition of TNSALP function suppresses TNSALP mRNA
expression and mineralization [43 ]. The
physiological substrates of TNSALP, apart from PPi, are thought to be pyridoxal
5′-phosphate (PLP) and, probably, phosphoethanolamine (PEA) [43 ]. As a result, they accumulate in cases
of TNSALP deficiency. Recent studies have also added adenosine
triphosphate (ATP), di-phosphoryl lipopolysaccharide (LPS), and phosphorylated
osteopontin (p-OPN) as alternative natural substrates [40 ]. PLP, an activated form of vitamin B6,
is involved as a cofactor in the formation of neurotransmitters in neuronal
cells, such as dopamine, serotonin, and γ-aminobutyric acid (GABA) [43 ]. Normally, PLP is not transportable
into neuronal cells, but TNSALP transforms PLP to pyridoxal, thus, enables it to
enter cells. Perhaps that explains the fact that inactivation of TNSALP gene in
mice results in seizures [44 ].
Hypophosphatasia (HPP)
Hypophosphatasia (HPP) is a rare inherited metabolic disease that results from
inborn mutations in the TNSALP gene (ALPL). So far, there have been reported
over 400 mutations, that cause exceptional clinical heterogeneity,
ranging from preterm intrauterine death to only dental complications or even
asymptomatic carriers [41 ]. The model of
inheritance may be autosomal recessive, that usually causes a more severe
phenotype, or autosomal dominant, that likely appears with variable penetrance
[45 ]. The incidence of the severe
phenotype is low and considered to be approximately 1:100 000 in Canada [46 ] and 1:300 000 in Europe [41 ]. The mutated TNSALP performs diminished
activity and leads to accumulation of its main substrates, PPi and PLP [41 ]. The inhibitory role of PPi in
mineralization along with the role of PLP as a cofactor for many enzymatic
procedures explain the musculoskeletal and systemic features of the disorder.
The predominant manifestations of the disease consist of impaired bone
mineralization, deformities, fractures, delayed fracture healing, premature
tooth loss, muscle weakness and musculoskeletal pain [8 ]. Other non-bone specific manifestations
include pulmonary abnormalities, seizures, impaired motor skills and
nephrocalcinosis [46 ].
HPP may manifest through a variety of phenotypes, from lethal disease in early
perinatal age, to isolated dental complications and premature tooth loss.
According to patient’s age of first appearance and the severity of
symptoms, HPP is classified as perinatal, infantile, childhood or adult, and
there is also the type of odontohypophosphatasia [40 ]
[41 ]. Perinatal disease is lethal and is presented with almost total
absence of bone mineralization, severe pulmonary manifestations and seizures.
There are some cases that are categorized as “benign prenatal
HPP” and include only long bone deformities, which may improve
spontaneously [40 ]. The infantile type
appears about six months after birth and includes rickets-like signs, failure to
thrive and pulmonary complications [41 ].
The childhood type may be mild to moderate in severity, according to the extent
of impaired physical function and skeletal deformities. The adult type affects
middle-aged adults and shows moderate symptoms of musculoskeletal pain, a mild
history of osteomalacia or fractures/pseudofractures, with recurrent
metatarsal stress fractures being characteristic [40 ]
[41 ]
[46 ]. Although dental
complications may be present in any HPP type, defective tooth formation is the
only clinical abnormality found in odontohypophosphatasia, resulting in loss of
primary teeth in children and early loss of permanent teeth or periodontal
pathology in adults [40 ].
Medical history, physical examination, and routine laboratory and radiographic
imaging indicative of osteomalacia are matched to elevated substrates of TNSALP
and the hallmark of low serum ALP for the diagnosis of HPP [47 ]. Low levels of ALP below
100U/ml in a child with rickets and less than 40 U/l in adults
of both sexes are highly suspicious of HPP [18 ]
[41] ]. Genetic detection of
the mutated ALPL gene sets the definite diagnosis, yet usually unnecessary [9 ]. Despite the fact that nearly all cases
of rickets/osteomalacia suggest hypocalcemia and/or
hypophosphatemia, in HPP, the potential hypercalcemia explains the suppression
of PTH and the consequent hyperphosphatemia in these patients [47 ].
Supportive therapy and experimental treatments
Attempts have been pointed towards improvement of prognosis and quality of life
for patients with HPP, for many years. Nonetheless, the supportive treatment
offered so far failed to increase the survival rate of severe perinatal cases.
Mechanical ventilation can be challenging due to thoracic deformity or pulmonary
hypoplasia; pyridoxine administration has little role in restraining severe
neurological manifestations due to PPi excess; hydration, dietary calcium
restriction or loop diuretic agents may improve mild hypercalcemia;
non-steroidal anti-inflammatory treatment, physical exercise and physiotherapy
may be useful for musculoskeletal symptoms; several surgical procedures may be
necessary, including craniotomy, postosteotomy, dental procedures and scoliosis
operations [47 ]. Antiresorptives have not
only failed to improve the impaired mineralization, but they also prove to be
harmful. Biphosphonates are PPi analogs that have high affinity with
hydroxyapatite. N-containing BPs directly inhibit bone TNSALP activity, by
binding both Zn2+ and Mg2+
[48 ]. Indeed, a case series of multiple
atypical femoral fractures, associated with bisphosphonate therapy in an
adult-onset HPP patient has been reported in literature [49 ]. Vitamin D, phosphate and calcium
should be administered at a necessary minimum, to ensure the prevention of
rickets and as well diminish the risk of hypercalcemia, hypercalciuria and
nephrocalcinosis. Another treatment that failed to avoid substantial morbidity,
is allogenic stem cell transplantation, whereas anabolic treatment with the
parathyroid hormone analogue teriparatide remains controversial [45 ]
[46 ]. Although teriparatide has been shown to improve mobility and
bone pain, increase ALP levels and accelerate fracture healing in HPP cases
[50 ]
[51 ], these benefits were not guaranteed in every case [52 ] and were not sustained over time [53 ]. Teriparatide is contraindicated in
children [54 ].
There are some promising data regarding the use of sclerostin-antibody
romosozumab in HPP patients. Sclerostin is a protein encoded by the SOST gene,
is expressed in osteocytes and osteoblasts, and interferes with the Wnt
signaling pathway, by preventing low-density-lipoprotein-related protein 5
receptor (LRP5 receptor) from interacting with the frizzled receptor, thus
inhibiting osteoblastic bone formation [55 ]. The phenotype of human genetic disorder of sclerosteosis was
reproduced in SOST-knock out mice, with high bone mass being the prevalent
characteristic [56 ]. Both in preclinical
and clinical studies, administration of monoclonal antibodies against
sclerostin (BPS804) was shown to improve bone mineral density, fracture healing
and bone formation markers in healthy and osteoporotic individuals [55 ]
[57 ]. Seefried et al. enrolled 8 adult patients with HPP in a phase
IIA open-label study and assessed the beneficial effect of BPS804 regarding bone
formation, bone mineral density (BMD), bone biomarkers and ALP activity [58 ]. Patients received 3 ascending iv.
doses of BPS804 and were followed for 16 weeks after the last dose on day 29.
BPS804 administration was associated with increased mean ALP, bone-specific ALP
activity, bone formation markers and lumbar spine BMD, whereas no serious safety
issues were noted [58 ]. Despite the
promising results, romosozumab is not approved for HPP patients, while long-term
and large-scale results are not yet available [45 ].
Asfotase alfa: recombinant enzyme replacement therapy in HPP
After ineffective efforts to ameliorate clinical symptoms and radiographic signs
of HPP using iv. infusions of soluble ALP [46 ], it has been well-recognized that targeted correction of ALP
activity within skeletal tissue was necessary. Asfotase alfa is a human
recombinant replacement therapy that targets the bone, by having a
deca-aspartate motif that enhances its binding to hydroxyapatite. It also
includes the catalytic domain of TNSALP, responsible for its enzymatic function,
and the Fc fragment of IgG1, which prolongs its circulating half-life [41 ]. In 2008, a preliminary animal study in
TNSALP knockout mice (a good model for the infantile form of HPP [59 ]), demonstrated that administration of
human TNSALP enzyme replacement prevented the onset of HPP [60 ]. Notably, treated mice presented normal
growth and had normal biochemical, radiographic and clinical parameters, when
sc. administration of therapy was initiated at birth [60 ]. Later, in 2011, Mckee et al.
confirmed, with micro-computed tomography and histology, the dose-dependent
favorable effect of mineral-targeting human TNSALP on mineralization of alveolar
bone, dentin and cementum in treated knockout mice [61 ].
In 2012, the first clinical trial of asfotase alfa was conducted in 11 patients
aged less than 3 years old with perinatal or infantile HPP and severe associated
symptoms [62 ]. Asfotase alfa was
administered by one iv. infusion followed by numerous sc. injections for up to 1
year and led to improvement of skeletal manifestations as well as respiratory
status, motor function and cognitive development. These outcomes were evaluated
using RGI-C score, RSS and the Bayley Scales of Infant and Toddler Development
third edition (Bayley-III). The most common side effects related to treatment
were localized, mild-to-moderate in severity, ISRs [62 ]. An extension study showed similar
efficacy and safety as well as sustained improvements for up to 7 years of
treatment. The most frequent side effects were pyrexia and upper respiratory
tract infection [63 ]. When 69 pediatric
patients with severe HPP of infantile-onset were evaluated with RGI-C score,
after therapy with asfotase alfa up to 6 years, they showed early and sustained
radiographic and clinical improvement [64 ]. Moreover, positive results regarding respiratory status, growth, ALP
activity and PPi concentration were observed. The drug was generally
well-tolerated and, although all patients had at least one treatment-related
adverse event, they were mostly mild and accounted mainly for ISRs (erythema,
discoloration, induration, hematoma) or injection associated reactions (pyrexia,
chills, rash, anaphylactic reaction). Some cases of abnormal calcium levels were
recorded, and anti-asfotase alfa antibodies were identified in 88% of
patients, 67% of which were neutralizing, but were not related to
adverse events [64 ]. Therapy with asfotase
alfa has also shown a significantly favorable effect in extending the survival
of patients with perinatal and infantile HPP, according to data obtained from
two multicenter, phase II interventional studies, compared to historical
controls [65 ]. Treated patients received
the drug as sc. injection either 1 mg/kg six times per week or
2 mg/kg thrice weekly. 95% of them were alive at 1 year
of age, comparing to only 41% of untreated patients. That difference
increased more at 5 years of age, when 84% of treated patients survived.
Most common AEs were mild to moderate ISRs. No clinically important ectopic
calcification (5 patients had probably related to treatment calcium deposits on
conjunctiva or cornea) or lipohypertrophy occurred. A recent study assessed the
effect of asfotase alfa on the development and exfoliation patterns of primary
and permanent teeth in 11 infants and children with early-onset HPP. It
concluded that early initiation and continuation of therapy is superior
regarding oral health and the amount of prematurely exfoliated primary teeth
[66 ].
Asfotase alfa also seems to be beneficial for adult patients with pediatric-onset
HPP, improving the biochemical characteristics of the disease: it increases
TNSALP activity and lowers both PLP and PPi levels [67 ]
[68 ]
[69 ], with no significant
difference between the ALPL variant states [70 ]. When a group of adolescents and adults with childhood-onset HPP,
treated with asfotase alfa, was compared to an untreated control group, after 6
months of therapy, there was found a higher TNSALP activity, lower PLP and PPi
levels and significant improvement in the 6-minute-walk test [67 ]. Bone healing is delayed and
compromised in patients with HPP. Asfotase alfa treatment of adult patients was
found to remarkably improve osseous consolidation in the region of the
non-healing bone, by rapidly increasing levels of bone volume per tissue volume
(BV/TV) [68 ]. Moreover, it rapidly
increases the callus formation at the osteotomy site [71 ], increases mineral maturation,
ameliorates bone microarchitecture deterioration and mineralization
heterogeneity, improving volumetric bone density, structure and strength
parameters [72 ]
[73 ]. In an observational, prospective
single-center study, fourteen patients with pediatric-onset HPP, 19 to
78 years old, received asfotase alfa for at least 12 months,
with a dosing regimen of either 2 mg/kg sc. three times a week,
or 1 mg/kg sc. six times a week [74 ]. Treated patients demonstrated improved physical function, as
assessed by many tests (e. g., 6–minute walk test, timed
up-and-go test, Short Physical Performance Battery) as well as better
health-related quality of life, including significant decrease in pain intensity
at 6 months of treatment. No new safety concerns were identified [74 ]. Seefried et al. included in their
analysis 21 adult patients, with genetically confirmed pediatric-onset HPP, who
were treated with asfotase alfa for 24 months. They assessed the changes of many
bone turnover and mineral metabolism parameters [PLP, urine
PEA/creatinine (Cr) ratio, serum PTH, calcium, phosphate, FGF23,
osteocalcin, procollagen type 1 N-propeptide (P1NP), tartrate-resistant acid
phosphatase 5b (TRAP5b), N-terminal telopeptide of type 1 collagen (NTx)] as
well as BMD T-scores at baseline and 3, 6, 12, 18, and 24 months after treatment
initiation. Lumbar spine BMD T-scores continued to increase throughout the
follow up. They concluded that asfotase alfa mediates bone mineralization and
bone turnover on previously inaccessible bone [69 ] ([Fig. 3 ]).
Fig. 3 Pathophysiology and therapeutic approach in
hypophosphatasia. a : Alkaline phosphatase (ALP) hydrolyzes
inorganic pyrophosphate (PPi) to inorganic phosphate (Pi) and pyridoxal
phosphate (PLP) to pyridoxal. It promotes bone mineralization, as PPi is
a strong inhibitor of hydroxyapatite formation. Phosphoethanolamine
(PEA) is considered another natural substrate. b : In
hypophosphatasia, ALP deficiency leads to ALP substrates accumulation in
blood and impaired bone mineralization. c : Asfotase alfa (AA) is
a recombinant enzyme mimicking the natural effect of ALP in bone and
allowing bone mineralization.
To date, little data are available, concerning asfotase alfa use in adult-onset
HPP. Magdaleno et al. presented a unique case of a woman, whose symptoms mainly
presented in adulthood [75 ]. She was
started on off-label use of asfotase alfa at a dose of 1 mg/kg 3
times weekly sc. and within a few months she presented significant improvements
in physical function, bone pain and healing [75 ]. In Japan, a patient with adult-onset HPP was included in a
multicenter prospective trial, among other 12 child patients with perinatal,
infantile or childhood form. All patients survived and had significant benefits
regarding their radiographic, developmental, physical and respiratory status
[76 ]. Only 2% of a total 195
adverse events were severe, and only 2 were related to treatment (convulsion due
to hypocalcemia) [76 ].
In 2015, asfotase alfa (Strensiq) was approved for patients of any age with
pediatric-onset HPP to treat bone manifestations of the disease, first in Japan
then in Canada, European Union, United States and elsewhere [47 ]. Asfotase alfa is also approved for
adult form of HPP [41 ]. Current
recommendations suggest that it must be administered sc., with a dosage regimen
of either 2 mg/kg/d three days a week or
1 mg/kg/d six days a week [77 ]. A maximum dose of
9 mg/kg a week may be a choice for patients who respond poorly,
but only after excluding other potential reasons for treatment failure [77 ]. Suboptimal or discontinued treatment
leads to HPP symptoms regression, with weakness and radiographic deterioration
[78 ]. Thus, a strict follow-up is
required to adjust treatment dose, and to perform functional, growth,
radiographic and biochemical assessment [77 ]. The criteria established by Khan et al., that make adult
patients with HPP potential candidates for enzyme-replacement treatment are the
presence of osteomalacia and its complications; major osteoporotic fractures,
pseudofractures or delayed/incomplete fracture healing; intractable
musculoskeletal pain or chondrocalcinosis requiring or unresponsive to opioids;
impaired physical function and mobility [79 ]. The pathophysiology and therapeutic approach of HPP are
summarized in [Fig. 3 ] and [Table 2 ].
