Vosoritide (Voxzogo): The CNP Analog for Achondroplasia, Not a GH Peptide

Vosoritide is a 39-amino-acid analog of C-type natriuretic peptide (CNP), sold as Voxzogo. FDA approved it on November 19, 2021 under accelerated approval to increase linear growth in children with achondroplasia who still have open epiphyses (FDA, 2021). It is not growth hormone, not a GH secretagogue, and not a research-catalog CNP fragment.

TL;DR

• Achondroplasia is a gain-of-function FGFR3 problem. Overactive MAPK signaling slows endochondral ossification at the growth plate (FDA label).
• Native CNP hits natriuretic peptide receptor-B (NPR-B) and can brake that MAPK cascade, but IV half-life is about 2 minutes because neutral endopeptidase (NEP) eats it (Lorget et al., 2012).
• Vosoritide adds Pro-Gly at the N-terminus of the CNP53 C-terminal 37 residues. That NEP-resistant 39-mer is 4.1 kDa, made in E. coli (FDA label).
• Phase 3 (n=121): adjusted mean annualized growth velocity (AGV) difference 1.57 cm/year versus placebo at 52 weeks (95% CI 1.22 to 1.93, p<0.0001) (Savarirayan et al., 2020).
• A research vial marked “CNP” is not Voxzogo. It is also not a gym peptide.

Educational only. Not medical advice, not a protocol, and not a reason to inject a catalog analog.

How is vosoritide different from a GH peptide?

Growth hormone secretagogues and IGF-1 analogs work through the GH/IGF axis. Vosoritide does not. It binds NPR-B (also called NPR2 or GC-B) on growth-plate chondrocytes.

That receptor raises cGMP. cGMP, through protein kinase G, inhibits RAF-1 and thereby the ERK1/2 MAPK cascade that FGFR3 keeps turned on (FDA label). The peptide is a MAPK brake, not a pituitary secretagogue.

Lorget’s group showed the same crosstalk in human achondroplasia chondrocytes carrying p.Gly380Arg FGFR3: BMN 111 cut FGF-driven ERK1/2 phosphorylation and did not appear to cut STAT3 (Lorget et al., 2012). NPR-B talks to MAPK. It is not a general GH replacement.

For the amino-acid primer, see Peptides 101. For GH-axis peptides, see the ipamorelin guide.

Why does native CNP fail as a drug?

NEP eats it. Lorget cited a roughly 2-minute IV half-life for native CNP and argued that a pediatric therapy could not be a continuous infusion (Lorget et al., 2012). The analog keeps CNP pharmacology at NPR-B while resisting that peptidase.

The chemistry is a two-residue N-terminal extension, not PEGylation or a fatty acid. The sequence is the 37 C-terminal amino acids of human CNP53 plus Pro-Gly, 4.1 kDa, made in E. coli (FDA label).

After a subcutaneous shot, median Tmax is 15 minutes. Mean half-life in the label ranged from 21.0 plus or minus 4.7 to 27.9 plus or minus 9.9 minutes, with no accumulation on once-daily 15 mcg/kg in the phase 3 program (FDA label; Chan et al., 2022). Daily injection is the price of not PEGylating it.

See the DAC, PEGylation, and lipidation guide for those longer clocks.

What did the phase 3 trial actually measure?

Study 1 (NCT03197766) was a 52-week, multicenter, randomized, double-blind, placebo-controlled trial in 121 children with genetically confirmed achondroplasia and open epiphyses. Ages ran from 5.1 to 14.9 years (mean 8.7). Sixty received vosoritide 15 mcg/kg subcutaneously once daily, and 61 received placebo (FDA label).

The primary endpoint was change from baseline in AGV at week 52, not adult height.

In the Lancet report, 119 of 121 completed 52 weeks. The 1.57 cm/year gap is an adjusted mean difference, not final adult height (Savarirayan et al., 2020). FDA said so explicitly: accelerated approval rested on AGV as an intermediate endpoint, with a post-marketing look at adult height still required (FDA, 2021).

The 2021 label limited the indication to ages 5 and older. BioMarin later announced an October 2023 sNDA expansion to pediatric patients of all ages with open epiphyses, still under accelerated approval (BioMarin, 2023). The labeled product stops when epiphyses close, because there is no growth plate left to recruit.

What does NPR-B signaling cost?

CNP biology is natriuretic. Transient blood-pressure drops showed up in the trial, which is why the label wants food and about 240 to 300 mL of fluid in the hour before a dose.

Two of 60 vosoritide-treated children (3%) had a symptomatic low-pressure episode with vomiting and/or dizziness, versus none of 61 on placebo. Median onset was 31 minutes (range 18 to 120). Resolution was within 31 minutes (5 to 90) (FDA label).

Common adverse reactions at more than 10% included injection-site erythema, swelling, urticaria, vomiting, arthralgia, decreased blood pressure, and gastroenteritis. Injection-site reactions were common on both arms (85% vs 82%), but event rates were much higher on drug (120.4 vs 29.2 per person-year). Anti-drug antibodies appeared in 35% of evaluable subjects treated up to 240 weeks, none neutralizing, with no obvious hit to AGV or PK on the label (FDA label).

This is a growth-plate drug in a skeletal dysplasia, not a recomp protocol, and not indicated once the plates are fused.

Is a catalog CNP vial the same peptide?

No. Voxzogo is a specific 39-residue analog with a defined Pro-Gly N-terminus, recombinant manufacture, labeled reconstitution kit, and a pediatric indication tied to open epiphyses. A lyophilized “CNP-22” or “CNP-53” research vial is a different sequence, a different process, and not an FDA-approved drug.

How a lab builds a research peptide is a different conversation from how BioMarin builds Voxzogo. See how peptides are made, the reconstitution guide, and the calculator for dissolving a lyophilized vial. Licensed product still comes through a prescriber and the provider directory, not a research catalog.

Bottom line

Vosoritide is a short-acting, NEP-resistant CNP analog that brakes FGFR3-driven MAPK at the growth plate. The phase 3 number that won accelerated approval is 1.57 cm/year of extra AGV, not adult height, and a catalog CNP vial is not that product.

Educational only. Not medical advice.