Structure–Activity Relationship Studies of a Macrocyclic AGRP-Mimetic Scaffold c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-DPro] Yield Potent and Selective Melanocortin-4 Receptor Antagonists and Melanocortin-5 Receptor Inverse Agonists That Increase Food Intake in Mice

Abstract

The melanocortin system has five receptors, and antagonists of the central melanocortin receptors (MC3R, MC4R) are postulated to be viable therapeutics for disorders of negative energy balance such as anorexia, cachexia, and failure to thrive. Agouti-related protein (AGRP) is an antagonist of the MC3R and an antagonist/inverse agonist of the MC4R. Biophysical NMR-based structural studies have demonstrated that the active sequence of this hormone, Arg-Phe-Phe, is located on an exposed β-hairpin loop. It has previously been demonstrated that the macrocyclic octapeptide scaffold c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-DPro] is 16-fold less potent than AGRP at the mouse MC4R (mMC4R). Herein it was hypothesized that the Phe position may be substituted to produce more potent and/or selective melanocortin receptor antagonist ligands based on this template. A 10-membered library was synthesized that substituted small (Gly), polar (Ser), acidic (Asp), basic (Lys), aliphatic (Leu, Nle, and Cha), and aromatic (Trp, Tyr, hPhe) amino acids to explore potential modifications at the Phe position. The most potent mMC4R antagonist contained a Nle substitution, was equipotent to the lead ligand 200-fold selective for the mMC4R over the mMC3R, and caused a significant increase in food intake when injected intrathecally into male mice. Three compounds possessed sigmoidal dose-response inverse agonist curves at the mMC5R, while the remaining seven decreased cAMP production from basal levels at a concentration of 100 μM. These findings will add to the knowledge base toward the development of potent and selective probes to study the role of the melanocortin system in diseases of negative energy balance and can be useful in the design of molecular probes to examine the physiological functions of the mMC5R.

Synergistic Multiresidue Substitutions of a Macrocyclic c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-dPro] Agouti-Related Protein (AGRP) Scaffold Yield Potent and >600-Fold MC4R versus MC3R Selective Melanocortin Receptor Antagonists

“…However, Schild analysis of antagonist potency has indicated that AGRP(109–118) possesses submicromolar antagonist potency at the MC4R 46 – 47 . Many subsequent studies have observed similar melanocortin receptor pharmacology, including agonist activity at the mMC1R, micromolar potency at the mMC3R, and a range of potencies at the mMC4R (generally ranging from micromolar to nanomolar) 41 – 43 …”

“…Although peptide 1 possessed increased potency compared to the disulfide cyclized peptide at the mMC4R, it still does not match the potency of the native hormone AGRP (pA 2 = 8.7–9.1) 42 . Therefore, the SAR around this octapeptide scaffold has been explored in order to generate more potent and/or selective MC4R antagonists 41 – 43 . These SAR analyses have been performed on the Phe 3 , Phe 4 , Asn 5 , Ala 6 , and Phe 7 positions.…”

Arg-Phe-Phe d-Amino Acid Stereochemistry Scan in the Macrocyclic Agouti-Related Protein Antagonist Scaffold c[Pro-Arg-Phe-Phe-Xxx-Ala-Phe-DPro] Results in Unanticipated Melanocortin-1 Receptor Agonist Profiles

“…41, 42 These macrocyclic ligands possessed >100-fold decreased antagonist potency at the MC3R compared to AGRP, micromolar agonist potency or higher at the MC1R, and select compounds were apparent inverse agonists at the MC5R. 40 – 42…”

Potential role of the melanocortin signaling system interference in the excess weight gain associated to some antiretroviral drugs in people living with HIV

“…Distinct large-scale genome-wide association study analysis unveiled MC4R polymorphic gene variants associated with weight gain [12]. Pharmacological activators of MC4R are used to treat obese patients with MC4R genetic defects, and recently approved drugs for obesity (bupropion and naltrexone) activate the POMC system [13]. Conversely, MC4R antagonists or inverse agonists to treat anorexia cause increased food intake in animal models [13].…”