Evaluation of the therapeutic potential of a CNP analog in a Fgfr3 mouse model recapitulating achondroplasia

Sustained growth-promoting effects of vosoritide in children with achondroplasia from an ongoing phase 3 extension study

BACKGROUND

Vosoritide is a C-type natriuretic peptide analog that addresses an underlying pathway causing reduced bone growth in achondroplasia. Understanding the vosoritide treatment effect requires evaluation over an extended duration and comparison with outcomes in untreated children.

METHODS

After completing ≥6 months of a baseline observational growth study and 52 weeks in a double-blind, placebo-controlled study (ClinicalTrials.gov: NCT03197766), participants were eligible to continue treatment in an open-label extension (ClinicalTrials.gov: NCT03424018) wherein all received 15 μg/kg vosoritide daily. Data from the CLARITY achondroplasia study provided an external untreated control population and reference data.

FINDINGS

The population comprised 119 participants. Annualized growth velocity with vosoritide was similar to the average-stature population before puberty. The mean (SD) differences in annualized growth velocity across each integer age (6-16 years) between treated and untreated children were 1.84 (0.38) cm/year in boys and 1.44 (0.63) cm/year in girls. Three-year comparisons of treated versus untreated children demonstrated an additional height gain of 5.75 cm (95% confidence interval [CI]: 4.93, 6.57) with vosoritide. A significant improvement in upper-to-lower body segment ratio at 3 years of treatment was observed for participants with assessments at age <11 (females) and <12 years (males) versus population-level, age-matched, untreated controls (p = 0.0087). The arm span-to-standing height ratio remained consistent with untreated participants. Vosoritide had a favorable safety profile with continuous treatment for up to 6 years (464.05 person years of exposure). No long-term harms or deaths were observed.

CONCLUSIONS

Vosoritide treatment was well tolerated and had sustained growth-promoting effects in children with achondroplasia treated for up to 6 years.

FUNDING

This work was funded by BioMarin Pharmaceutical.

TYRA-300, an FGFR3-selective inhibitor, promotes bone growth in two FGFR3-driven models of chondrodysplasia

Achondroplasia (ACH) and hypochondroplasia (HCH), the two most common types of dwarfism, are each caused by FGFR3 gain-of-function mutations that result in increased FGFR3 signaling, which disrupts chondrogenesis and osteogenesis, resulting in disproportionately shortened long bones. In this study, TYRA-300, a potent and selective FGFR3 inhibitor, was evaluated in 3 genetic contexts: wild-type mice, the Fgfr3Y367C/+ mouse model of ACH, and the Fgfr3N534K/+ mouse model of HCH. In each model, TYRA-300 treatment increased nasoanal length and tibia and femur length. In the two FGFR3-altered models, TYRA-300-induced growth partially restored the disproportionality of long bones. Histologic analysis of the growth plate in Fgfr3Y367C/+ mice revealed that TYRA-300 mechanistically increased both proliferation and differentiation of chondrocytes. Importantly, children with ACH can experience medical complications due to foramen magnum stenosis, and TYRA-300 significantly improved the size and shape of the skull and foramen magnum in Fgfr3Y367C/+ mice. Spinal stenosis is also a frequent complication, and TYRA-300 increased the lumbar vertebrae length and improved the shape of the intervertebral discs in both models. Taken together, these studies demonstrate that the selective FGFR3 inhibitor TYRA-300 led to a significant increase in bone growth in two independent FGFR3-driven preclinical models as well as in wild-type mice.

Real-world Outcome of Vosoritide Treatment in Children With Achondroplasia: A 12-month Retrospective Observational Study

Context

Vosoritide is the first approved targeted therapy for achondroplasia (ACH) based on increased annualized growth velocity in clinical trials. The aim of our project was an assessment of the real-world setting and treatment with vosoritide.

Design

This was a 12-month, retrospective observational study on an inception cohort of 34 patients with ACH treated with vosoritide.

Patients and Methods

Thirty-four patients with ACH (22 males; aged 2.8 to 15.3 years at treatment initiation) who received vosoritide treatment for at least 12 months at a specialized clinic for skeletal dysplasia in childhood were included in the analysis. Auxological measurements at baseline and after 12 months of therapy were converted into disease-specific (ACH) and general population [Centers for Disease Control and Prevention (CDC)] z-scores. Physical function assessed by a 6-minute walk test was converted into z-scores and compared to an unaffected reference cohort.

Results

After 12 months of treatment, both ACH and CDC height z-scores showed significant increases, with mean changes (mean ± SD) of 0.52 ± 0.35 and 0.38 ± 0.44, respectively (both P < .0001). The annualized growth velocity exceeded reference values for untreated children with ACH. No significant changes were observed in body mass index, upper to lower body segment ratio (sitting height/height), or head circumference. The 6-minute walking distance improved, with z-scores increasing from -2.00 ± 1.12 to -1.39 ± 1.23 (P = .0215).

Conclusion

In a real-world setting, children with ACH showed significant improvements in growth and physical function after 12 months of treatment with vosoritide.

CDK8 inhibitor KY-065 rescues skeletal abnormalities in achondroplasia model mice

Cyclin-dependent kinase 8 (CDK8) is a transcription-related CDK family member implicated in the regulation of bone homeostasis, and we recently demonstrated that our internally developed CDK8 inhibitor KY-065 can prevent postmenopausal osteoporosis in a mouse model. Achondroplasia (ACH), the most common form of genetic dwarfism in humans, is caused by a gain-of-function mutation in fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase that activates downstream mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription (STAT) signaling pathways. The first precision drug approved for the treatment of ACH in children, the C-type natriuretic peptide analog vosoritide, antagonizes the MAPK pathway, while there are currently no effective and safe medications targeting the STAT1 pathway. Here, we demonstrate that KY-065 rescues impaired chondrogenesis and stunted long bone growth in the Fgfr3Ach mouse model of ACH. KY-065 inhibited CDK8 with high affinity in vitro by competing with ATP. The CDK8 expression and STAT1Ser727 phosphorylation were upregulated in chondrocytes isolated from ACH model mice, and KY-065 repressed its phosphorylation and restored normal chondrogenic differentiation without affecting MAPK activation. Moreover, daily administration of 10 mg/kg KY-065 to Fgfr3Ach mice (yielding a peak concentration of 22.0 ± 1.47 μM in plasma) resulted in significant elongation of long bone and improved growth plate cytoarchitecture. Collectively, these findings identify the CDK8 in chondrocytes as a potential therapeutic target for ACH and KY-065 as a promising candidate drug treatment for this debilitating skeletal disease.

FGFR antagonists restore defective mandibular bone repair in a mouse model of osteochondrodysplasia

Gain-of-function mutations in fibroblast growth factor receptor (FGFR) genes lead to chondrodysplasia and craniosynostoses. FGFR signaling has a key role in the formation and repair of the craniofacial skeleton. Here, we analyzed the impact of Fgfr2- and Fgfr3-activating mutations on mandibular bone formation and endochondral bone repair after non-stabilized mandibular fractures in mouse models of Crouzon syndrome (Crz) and hypochondroplasia (Hch). Bone mineralization of the calluses was abnormally high in Crz mice and abnormally low in Hch mice. The latter model presented pseudarthrosis and impaired chondrocyte differentiation. Spatial transcriptomic analyses of the Hch callus revealed abnormally low expression of Col11, Col1a, Dmp1 genes in mature chondrocytes. We found that the expression of genes involved in autophagy and apoptosis (Smad1, Comp, Birc2) was significantly perturbed and that the Dusp3, Dusp9, and Socs3 genes controlling the mitogen-activated protein kinase pathway were overexpressed. Lastly, we found that treatment with a tyrosine kinase inhibitor (BGJ398, infigratinib) or a C-type natriuretic peptide (BMN111, vosoritide) fully rescued the defective endochondral bone repair observed in Hch mice. Taken as a whole, our findings show that FGFR3 is a critical orchestrator of bone repair and provide a rationale for the development of potential treatments for patients with FGFR3-osteochondrodysplasia.

Progress in managing children with achondroplasia

INTRODUCTION

Achondroplasia is a heritable disorder of the skeleton that affects approximately 300,000 individuals worldwide. Until recently, treatment for this condition has been purely symptomatic. Efficacious treatment options for children are now approved or are in clinical trials.

AREAS COVERED

This review discusses key advances in the therapeutic management of children with achondroplasia, including vosoritide, the first approved drug, and other emerging precision therapies. These include navepegritide, a long-acting form of C-type natriuretic peptide, and infigratinib, a tyrosine kinase receptor inhibitor, summarizing trial outcomes to date.

EXPERT OPINION

The advent of the first approved precision therapy for achondroplasia in vosoritide has been a paradigm shifting advance for children affected by this condition. In addition to changing their natural growth history, it is hoped that it will decrease their medical complications and enhance functionality. These new treatment options highlight the importance of prompt prenatal identification and subsequent testing of a suspected fetus with achondroplasia and counseling of families. It is hoped that, in the near future, families will have the option to consider a range of effective targeted therapies that best suit their child with achondroplasia, starting from birth should they choose.

Cellular and molecular mechanisms that shape the development and evolution of tail vertebral proportion in mice and jerboas

Despite the functional importance of the vertebral skeleton, little is known about how individual vertebrae elongate or achieve disproportionate lengths as in the giraffe neck. Rodent tails are an abundantly diverse and more tractable system to understand mechanisms of vertebral growth and proportion. In many rodents, disproportionately long mid-tail vertebrae form a 'crescendo-decrescendo' of lengths in the tail series. In bipedal jerboas, these vertebrae grow exceptionally long such that the adult tail is 1.5x the length of a mouse tail, relative to body length, with four fewer vertebrae. How do vertebrae with the same regional identity elongate differently from their neighbors to establish and diversify adult proportion? Here, we find that vertebral lengths are largely determined by differences in growth cartilage height and the number of cells progressing through endochondral ossification. Hypertrophic chondrocyte size, a major contributor to differential elongation in mammal limb bones, differs only in the longest jerboa mid-tail vertebrae where they are exceptionally large. To uncover candidate molecular mechanisms of disproportionate vertebral growth, we performed intersectional RNA-Seq of mouse and jerboa tail vertebrae with similar and disproportionate elongation rates. Many regulators of posterior axial identity and endochondral elongation are disproportionately differentially expressed in jerboa vertebrae. Among these, the inhibitory natriuretic peptide receptor C (NPR3) appears in multiple studies of rodent and human skeletal proportion suggesting it refines local growth rates broadly in the skeleton and broadly in mammals. Consistent with this hypothesis, NPR3 loss of function mice have abnormal tail and limb proportions. Therefore, in addition to genetic components of the complex process of vertebral evolution, these studies reveal fundamental mechanisms of skeletal growth and proportion.

Phase 2 Trial of Vosoritide Use in Patients with Hypochondroplasia: A Pharmacokinetic/Pharmacodynamic Analysis

INTRODUCTION

Vosoritide is a C-type natriuretic peptide (CNP) analog that binds its receptor on chondrocytes, promoting growth by inhibiting the ERK1/2-MAPK pathway. We previously reported the results of a phase II study in children with hypochondroplasia. Vosoritide led to an average increase in annualized height velocity (AHV) of 1.81 cm/year and gain of 0.36 in height standard deviation (SD) over 12 months. We present here the pharmacokinetic/pharmacodynamic (PK/PD) data from this study and examine the correlations between these parameters and growth outcomes.

METHODS

We conducted a phase II trial of daily subcutaneous vosoritide (15 μg/kg/day) in 24 prepubertal subjects with hypochondroplasia (12 females, mean age 5.9 ± 2.3 years, mean height -3.29 + 0.68 SD). Plasma vosoritide levels were assayed using an electrochemiluminescence assay. PD markers including serum collagen X biomarker (CXM) and urine cyclic guanosine monophosphate (cGMP) production were measured at day 1, month 6, and month 12 visits. Pearson correlations and regression analyses were performed between PK and PD parameters and growth outcomes.

RESULTS

Vosoritide PK parameters were similar to those previously reported in patients with achondroplasia. CXM levels increased from a baseline mean of 22.5 ± 6.5 to 41.6 ± 15.9 ng/mL after 12 months of treatment (p < 0.0001). Urine cGMP increased within 1 h and peaked at 2 h after injection. The mean AUC for cGMP production was not significantly different at each study visit. The maximum change in cGMP AUC correlated with PK AUC (r = 0.46, p = 0.0001). However, drug exposure, as measured by average PK AUC, did not correlate with any growth outcome. CXM levels correlated with the prior 6-month interval height velocity (partial correlation coefficient = 0.40, p = 0.0048). However, change in CXM did not correlate with change in height velocity or change in height SD during treatment.

CONCLUSIONS

Vosoritide treatment showed improvement in AHV and height SD in children with hypochondroplasia. PK analysis indicates that drug exposure was correlated to global CNP activity as measured by urine cGMP but did not correlate with growth outcomes. More studies are needed to identify specific patient characteristics that can predict response to therapy and clinical outcomes.

Future Perspective: Harnessing the Power of Artificial Intelligence in the Generation of New Peptide Drugs

The expansive field of drug discovery is continually seeking innovative approaches to identify and develop novel peptide-based therapeutics. With the advent of artificial intelligence (AI), there has been a transformative shift in the generation of new peptide drugs. AI offers a range of computational tools and algorithms that enables researchers to accelerate the therapeutic peptide pipeline. This review explores the current landscape of AI applications in peptide drug discovery, highlighting its potential, challenges, and ethical considerations. Additionally, it presents case studies and future prospectives that demonstrate the impact of AI on the generation of new peptide drugs.

C-type natriuretic peptide (CNP): The cardiovascular system and beyond

C-type natriuretic peptide (CNP) represents the 'local' member of the natriuretic peptide family, functioning in an autocrine or paracrine capacity to modulate a hugely diverse portfolio of physiological processes. Whilst the best-characterised of these regulatory roles are in the cardiovascular system, akin to its predominantly endocrine siblings atrial (ANP) and brain (BNP) natriuretic peptides, CNP governs many additional, unrelated mechanisms including bone growth, gamete maturation, auditory processing, and neuronal integrity. Furthermore, there is currently great interest in mimicking the biological activity of CNP for therapeutic gain in many of these disparate organ systems. Herein, we provide an overview of the physiology, pathophysiology and pharmacology of CNP in both cardiovascular and non-cardiovascular settings.

International expert opinion on the considerations for combining vosoritide and limb surgery: a modified delphi study

BACKGROUND

Achondroplasia is the most common form of skeletal disorder with disproportionate short stature. Vosoritide is the first disease-specific, precision pharmacotherapy to increase growth velocity in children with achondroplasia. Limb surgery is a standard approach to increase height and arm span, improve proportionality and functionality, as well as correcting deformities. The aim of this study was to gain expert opinion on the combined use of vosoritide and limb surgery in children and adolescents with achondroplasia.

METHODS

An international expert panel of 17 clinicians and orthopaedic surgeons was convened, and a modified Delphi process undertaken. The panel reviewed 120 statements for wording, removed any unnecessary statements, and added any that they felt were missing. There were 26 statements identified as facts that were not included in subsequent rounds of voting. A total of 97 statements were rated on a ten-point scale where 1 was 'Completely disagree' and 10 'Completely agree'. A score of ≥ 7 was identified as agreement, and ≤ 4 as disagreement. All experts who scored a statement ≤ 4 were invited to provide comments.

RESULTS

There was 100% agreement with several statements including, "Achieve a target height, arm span or upper limb length to improve daily activities" (mean level of agreement [LoA] 9.47, range 8-10), the "Involvement of a multidisciplinary team in a specialist centre to follow up the patient" (mean LoA 9.67, range 7-10), "Planning a treatment strategy based on age and pubertal stage" (mean LoA 9.60, range 8-10), and "Identification of short- and long-term goals, based on individualised treatment planning" (mean LoA 9.27, range 7-10), among others. The sequence of a combined approach and potential impact on the physes caused disagreement, largely due to a lack of available data.

CONCLUSIONS

It is clear from the range of responses that this modified Delphi process is only the beginning of new considerations, now that a medical therapy for achondroplasia is available. Until data on a combined treatment approach are available, sharing expert opinion is a vital way of providing support and guidance to the clinical community.

Low-dose infigratinib increases bone growth and corrects growth plate abnormalities in an achondroplasia mouse model

Achondroplasia (ACH), the most common form of disproportionate short stature, is caused by gain-of-function point mutations in fibroblast growth factor receptor 3 (FGFR3). Abnormally elevated activation of FGFR3 modulates chondrocyte proliferation and differentiation via multiple signaling pathways, such as the MAPK pathway. Using a mouse model mimicking ACH (Fgfr3Y367C/+), we have previously shown that daily treatment with infigratinib (BGJ398), a selective and orally bioavailable FGFR1-3 inhibitor, at a dose of 2 mg/kg, significantly increased bone growth. In this study, we investigated the activity of infigratinib administered at substantially lower doses (0.2 and 0.5 mg/kg, given once daily) and using an intermittent dosing regimen (1 mg/kg every 3 days). Following a 15-day treatment period, these low dosages were sufficient to observe significant improvement of clinical hallmarks of ACH such as growth of the axial and appendicular skeleton and skull development. Immunohistological labeling demonstrated the positive impact of infigratinib on chondrocyte differentiation in the cartilage growth plate and the cartilage end plate of the vertebrae. Macroscopic and microcomputed analyses showed enlargement of the foramen magnum area at the skull base, thus improving foramen magnum stenosis, a well-recognized complication in ACH. No changes in FGF23 or phosphorus levels were observed, indicating that the treatment did not modify phosphate homeostasis. This proof-of-concept study demonstrates that infigratinib administered at low doses has the potential to be a safe and effective therapeutic option for children with ACH.

Molecular therapy and nucleic acid adeno-associated virus-based gene therapy delivering combinations of two growth-associated genes to MPS IVA mice

Mucopolysaccharidosis type IVA (MPS IVA) is caused by a deficiency of the galactosamine (N-acetyl)-6-sulfatase (GALNS) enzyme responsible for the degradation of specific glycosaminoglycans (GAGs). The progressive accumulation of GAGs leads to various skeletal abnormalities (short stature, hypoplasia, tracheal obstruction) and several symptoms in other organs. To date, no treatment is effective for patients with bone abnormalities. To improve bone pathology, we propose a novel combination treatment with the adeno-associated virus (AAV) vectors expressing GALNS enzyme and a natriuretic peptide C (CNP; NPPC gene) as a growth-promoting agent for MPS IVA. In this study, an MPS IVA mouse model was treated with an AAV vector expressing GALNS combined with another AAV vector expressing NPPC gene, followed for 12 weeks. After the combination therapy, bone growth in mice was induced with increased enzyme activity in tissues (bone, liver, heart, lung) and plasma. Moreover, there were significant changes in bone morphology in CNP-treated mice with increased CNP activity in plasma. Delivering combinations of CNP and GALNS gene therapies enhanced bone growth in MPS IVA mice more than in GALNS gene therapy alone. Enzyme expression therapy alone fails to reach the bone growth region; our results indicate that combining it with CNP offers a potential alternative.

Radiological characteristics of skeletal growth in neonates and infants with achondroplasia

Achondroplasia (ACH) is the most common form of skeletal dysplasia characterized by a rhizomelic short stature. Radiological skeletal findings in pediatric and adult patients with ACH include short long bones, a relatively longer fibula compared to the tibia, a narrow lumbar interpedicular distance, and a hypoplastic iliac wing. Nonetheless, the characteristics of skeletal growth during the neonatal and infantile periods have scarcely been explored. Therefore, this retrospective study aimed to analyze the radiological skeletal growth during the neonatal and infantile periods in 41 Japanese patients with genetically confirmed ACH. The length of long bones in the upper and lower limbs and the lumbar interpedicular distances at L1 and L4 were measured. These parameters showed significant positive correlations with age. The upper segment-to-lower segment ratio in the lower limbs resembled the data of healthy controls from previous reports. The L1/L4 and fibula/tibia ratios increased with age, suggesting that some representative skeletal phenotypes of ACH were less distinct during the neonatal and infantile periods. In conclusion, for the first time, this study radiologically characterized skeletal growth during the neonatal and infantile periods of patients with genetically confirmed ACH.

Novel therapies for growth disorders

As we continue to understand more about the complex mechanism of growth, a plethora of novel therapies have recently been developed that aim to address barriers and optimize efficacy. This review aims to explore these novel therapies and provide a succinct review based on the latest clinical studies in order to introduce clinicians to therapies that will soon constitute the future in the field of short stature.  Conclusion: The review focuses on long-acting growth hormone formulations, a novel growth hormone oral secretagogue, novel treatments for children with achondroplasia, and targeted therapies for rare forms of skeletal dysplasias. What is Known: • Recombinant human growth hormone has been the mainstay of treatment for children with short stature for years. • Such therapy is not always effective based on the underlying diagnosis (e.g achondroplasia, Turner syndrome). Compliance with daily injections is challenging and can directly affect efficacy. What is New: • Recent development of long-acting growth hormone regimens and oral secretagogues can overcome some of these barriers, however several limitations need to be taken into consideration. • Newer therapies for achondroplasia, and other rare forms of skeletal dysplasias introduce us to a new era of targeted therapies for children with short stature. Clinicians ought to be aware of pitfalls and caveats before introducing these novel therapies to every day practice.

Novel therapeutic perspectives in Noonan syndrome and RASopathies

Noonan syndrome belongs to the family of RASopathies, a group of multiple congenital anomaly disorders caused by pathogenic variants in genes encoding components or regulators of the RAS/mitogen-activated protein kinase (MAPK) signalling pathway. Collectively, all these pathogenic variants lead to increased RAS/MAPK activation. The better understanding of the molecular mechanisms underlying the different manifestations of NS and RASopathies has led to the identification of molecular targets for specific pharmacological interventions. Many specific agents (e.g. SHP2 and MEK inhibitors) have already been developed for the treatment of RAS/MAPK-driven malignancies. In addition, other molecules with the property of modulating RAS/MAPK activation are indicated in non-malignant diseases (e.g. C-type natriuretic peptide analogues in achondroplasia or statins in hypercholesterolemia).  Conclusion: Drug repositioning of these molecules represents a challenging approach to treat or prevent medical complications associated with RASopathies. What is Known: • Noonan syndrome and related disorders are caused by pathogenic variants in genes encoding components or regulators of the RAS/mitogen-activated protein kinase (MAPK) signalling pathway, resulting in increased activation of this pathway. • This group of disorders is now known as RASopathies and represents one of the largest groups of multiple congenital anomaly diseases known. What is New: • The identification of pathophysiological mechanisms provides new insights into the development of specific therapeutic strategies, in particular treatment aimed at reducing RAS/MAPK hyperactivation. • Drug repositioning of specific agents already developed for the treatment of malignant (e.g. SHP2 and MEK inhibitors) or non-malignant diseases (e.g. C-type natriuretic peptide analogues in achondroplasia or statins in hypercholesterolaemia) represents a challenging approach to the treatment of RASopathies.

The Treatment of Growth Disorders in Childhood and Adolescence

BACKGROUND

3% of all children are unusually short, and 3% are unusually tall. New approaches have broadened the range of therapeutic options in treating growth disorders.

METHODS

This review is based on publications retrieved by a selective review of the literature and on the authors' clinical experience.

RESULTS

Pituitary growth hormone deficiency is treated with recombinant growth hormone. Long-acting preparations of this type became available recently, but their long-term safety and efficacy are still unknown. Vosoritide, a CNP analogue, has also been approved for the treatment of achondroplasia, and severe primary deficiency of insulin-like growth factor 1 (IGF-1) can be treated with recombinant IGF-1. In the treatment of excessively tall stature, new information on the safety of growth-attenuating treatment and an altered perception of above-average height in society have led to a change in management.

CONCLUSION

There are new options for the treatment of rare causes of short stature, while new information on the safety of treatment strategies for excessive tallness have led to a reconsideration of surgical intervention. There is insufficient evidence on the benefits and risks of supraphysiological GH therapy and of newer treatment options for which there are as yet no robust data on adult height. Therefore, before any treatment is provided, physicians should give patients and their families detailed information and discuss their expectations from treatment and the goals that treatment can be expected to achieve.

C-Type Natriuretic Peptide Analogs: Current and Future Therapeutic Applications

BACKGROUND

Short stature is one of the most common reasons for referral to a pediatric endocrinologist that can be due to multitude of conditions, including an ever-growing list of genetic etiologies. Despite the numerous different causes, options for medical therapy remain quite limited, with the primary medication available being recombinant human growth hormone. A second option is recombinant insulin-like growth factor 1 (rIGF-1) in select patients with severe primary IGF-1 deficiency. Alternative strategies to increase height have been attempted such as delaying the onset of puberty with a gonadotropin-releasing hormone agonist or delaying epiphyseal fusion with an aromatase inhibitor. However, these options focus on increasing the duration of growth as opposed to directly stimulating growth at the growth plate.

SUMMARY

Novel approaches to growth promotion have recently been developed, including analogs of C-type natriuretic peptide (CNP). The purpose of this study is to review the function of CNP and its potential use in different conditions.

KEY MESSAGES

Alterations in the CNP/FGFR3 pathway can lead to multiple defined genetic causes of short stature. The CNP pathway has become the focus for treatment of children with short stature that suffer from such genetic conditions, with promising outcomes.

Vosoritide therapy in children with achondroplasia aged 3-59 months: a multinational, randomised, double-blind, placebo-controlled, phase 2 trial

BACKGROUND

Vosoritide is a recombinant C-type natriuretic peptide analogue that increases annualised growth velocity in children with achondroplasia aged 5-18 years. We aimed to assess the safety and efficacy of vosoritide in infants and children younger than 5 years.

METHODS

This double-blind, randomised, placebo-controlled, phase 2 trial was done in 16 hospitals across Australia, Japan, the UK, and the USA. Children younger than 60 months with a clinical diagnosis of achondroplasia confirmed by genetic testing and who had completed a baseline growth study or observation period were enrolled into one of three sequential cohorts based on age at screening: 24-59 months (cohort 1); 6-23 months (cohort 2); and 0-5 months (cohort 3). Each cohort included sentinels who received vosoritide to determine appropriate daily drug dose, with the remainder randomly assigned (1:1) within each age stratum (except in Japan, where participants were randomly assigned within each cohort) to receive daily subcutaneous injections of vosoritide (30·0 μg/kg for infants aged 0-23 months; 15·0 μg/kg for children aged 24-59 months) or placebo for 52 weeks. Participants, caregivers, investigators, and the sponsor were masked to treatment assignment. The first primary outcome was safety and tolerability, assessed in all participants who received at least one study dose. The second primary outcome was change in height Z score at 52 weeks from baseline, analysed in all randomly assigned participants. This trial is registered with EudraCT, 2016-003826-18, and ClinicalTrials.gov, NCT03583697.

FINDINGS

Between May 13, 2018, and March 1, 2021, 75 participants were recruited (37 [49%] females). 11 were assigned as sentinels, whereas 32 were randomly assigned to receive vosoritide and 32 placebo. Two participants discontinued treatment and the study: one in the vosoritide group (death) and one in the placebo group (withdrawal). Adverse events occurred in all 75 (100%) participants (annual rate 204·5 adverse events per patient in the vosoritide group and 73·6 per patient in the placebo group), most of which were transient injection-site reactions and injection-site erythema. Serious adverse events occurred in three (7%) participants in the vosoritide group (decreased oxygen saturation, respiratory syncytial virus bronchiolitis and sudden infant death syndrome, and pneumonia) and six (19%) participants in the placebo group (petit mal epilepsy, autism, gastroenteritis, vomiting and parainfluenza virus infection, respiratory distress, and skull fracture and otitis media). The least-squares mean difference for change from baseline in height Z score between the vosoritide and placebo groups was 0·25 (95% CI -0·02 to 0·53).

INTERPRETATION

Children with achondroplasia aged 3-59 months receiving vosoritide for 52 weeks had a mild adverse event profile and gain in the change in height Z score from baseline.

FUNDING

BioMarin Pharmaceutical.

Vosoritide Therapy in Children with Achondroplasia: Early Experience and Practical Considerations for Clinical Practice

INTRODUCTION

Vosoritide is the first precision medical therapy approved to increase growth velocity in children with achondroplasia. Sharing early prescribing experiences across different regions could provide a framework for developing practical guidance for the real-world use of vosoritide.

METHODS

Two meetings were held to gather insight and early experience from experts in Europe, the Middle East, and the USA. The group comprised geneticists, pediatric endocrinologists, pediatricians, and orthopedic surgeons. Current practices and considerations for vosoritide were discussed, including administration practicalities, assessments, and how to manage expectations.

RESULTS

A crucial step in the management of achondroplasia is to determine if adequate multidisciplinary support is in place. Training for families is essential, including practical information on administration of vosoritide, and how to recognize and manage injection-site reactions. Advocated techniques include establishing a routine, empowering patients by allowing them to choose injection sites, and managing pain. Patients may discontinue vosoritide if they cannot tolerate daily injections or are invited to participate in a clinical trial. Clinicians in Europe and the Middle East emphasized the importance of assessing adherence to daily injections, as non-adherence may impact response and reimbursement. Protocols for monitoring patients receiving vosoritide may be influenced by regional differences in reimbursement and healthcare systems. Core assessments may include pubertal staging, anthropometry, radiography to confirm open physes, the review of adverse events, and discussion of concomitant or new medications-but timing of these assessments may also differ regionally and vary across institutions. Patients and families should be informed that response to vosoritide can vary in both magnitude and timing. Keeping families informed regarding vosoritide clinical trial data is encouraged.

CONCLUSION

The early real-world experience with vosoritide is generally positive. Sharing these insights is important to increase understanding of the practicalities of treatment with vosoritide in the clinical setting.

Cell-based screen identifies porphyrins as FGFR3 activity inhibitors with therapeutic potential for achondroplasia and cancer

Overactive fibroblast growth factor receptor 3 (FGFR3) signaling drives pathogenesis in a variety of cancers and a spectrum of short-limbed bone dysplasias, including the most common form of human dwarfism, achondroplasia (ACH). Targeting FGFR3 activity holds great promise as a therapeutic approach for treatment of these diseases. Here, we established a receptor/adaptor translocation assay system that can specifically monitor FGFR3 activation, and we applied it to identify FGFR3 modulators from complex natural mixtures. An FGFR3-suppressing plant extract of Amaranthus viridis was identified from the screen, and 2 bioactive porphyrins, pheophorbide a (Pa) and pyropheophorbide a, were sequentially isolated from the extract and functionally characterized. Further analysis showed that Pa reduced excessive FGFR3 signaling by decreasing its half-life in FGFR3-overactivated multiple myeloma cells and chondrocytes. In an ex vivo culture system, Pa alleviated defective long bone growth in humanized ACH mice (FGFR3ACH mice). Overall, our study presents an approach to discovery and validation of plant extracts or drug candidates that target FGFR3 activation. The compounds identified by this approach may have applications as therapeutics for FGFR3-associated cancers and skeletal dysplasias.

Once-weekly TransCon CNP (navepegritide) in children with achondroplasia (ACcomplisH): a phase 2, multicentre, randomised, double-blind, placebo-controlled, dose-escalation trial

Background

TransCon CNP (navepegritide) is an investigational prodrug of C-type natriuretic peptide (CNP) designed to allow for continuous CNP exposure with once-weekly dosing. This 52-week phase 2 (ACcomplisH) trial assessed the safety and efficacy of TransCon CNP in children with achondroplasia.

Methods

ACcomplisH is a global, randomised, double-blind, placebo-controlled, dose-escalation trial. Study participants were recruited between June 10, 2020, and September 24, 2021. Eligible participants were prepubertal, aged 2-10 years, with genetically confirmed achondroplasia, and randomised 3:1 to once-weekly subcutaneous injections of TransCon CNP (6, 20, 50, or 100 μg CNP/kg/week) or placebo for 52 weeks. Primary objectives were safety and annualised growth velocity (AGV). ACcomplisH is registered with ClinicalTrials.gov (NCT04085523) and Eudra (CT 2019-002754-22).

Findings

Forty-two participants received TransCon CNP at doses of 6 μg (n = 10; 7 female), 20 μg (n = 11; 3 female), 50 μg (n = 10; 3 female), or 100 μg (n = 11; 6 female) CNP/kg/week, with 15 receiving placebo (5 female). Treatment-emergent adverse events (TEAEs) were mild or moderate with no grade 3/4 events reported. There were 2 serious TEAEs that were assessed as not related to TransCon CNP. Eleven injection site reactions occurred in 8 participants receiving TransCon CNP and no symptomatic hypotension occurred. TransCon CNP demonstrated a dose-dependent improvement in AGV. At 52 weeks, TransCon CNP 100 μg CNP/kg/week significantly improved AGV vs placebo (least squares mean [95% CI] 5.42 [4.74-6.11] vs 4.35 [3.75-4.94] cm/year; p = 0.0218), and improved achondroplasia-specific height SDS from baseline (least squares mean [95% CI] 0.22 [0.02-0·41] vs -0·08 [-0.25 to 0.10]; p = 0.0283). All participants completed the randomised period and continued in the ongoing open-label extension period receiving TransCon CNP 100 μg CNP/kg/week.

Interpretation

This phase 2 trial suggests that TransCon CNP is effective, safe, with low injection site reaction frequency, and may provide a novel, once-weekly treatment option for children with achondroplasia. These results support TransCon CNP at 100 μg CNP/kg/week in the ongoing pivotal trial.

Funding

Ascendis Pharma, A/S.

Shedding New Light: Novel Therapies for Achondroplasia and Growth Disorders

Achondroplasia is the most common form of disproportionate severe short stature. Management of achondroplasia requires a multidisciplinary approach and has been largely symptomatic for medical complications and psychosocial implications. Increased understanding of genetic and molecular mechanisms of achondroplasia has led to the development of novel disease-modifying drugs. The current drugs under investigation target the growth plate to stimulate chondrocyte growth and development. These include analogs of C-type natriuretic peptide (CNP), FGFR3-selective tyrosine kinase inhibitors, anti-FGFR3 antibodies, aptamers against FGF2, and soluble forms of FGFR3. Long-term data on the effects of these therapies on medical comorbidities are pending at this time.

Pharmacological and Genetic Disruption of C-Type Natriuretic Peptide (nppcl) Expression in Zebrafish (Danio rerio) Causes Stunted Growth during Development

Human patients with mutations within NPPC or NPR2 genes (encoding C-type natriuretic peptide (CNP) and guanylyl cyclase-B (GC-B), respectively) display clinical signs associated with skeletal abnormalities, such as overgrowth or short stature. Mice with induced models of Nppc or Npr2 deletion display profound achondroplasia, dwarfism and early death. Recent pharmacological therapies to treat short stature are utilizing long-acting CNP analogues, but the effects of manipulating CNP expression during development remain unknown. Here, we use Danio rerio (zebrafish) as a model for vertebrate development, employing both pharmacological and reverse genetics approaches to alter expression of genes encoding CNP in zebrafish. Four orthologues of CNP were identified in zebrafish, and spatiotemporal expression profiling confirmed their presence during development. Bioinformatic analyses suggested that nppcl is the most likely the orthologue of mammalian CNP. Exogenous CNP treatment of developing zebrafish embryos resulted in impaired growth characteristics, such as body length, head width and eye diameter. This reduced growth was potentially caused by increased apoptosis following CNP treatment. Expression of endogenous nppcl was downregulated in these CNP-treated embryos, suggesting that negative feedback of the CNP system might influence growth during development. CRISPR knock-down of endogenous nppcl in developing zebrafish embryos also resulted in impaired growth characteristics. Collectively, these data suggest that CNP in zebrafish is crucial for normal embryonic development, specifically with regard to growth.

Bone Growth Induction in Mucopolysaccharidosis IVA Mouse

Mucopolysaccharidosis IVA (MPS IVA; Morquio A syndrome) is caused by a deficiency of the N-acetylgalactosamine-6-sulfate-sulfatase (GALNS) enzyme, leading to the accumulation of glycosaminoglycans (GAG), keratan sulfate (KS) and chondroitin-6-sulfate (C6S), mainly in cartilage and bone. This lysosomal storage disorder (LSD) is characterized by severe systemic skeletal dysplasia. To this date, none of the treatment options for the MPS IVA patients correct bone pathology. Enzyme replacement therapy with elosulfase alpha provides a limited impact on bone growth and skeletal lesions in MPS IVA patients. To improve bone pathology, we propose a novel gene therapy with a small peptide as a growth-promoting agent for MPS IVA. A small molecule in this peptide family has been found to exert biological actions over the cardiovascular system. This work shows that an AAV vector expressing a C-type natriuretic (CNP) peptide induces bone growth in the MPS IVA mouse model. Histopathological analysis showed the induction of chondrocyte proliferation. CNP peptide also changed the pattern of GAG levels in bone and liver. These results suggest the potential for CNP peptide to be used as a treatment in MPS IVA patients.

Current and emerging therapies for Achondroplasia: The dawn of precision medicine

Achondroplasia is a rare disease affecting bone growth and is caused by a missense mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. In the past few years, there were multiple experimental drugs entering into clinical trials for treating achondroplasia including vosoritide, the first precision medicine approved for this indication. This perspective presents the mechanism of action, benefit, and potential mechanistic limitation of the drugs currently being evaluated in clinical trials for achondroplasia. This article also discusses the potential impact of those drugs not only in increasing the growth of individuals living with achondroplasia but also in improving their quality of life.

Growth parameters in children with achondroplasia: A 7-year, prospective, multinational, observational study

PURPOSE

This study was undertaken to collect baseline growth parameters in children with achondroplasia who might enroll in interventional trials of vosoritide, and to establish a historical control.

METHODS

In this prospective, observational study, participants (≤17 years) underwent a detailed medical history and physical examination and were followed every 3 months until they finished participating in the study by enrolling in an interventional trial or withdrawing.

RESULTS

A total of 363 children were enrolled (28 centers, 8 countries). Mean (SD) follow up was 20.4 (15.0) months. In participants <1 year, mean annualized growth velocity (AGV) was 11.6 cm/year for girls and 14.6 cm/year for boys. By age 1 year, mean AGV decreased to 7.4 cm/year in girls and 7.1 cm/year in boys. By age 10 years, mean AGV decreased to 3.6 cm/year for both sexes. Mean height z-score in participants <1 year was -2.5 for girls and -3.2 for boys and decreased up to the age 5 years (-5.3 for girls; -4.6 for boys). Girls and boys had a disproportionate upper-to-lower body segment ratio. Mean ratio was highest in participants aged <1 year (2.9 for girls; 2.8 for boys) and decreased gradually to approximately 2 in both sexes from 4 years of age onward.

CONCLUSION

This study represents one of the largest datasets of prospectively collected medical and longitudinal growth data in children with achondroplasia. It serves as a robust historical control to measure therapeutic interventions against and to further delineate the natural history of this condition.

Phase 1 safety, tolerability, pharmacokinetics and pharmacodynamics results of a long-acting C-type natriuretic peptide prodrug, TransCon CNP

AIM

TransCon CNP is a novel prodrug designed to provide sustained release of C-type natriuretic peptide (CNP) for once-weekly therapy, addressing the pathology leading to aberrant skeletal development in achondroplasia. This phase 1 trial was initiated to assess the safety, tolerability, pharmacodynamics (PD) and pharmacokinetics (PK) of TransCon CNP.

METHODS

This randomized, placebo-controlled, single-ascending dose phase 1 trial was performed at two sites in Australia and enrolled 45 healthy adult males. Subjects received placebo or TransCon CNP (single-ascending dose cohorts [3, 10, 25, 75 or 150 μg CNP/kg]). The primary endpoint was frequency of adverse events and other safety outcomes. Other endpoints included PK and PD measured by cyclic guanosine-monophosphate (cGMP) and amino-terminal propeptide of CNP (NTproCNP).

RESULTS

TransCon CNP provided continuous systemic exposure to CNP over at least 7 days post-dose. Plasma and urine levels of cGMP were significantly increased in subjects administered TransCon CNP at 75-150 μg CNP/kg, indicating target engagement of active CNP at the natriuretic peptide receptor-B (NPR-B) for at least 1 week post-dose. TransCon CNP was well-tolerated, with no serious treatment-emergent adverse events or discontinuations. Extensive cardiac safety assessments did not reveal any clinically relevant effects on electrocardiogram parameters, including heart rate, PR, QRS and QTcF intervals.

CONCLUSIONS

Safety and PD data from this phase 1 trial support that TransCon CNP is well tolerated, with a PK profile compatible with a once-weekly dosing regimen. Further studies are ongoing to evaluate the potential of TransCon CNP to positively impact abnormal endochondral ossification in children with achondroplasia.

Guanylyl Cyclase-B Dependent Bone Formation in Mice is Associated with Youth, Increased Osteoblasts, and Decreased Osteoclasts

C-type natriuretic peptide (CNP) activation of guanylyl cyclase-B (GC-B) catalyzes the synthesis of cGMP in chondrocytes and osteoblasts. Elevated cGMP stimulates long bone growth, and inactivating mutations in CNP or GC-B reduce cGMP, which causes dwarfism. GC-B^7E/7E mice that express a GC-B mutant that cannot be inactivated by dephosphorylation exhibit increased CNP-dependent GC-B activity, which increases bone length, as well as bone mass and strength. Importantly, how GC-B increases bone mass is not known. Here, we injected 12-week-old, wild type mice once daily for 28 days with or without BMN-111 (Vosoritide), a proteolytically resistant CNP analog. We found that BMN-111 treated mice had elevated levels of osteocalcin and collagen 1 C-terminal telopeptide (CTX) as well as increased osteoblasts and osteoclasts. In BMN-111 injected mice, tibial mRNAs for Rank ligand and osteoprotegrin were increased and decreased, respectively, whereas sclerostin mRNA was elevated 400-fold, consistent with increased osteoclast activity and decreased osteoblast activity. Mineral apposition rates and trabecular bone mass were not elevated in response to BMN-111. Because 9-week-old male GC-B^7E/7E mice have increased bone mass but do not exhibit increased mineral apposition rates, we examined 4-week-old male GC-B^7E/7E mice and found that these animals had increased serum osteocalcin, but not CTX. Importantly, tibias from these mice had 37% more osteoblasts, 26% fewer osteoclasts as well as 36% and 40% higher mineral apposition and bone formation rates, respectively. We conclude that GC-B-dependent bone formation is coupled to an early juvenile process that requires both increased osteoblasts and decreased osteoclasts.

Literature review and expert opinion on the impact of achondroplasia on medical complications and health-related quality of life and expectations for long-term impact of vosoritide: a modified Delphi study

BACKGROUND

Achondroplasia is associated with disproportionate short stature and significant and potentially severe medical complications. Vosoritide is the first medicine to treat the underlying cause of achondroplasia and data from phase 3 and phase 2 extension studies showed effects on growth and body proportions. However, there are currently no long-term data available on the direct impact on endpoints such as medical complications and health-related quality of life (HRQoL). This study explored the perceived impact of achondroplasia on medical complications, HRQoL, healthcare resource use and mortality, and potential modifying effects of vosoritide, based on published evidence and expert opinion. Structured expert opinion was obtained by an international modified Delphi study among 14 experts in managing achondroplasia performed on a virtual platform and consisting of an explorative phase followed by an anonymous individual rating round.

RESULTS

Overall, the panelists expect that in individuals starting long-term treatment between 2 years of age and puberty, growth velocity increases observed in the clinical trials will be maintained until final height is reached (92% agreement) and will likely result in clinically meaningful improvements in upper-to-lower body segment ratio (85%). Earlier treatment initiation will likely result in a greater final height (100%) and more likely improve proportionality (92%) than later treatment. Although current data are limited, ≥ 75% of panelists find it conceivable that the earlier long-term treatment is started, the greater the probability of a positive effect on the lifetime incidence of symptomatic spinal stenosis, kyphosis, obstructive sleep apnea, and foramen magnum stenosis. These are among the most clinically important complications of achondroplasia because of their high impact on comorbidity, mortality, and/or HRQoL. A positive effect of vosoritide on the incidence of surgeries through lifetime was considered more likely with earlier long-term treatment (90%).

CONCLUSIONS

This explorative study, based on international expert opinion, provides further insight into the medical and functional impacts of achondroplasia and how these might be modified through long-term use of vosoritide. The results can be used to guide the direction and design of future research to validate the assumptions and to discuss potential treatment outcomes with disease modifying therapies with families and clinicians.

Natriuretic Peptide-Based Novel Therapeutics: Long Journeys of Drug Developments Optimized for Disease States

Simple Summary

Natriuretic peptides are endogenous hormones produced in the heart and vascular endothelium, and they enable cardiorenal protective actions or bone growth via cGMP stimulation through their receptor guanylyl cyclase receptor A or B. To optimize the drug for each disease state, we must consider drug metabolism, delivery systems, and target receptor(s). This review summarizes attempts to develop novel natriuretic peptide-based therapeutics, including novel designer natriuretic peptides and oral drugs to enhance endogenous natriuretic peptides. We introduce some therapeutics that have been successful in clinical practice, as well as the prospective drug developments in the natriuretic peptide system for disease states.

Abstract

The field of natriuretic peptides (NPs) as an endocrine hormone has been developing since 1979. There are three peptides in humans: atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), which bind to the guanylyl cyclase-A (GC-A) receptor (also called natriuretic peptide receptor-A (NPR-A)), and C-type natriuretic peptide (CNP), which binds to the GC-B receptor (also called the NPR-B) and then synthesizes intracellular cGMP. GC-A receptor stimulation has natriuretic, vasodilatory, cardiorenal protective and anti-renin–angiotensin–aldosterone system actions, and GC-B receptor stimulation can suppress myocardial fibrosis and can activate bone growth before epiphyseal plate closure. These physiological effects are useful as therapeutics for some disease states, such as heart failure, hypertension, and dwarfism. To optimize the therapeutics for each disease state, we must consider drug metabolism, delivery systems, and target receptor(s). We review the cardiac NP system; new designer NPs, such as modified/combined NPs and modified peptides that can bind to not only NP receptors but receptors for other systems; and oral drugs that enhance endogenous NP activity. Finally, we discuss prospective drug discoveries and the development of novel NP therapeutics.

Expanding horizons of achondroplasia treatment: current options and future developments

Activating mutations in the FGFR3 receptor tyrosine kinase lead to most prevalent form of genetic dwarfism in humans, the achondroplasia. Many features of the complex function of FGFR3 in growing skeleton were characterized, which facilitated identification of therapy targets, and drove progress toward treatment. In August 2021, the vosoritide was approved for treatment of achondroplasia, which is based on a stable variant of the C-natriuretic peptide. Other drugs may soon follow, as several conceptually different inhibitors of FGFR3 signaling progress through clinical trials. Here, we review the current achondroplasia therapeutics, describe their mechanisms, and illuminate motivations leading to their development. We also discuss perspectives of curing achondroplasia, and options for repurposing achondroplasia drugs for dwarfing conditions unrelated to FGFR3.

Therapeutic peptides: current applications and future directions

Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.

Theobroma cacao improves bone growth by modulating defective ciliogenesis in a mouse model of achondroplasia

A gain-of-function mutation in the fibroblast growth factor receptor 3 gene (FGFR3) results in achondroplasia (ACH), the most frequent form of dwarfism. Constitutive activation of FGFR3 impairs bone formation and elongation and many signal transduction pathways. Identification of new and relevant compounds targeting the FGFR3 signaling pathway is of broad importance for the treatment of ACH, and natural plant compounds are prime drug candidate sources. Here, we found that the phenolic compound (-)-epicatechin, isolated from Theobroma cacao, effectively inhibited FGFR3's downstream signaling pathways. Transcriptomic analysis in an Fgfr3 mouse model showed that ciliary mRNA expression was modified and influenced significantly by the Indian hedgehog and PKA pathways. (-)-Epicatechin is able to rescue mRNA expression impairments that control both the structural organization of the primary cilium and ciliogenesis-related genes. In femurs isolated from a mouse model (Fgfr3^Y367C/+) of ACH, we showed that (-)-epicatechin eliminated bone growth impairment during 6 days of ex vivo culture. In vivo, we confirmed that daily subcutaneous injections of (-)-epicatechin to Fgfr3^Y367C/+ mice increased bone elongation and rescued the primary cilium defects observed in chondrocytes. This modification to the primary cilia promoted the typical columnar arrangement of flat proliferative chondrocytes and thus enhanced bone elongation. The results of the present proof-of-principle study support (-)-epicatechin as a potential drug for the treatment of ACH.

Emerging therapies for the treatment of rare pediatric bone disorders

In recent years, new therapies for the treatment of rare pediatric bone disorders have emerged, guided by an increasing understanding of the genetic and molecular etiology of these diseases. Herein, we review three such disorders, impacted by debilitating deficits in bone mineralization or cartilage ossification, as well as the novel disease-modifying drugs that are now available to treat these conditions. Specifically, we discuss asfotase alfa, burosumab-twza, and vosoritide, for the treatment of hypophosphatasia, X-linked hypophosphatemia and achondroplasia, respectively. For each skeletal disorder, an overview of the clinical phenotype and natural history of disease is provided, along with a discussion of the clinical pharmacology, mechanism of action and FDA indication for the relevant medication. In each case, a brief review of clinical trial data supporting drug development for each medication is provided. Additionally, guidance as to drug dosing and long-term monitoring of adverse events and pediatric efficacy is presented, to aid the clinician seeking to utilize these novel therapies in their practice, or to become familiar with the healthcare expectations for children receiving these medications through specialized multidisciplinary clinics. The availability of these targeted therapies now significantly augments treatment options for conditions in which past therapy has relied upon less specific, symptomatic medical and orthopedic care.

Non-GH Agents and Novel Therapeutics in the Management of Short Stature

Short stature is one of the most common reasons for referral to pediatric endocrinologists. The vast majority of short children do not have growth hormone (GH) deficiency or another pathologic process that is interfering with normal growth. While GH has been approved in the US for several etiologies of non-GH deficient short stature, its high cost and need for daily injections represent barriers for many families. Alternative agents for the management of short stature include the use of gonadotropin releasing hormone analogs (GnRHas) to delay puberty, and aromatase inhibitors (AIs) in boys to postpone epiphyseal fusion. The results of studies employing GnRHas as either monotherapy or combined with GH are mixed, and there is a dearth of rigorously designed clinical trials that have followed patients to adult height. While AIs have been found to result in modest increases in adult height in some studies, important questions about their long-term safety exist. The C-type natriuretic peptide analog vosoritide is an experimental agent that is emerging as a potential treatment for a few specific conditions including achondroplasia, although its efficacy in attenuating disproportionality is as yet unproven. While each of these therapeutic strategies holds promise, none are currently considered standard of care and several important questions remain. These include the impact of these interventions on quality of life as well as long-term outcomes.

Safe and persistent growth-promoting effects of vosoritide in children with achondroplasia: 2-year results from an open-label, phase 3 extension study

PURPOSE

Achondroplasia is caused by pathogenic variants in the fibroblast growth factor receptor 3 gene that lead to impaired endochondral ossification. Vosoritide, an analog of C-type natriuretic peptide, stimulates endochondral bone growth and is in development for the treatment of achondroplasia. This phase 3 extension study was conducted to document the efficacy and safety of continuous, daily vosoritide treatment in children with achondroplasia, and the two-year results are reported.

METHODS

After completing at least six months of a baseline observational growth study, and 52 weeks in a double-blind, placebo-controlled study, participants were eligible to continue treatment in an open-label extension study, where all participants received vosoritide at a dose of 15.0 μg/kg/day.

RESULTS

In children randomized to vosoritide, annualized growth velocity increased from 4.26 cm/year at baseline to 5.39 cm/year at 52 weeks and 5.52 cm/year at week 104. In children who crossed over from placebo to vosoritide in the extension study, annualized growth velocity increased from 3.81 cm/year at week 52 to 5.43 cm/year at week 104. No new adverse effects of vosoritide were detected.

CONCLUSION

Vosoritide treatment has safe and persistent growth-promoting effects in children with achondroplasia treated daily for two years.

Emerging therapies for Achondroplasia: changing the rules of the game

INTRODUCTION

Achondroplasia is the most common genetic cause of disproportionate short stature, affecting over 360,000 individuals. Serious complications contributing to significant morbidity in affected individuals include cranio-cervical junction compression and obstructive sleep apnoea. Current clinically available treatments are predominantly symptomatic, and associated with variable outcomes. We summarise the new precision investigational products that are currently in Phase 2 and Phase 3 clinical trials for the treatment of individuals with achondroplasia.

AREAS COVERED

Fibroblast growth factor receptor 3 (FGFR3), a membrane-spanning tyrosine kinase receptor, binds various fibroblast growth factors (FGF) to regulate the normal process of endochondral bone growth. Gain of FGFR3 function in individuals with achondroplasia results in inhibition of normal endochondral ossification. A greater understanding of these molecular pathways through animal models has led to the development of several targeted therapies being tested in children, which we discuss in this review.

EXPERT OPINION

The last decade has been game-changing in terms of new precision therapies for children with achondroplasia that have the potential to fundamentally change the natural history of this condition. The next decade will see how these therapies compare, if they might be used in combination, and evaluate the balance of their long-term benefits and harms.

Vosoritide: First Approval

Vosoritide (VOXZOGO^®) is a modified recombinant human C-type natriuretic peptide (CNP) analogue, being developed by BioMarin Pharmaceutical for the treatment of achondroplasia. Achondroplasia is caused by a gain-of-function mutation in the fibroblast growth factor receptor 3 gene (FGFR3), which is a negative regulator of bone growth. Vosoritide acts to restore chondrogenesis through its binding to natriuretic peptide receptor B (NPR-B), resulting in the inhibition of downstream signalling pathways of the overactive FGFR3 gene. Vosoritide was approved in August 2021 in the EU for the treatment of achondroplasia in patients aged ≥ 2 years whose epiphyses are not closed; the diagnosis of achondroplasia should be confirmed by appropriate genetic testing. The drug is also under regulatory review in the USA for the treatment of achondroplasia and clinical development is underway in several countries. This article summarizes the milestones in the development of vosoritide leading to this first approval for achondroplasia in patients aged ≥ 2 years whose epiphyses are not closed.

Pharmacokinetics and Exposure-Response of Vosoritide in Children with Achondroplasia

BACKGROUND AND OBJECTIVE

Vosoritide, an analog of C-type natriuretic peptide, has been developed for the treatment of children with achondroplasia. The pharmacokinetics of vosoritide and relationships between plasma exposure and efficacy, biomarkers, and safety endpoints were evaluated in a phase II, open-label, dose-escalation study (N = 35 patients aged 5-14 years who received daily subcutaneous injections for 24 months) and a phase III, double-blind, placebo-controlled study (N = 60 patients aged 5-18 years randomized to receive daily subcutaneous injections for 52 weeks).

METHODS

Pharmacokinetic parameters for both studies were obtained from non-compartmental analysis. Potential correlations between vosoritide exposure and changes in annualized growth velocity, collagen type X marker (CXM; a biomarker of endochondral ossification), cyclic guanosine monophosphate (cGMP; a biomarker of pharmacological activity), heart rate, and systolic and diastolic blood pressures were then evaluated.

RESULTS

The exposure-response relationships for changes in both annualized growth velocity and the CXM biomarker saturated at 15 μg/kg, while systemic pharmacological activity, as measured by urinary cGMP, was near maximal or saturated at exposures obtained at the highest dose studied (i.e. 30 μg/kg). This suggested that the additional bioactivity was likely in tissues not related to endochondral bone formation. In the phase III study, following subcutaneous administration at the recommended dose of 15 μg/kg to patients with achondroplasia aged 5-18 years, vosoritide was rapidly absorbed with a median time to maximal plasma concentration (C_max) of 15 minutes, and cleared with a mean half-life of 27.9 minutes after 52 weeks of treatment. Vosoritide exposure (C_max and area under the concentration-time curve [AUC]) was consistent across visits. No evidence of accumulation with once-daily dosing was observed. Total anti-vosoritide antibody (TAb) responses were detected in the serum of 25 of 60 (42%) treated patients in the phase III study, with no apparent impact of TAb development noted on annualized growth velocity or vosoritide exposure. Across the exposure range obtained with 15 µg/kg in the phase III study, no meaningful correlations between vosoritide plasma exposure and changes in annualized growth velocity or CXM, or changes from predose heart rate, and systolic or diastolic blood pressures were observed.

CONCLUSIONS

The results support the recommended dose of vosoritide 15 µg/kg for once-daily subcutaneous administration in patients with achondroplasia aged ≥ 5 years whose epiphyses are not closed.

CLINICAL TRIALS REGISTRATION

NCT02055157, NCT03197766, and NCT01603095.

An RNA aptamer restores defective bone growth in FGFR3-related skeletal dysplasia in mice

Achondroplasia is the most prevalent genetic form of dwarfism in humans and is caused by activating mutations in FGFR3 tyrosine kinase. The clinical need for a safe and effective inhibitor of FGFR3 is unmet, leaving achondroplasia currently incurable. Here, we evaluated RBM-007, an RNA aptamer previously developed to neutralize the FGFR3 ligand FGF2, for its activity against FGFR3. In cultured rat chondrocytes or mouse embryonal tibia organ culture, RBM-007 rescued the proliferation arrest, degradation of cartilaginous extracellular matrix, premature senescence, and impaired hypertrophic differentiation induced by FGFR3 signaling. In cartilage xenografts derived from induced pluripotent stem cells from individuals with achondroplasia, RBM-007 rescued impaired chondrocyte differentiation and maturation. When delivered by subcutaneous injection, RBM-007 restored defective skeletal growth in a mouse model of achondroplasia. We thus demonstrate a ligand-trap concept of targeting the cartilage FGFR3 and delineate a potential therapeutic approach for achondroplasia and other FGFR3-related skeletal dysplasias.

Etiology and Treatment of Growth Delay in Noonan Syndrome

Noonan syndrome is characterized by multiple phenotypic features, including growth retardation, which represents the main cause of consultation to the clinician. Longitudinal growth during childhood and adolescence depends on several factors, among them an intact somatotrophic axis, which is characterized by an adequate growth hormone (GH) secretion by the pituitary, subsequent binding to its receptor, proper function of the post-receptor signaling pathway for this hormone (JAK-STAT5b and RAS/MAPK), and ultimately by the production of its main effector, insulin like growth factor 1 (IGF-1). Several studies regarding the function of the somatotrophic axis in patients with Noonan syndrome and data from murine models, suggest that partial GH insensitivity at a post-receptor level, as well as possible derangements in the RAS/MAPK pathway, are the most likely causes for the growth failure in these patients. Treatment with recombinant human growth hormone (rhGH) has been used extensively to promote linear growth in these patients. Numerous treatment protocols have been employed so far, but the published studies are quite heterogeneous regarding patient selection, length of treatment, and dose of rhGH utilized, so the true benefit of GH therapy is somewhat difficult to establish. This review will discuss the possible etiologies for the growth delay, as well as the outcomes following rhGH treatment in patients with Noonan syndrome.

Phosphatase inhibition by LB-100 enhances BMN-111 stimulation of bone growth

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) and inactivating mutations in the natriuretic peptide receptor 2 (NPR2) guanylyl cyclase both result in decreased production of cyclic GMP in chondrocytes and severe short stature, causing achondroplasia (ACH) and acromesomelic dysplasia, type Maroteaux, respectively. Previously, we showed that an NPR2 agonist BMN-111 (vosoritide) increases bone growth in mice mimicking ACH (Fgfr3^Y367C/+). Here, because FGFR3 signaling decreases NPR2 activity by dephosphorylating the NPR2 protein, we tested whether a phosphatase inhibitor (LB-100) could enhance BMN-111–stimulated bone growth in ACH. Measurements of cGMP production in chondrocytes of living tibias, and of NPR2 phosphorylation in primary chondrocytes, showed that LB-100 counteracted FGF-induced dephosphorylation and inactivation of NPR2. In ex vivo experiments with Fgfr3^Y367C/+ mice, the combination of BMN-111 and LB-100 increased bone length and cartilage area, restored chondrocyte terminal differentiation, and increased the proliferative growth plate area, more than BMN-111 alone. The combination treatment also reduced the abnormal elevation of MAP kinase activity in the growth plate of Fgfr3^Y367C/+ mice and improved the skull base anomalies. Our results provide a proof of concept that a phosphatase inhibitor could be used together with an NPR2 agonist to enhance cGMP production as a therapy for ACH.

Prevention of guanylyl cyclase-B dephosphorylation rescues achondroplastic dwarfism

Activating mutations in the fibroblast growth factor receptor 3 (FGFR3) or inactivating mutations in guanylyl cyclase-B (GC-B), also known as NPR-B or Npr2, cause short-limbed dwarfism. FGFR3 activation causes dephosphorylation and inactivation of GC-B, but the contribution of GC-B dephosphorylation to achondroplasia (ACH) is unknown. GC-B7E/7E mice that express a glutamate-substituted version of GC-B that cannot be inactivated by dephosphorylation were bred with mice expressing FGFR3-G380R, the most common human ACH mutation, to determine if GC-B dephosphorylation is required for ACH. Crossing GC-B7E/7E mice with FGFR3G380R/G380R mice increased naso-anal and long (tibia and femur), but not cranial, bone length twice as much as crossing GC-B7E/7E mice with FGFR3WT/WT mice from 4 to 16 weeks of age. Consistent with increased GC-B activity rescuing ACH, long bones from the GC-B7E/7E/FGFR3G380R/G380R mice were not shorter than those from GC-BWT/WT/FGFR3WT/WT mice. At 2 weeks of age, male but not female FGFR3G380R/G380R mice had shorter long bones and smaller growth plate hypertrophic zones, whereas female but not male GC-B7E/7E mice had longer bones and larger hypertrophic zones. In 2-week-old males, crossing FGFR3G380R/G380R mice with GC-B7E/7E mice increased long bone length and hypertrophic zone area to levels observed in mice expressing WT versions of both receptors. We conclude that preventing GC-B dephosphorylation rescues reduced axial and appendicular skeleton growth in a mouse model of achondroplasia.

Identifying therapeutic drug targets using bidirectional effect genes

Prioritizing genes for translation to therapeutics for common diseases has been challenging. Here, we propose an approach to identify drug targets with high probability of success by focusing on genes with both gain of function (GoF) and loss of function (LoF) mutations associated with opposing effects on phenotype (Bidirectional Effect Selected Targets, BEST). We find 98 BEST genes for a variety of indications. Drugs targeting those genes are 3.8-fold more likely to be approved than non-BEST genes. We focus on five genes (IGF1R, NPPC, NPR2, FGFR3, and SHOX) with evidence for bidirectional effects on stature. Rare protein-altering variants in those genes result in significantly increased risk for idiopathic short stature (ISS) (OR = 2.75, p = 3.99 × 10-8). Finally, using functional experiments, we demonstrate that adding an exogenous CNP analog (encoded by NPPC) rescues the phenotype, thus validating its potential as a therapeutic treatment for ISS. Our results show the value of looking for bidirectional effects to identify and validate drug targets.

Sensitivity of the Natriuretic Peptide/cGMP System to Hyperammonaemia in Rat C6 Glioma Cells and GPNT Brain Endothelial Cells

C-type natriuretic peptide (CNP) is the major natriuretic peptide of the central nervous system and acts via its selective guanylyl cyclase-B (GC-B) receptor to regulate cGMP production in neurons, astrocytes and endothelial cells. CNP is implicated in the regulation of neurogenesis, axonal bifurcation, as well as learning and memory. Several neurological disorders result in toxic concentrations of ammonia (hyperammonaemia), which can adversely affect astrocyte function. However, the relationship between CNP and hyperammonaemia is poorly understood. Here, we examine the molecular and pharmacological control of CNP in rat C6 glioma cells and rat GPNT brain endothelial cells, under conditions of hyperammonaemia. Concentration-dependent inhibition of C6 glioma cell proliferation by hyperammonaemia was unaffected by CNP co-treatment. Furthermore, hyperammonaemia pre-treatment (for 1 h and 24 h) caused a significant inhibition in subsequent CNP-stimulated cGMP accumulation in both C6 and GPNT cells, whereas nitric-oxide-dependent cGMP accumulation was not affected. CNP-stimulated cGMP efflux from C6 glioma cells was significantly reduced under conditions of hyperammonaemia, potentially via a mechanism involving changed in phosphodiesterase expression. Hyperammonaemia-stimulated ROS production was unaffected by CNP but enhanced by a nitric oxide donor in C6 cells. Extracellular vesicle production from C6 cells was enhanced by hyperammonaemia, and these vesicles caused impaired CNP-stimulated cGMP signalling in GPNT cells. Collectively, these data demonstrate functional interaction between CNP signalling and hyperammonaemia in C6 glioma and GPNT cells, but the exact mechanisms remain to be established.

Natriuretic Peptide Expression and Function in GH3 Somatolactotropes and Feline Somatotrope Pituitary Tumours

Patients harbouring mutations in genes encoding C-type natriuretic peptide (CNP; NPPC) or its receptor guanylyl cyclase B (GC-B, NPR2) suffer from severe growth phenotypes; loss-of-function mutations cause achondroplasia, whereas gain-of-function mutations cause skeletal overgrowth. Although most of the effects of CNP/GC-B on growth are mediated directly on bone, evidence suggests the natriuretic peptides may also affect anterior pituitary control of growth. Our previous studies described the expression of NPPC and NPR2 in a range of human pituitary tumours, normal human pituitary, and normal fetal human pituitary. However, the natriuretic peptide system in somatotropes has not been extensively explored. Here, we examine the expression and function of the CNP/GC-B system in rat GH3 somatolactotrope cell line and pituitary tumours from a cohort of feline hypersomatotropism (HST; acromegaly) patients. Using multiplex RT-qPCR, all three natriuretic peptides and their receptors were detected in GH3 cells. The expression of Nppc was significantly enhanced following treatment with either 100 nM TRH or 10 µM forskolin, yet only Npr1 expression was sensitive to forskolin stimulation; the effects of forskolin and TRH on Nppc expression were PKA- and MAPK-dependent, respectively. CNP stimulation of GH3 somatolactotropes significantly inhibited Esr1, Insr and Lepr expression, but dramatically enhanced cFos expression at the same time point. Oestrogen treatment significantly enhanced expression of Nppa, Nppc, Npr1, and Npr2 in GH3 somatolactotropes, but inhibited CNP-stimulated cGMP accumulation. Finally, transcripts for all three natriuretic peptides and receptors were expressed in feline pituitary tumours from patients with HST. NPPC expression was negatively correlated with pituitary tumour volume and SSTR5 expression, but positively correlated with D2R and GHR expression. Collectively, these data provide mechanisms that control expression and function of CNP in somatolactotrope cells, and identify putative transcriptional targets for CNP action in somatotropes.