Evaluation of a melanocortin-4 receptor (MC4R) agonist (Setmelanotide) in MC4R deficiency

Abstract

Objective: Pro-opiomelanocortin (POMC)-derived peptides act on neurons expressing the Melanocortin 4 receptor (MC4R) to reduce body weight. Setmelanotide is a highly potent MC4R agonist that leads to weight loss in diet-induced obese animals and in obese individuals with complete POMC deficiency. While POMC deficiency is very rare, 1-5% of severely obese individuals harbor heterozygous mutations in MC4R. We sought to assess the efficacy of Setmelanotide in human MC4R deficiency.

Methods: We studied the effects of Setmelanotide on mutant MC4Rs in cells and the weight loss response to Setmelanotide administration in rodent studies and a human clinical trial. We annotated the functional status of 369 published MC4R variants.

Results: In cells, we showed that Setmelanotide is significantly more potent at MC4R than the endogenous ligand alpha-melanocyte stimulating hormone and can disproportionally rescue signaling by a subset of severely impaired MC4R mutants. Wild-type rodents appear more sensitive to Setmelanotide when compared to MC4R heterozygous deficient mice, while MC4R knockout mice fail to respond. In a 28-day Phase 1b clinical trial, Setmelanotide led to weight loss in obese MC4R variant carriers. Patients with POMC defects upstream of MC4R show significantly more weight loss with Setmelanotide than MC4R deficient patients or obese controls.

Conclusions: Setmelanotide led to weight loss in obese people with MC4R deficiency; however, further studies are justified to establish whether Setmelanotide can elicit clinically meaningful weight loss in a subset of the MC4R deficient obese population.

Keywords: Melanocortin 4 receptor; Obesity; Setmelanotide; Stratification.

Figure 1

Genetic and molecular classification of MC4R mutations. (A) MC4R is a seven transmembrane domain GPCR with 3 intracellular loops (ICL), 3 extracellular loops (ECL), and a C-terminal helix 8 (H8); schematic adapted from GPCRDB Tools . Colors denote amino acids that have been altered by mutations. Functional consequences of these mutations were classified following a systematic review of the literature (Figure S1). Where multiple variants affecting the same amino acid have been reported, the most severe loss of function (LOF) mutation is shown. (B–D) Functional characterization of 22 new MC4R mutations. α-MSH stimulated cAMP generation was assessed in transiently transfected cells using a CREB-luciferase reporter assay (Table S1). Data are mean ± SEM of 4 experiments. (E–G). Genotype-phenotype correlations in 249 children with complete or partial loss of function MC4R mutations (cLOF/pLOF respectively). Data are represented as mean ± SEM. (E) Body mass index (BMI) standard deviation scores (SDS) of children who were homozygous (Hom) for cLOF (n = 16) or pLOF mutations (n = 17) were compared to those who were heterozygous (Het) for cLOF (n = 131) or pLOF (n = 85) MC4R mutations. Homozygotes had a significantly higher BMI SDS than those with heterozygous MC4R mutations (p < 0.001 with and without adjustment for age, sex and ethnicity), while there was no statistically significant difference between those with cLOF or pLOF mutations within the same gene dosage group (both p = 1.00). (F) Ad libitum food intake adjusted for lean mass measured by dual energy X-ray absorptiometry in 57 children (age < 18 years) (Hom cLOF (n = 6); Hom pLOF (n = 3); Het cLOF (n = 29); Het pLOF (n = 19)). (G) Ad libitum food intake adjusted for lean mass in 35 children age < 10 years (Hom cLOF (n = 5); Hom pLOF (n = 2); Het cLOF (n = 17); Het pLOF (n = 11)) (data on obese children have been reported previously .

Figure 2

Effects of Setmelanotide in cells expressing different MC4R variants. The cAMP response to α-melanocyte stimulating hormone (α-MSH) and Setmelanotide was studied for wildtype (WT) and 17 mutant forms of MC4R; mean EC50 (concentration required to elicit 50% of the maximal response) of experiments performed in duplicate with 11 point dose titrations. Some variants (R18C, V50M, I102T, G231S, R305W) have been studied previously and values were taken from the relevant reference . The ability of Setmelanotide to rescue signaling by MC4R mutants was calculated (EC50 of α-MSH/EC50 of Setmelanotide) and varied considerably between mutants. The ratio could not be precisely determined for some variants (G55D, E61K, C271Y) because of the high concentrations of α-MSH needed.

Figure 3

Effect of Setmelanotide administration in mice lacking Mc4r.Mc4r^+/+, ^+/−, and ^−/− mice were infused with 1200 nmol/kg/day of Setmelanotide, or vehicle (saline), using an implanted subcutaneous minipump. (A) Body weight before (day 0) and after (day 9) among all treatment groups. (B) Cumulative 9-day weight change among all Setmelanotide treated groups. Each symbol represents a single test animal's change in body weight. Statistical significance between groups is calculated using one-way ANOVA with Bonferroni post-test. (C) Setmelanotide mediated 9-day weight improvement over saline treated groups. These values are calculated by subtracting the average body weight change in saline treated groups from the average body weight change in Setmelanotide treated groups (ΔSetmelanotide − ΔSaline). A treatment that causes no change compared to saline would be represented by a weight difference of zero. Values displayed are Mean +/− SD and statistical significance is calculated using a T-test versus saline controls. *P < 0.05; **P < 0.01.