MT-2: Research Chemistry and Literature

Origin and identification

Melanocortin peptide chemistry at the University of Arizona ran along two lines through the 1980s. One replaced individual residues in the open chain. The other closed rings. Cyclic lactam variants of Ac-[Nle4]alpha-MSH(4-11)-NH2 were reported in 1988, and the comparison was instructive: the L-Phe7 ring held its activity, while the D-Phe7 ring lost more than two orders of magnitude in the amphibian assay, with nuclear magnetic resonance pointing to a stabilised turn in the weaker of the two.

MT-2 followed in 1989. Bridge position and ring size were selected from quenched molecular dynamics simulations rather than by enumeration, and the peptide that came out of that exercise was described as both potent and long-acting. Cutting the parent from thirteen residues to seven while adding the constraint shows how little of the native sequence the receptor family actually requires.

Identification rests on three facts read together. The residue sequence, the position of the bridge, and the measured mass. Cyclisation removes one molecule of water relative to the open-chain precursor, so a linear contaminant sits eighteen mass units above the ring and is straightforward to detect.

Preclinical research context

The scaffold is best known for what one substitution does to it. Exchanging D-Phe7 for D-2-naphthylalanine or for D-p-iodophenylalanine converts the agonist into a compound that antagonises MC3R and MC4R while continuing to act as an agonist at MC1R and MC5R. Agonist and antagonist differ at a single position. That is why the two are run together when a response has to be assigned to a subtype.

Assays are performed on cloned receptors expressed in COS or HEK cell lines. Binding is measured by displacement of an iodinated melanocortin tracer and function by cyclic AMP accumulation, and the two readouts together separate affinity from efficacy. Constrained analogues built on the same principle have reached single-subtype selectivity, with binding at the human MC4R in the low nanomolar range and no measurable binding at the other three subtypes up to ten micromoles per litre.

Conformational analysis underpins the pharmacology. Solution structures of the agonist and its antagonist counterpart have been determined by NMR in water, in water with dimethyl sulfoxide, and in membrane-mimicking micelles, then docked into models of MC4R. The differences between the two are small and local, consistent with a receptor that reads the orientation of the aromatic residue at position 7 rather than the gross shape of the ring.

Rodent contributions to this literature are mainly receptor mapping and ex vivo pharmacology. Transcript distribution is traced across tissues, and cloned rodent receptors are compared against the human orthologues, whose affinities do not transfer between species.

Analytical characterisation

Reverse-phase HPLC on C18 with an acetonitrile gradient in dilute trifluoroacetic acid gives the chromatographic purity figure. A cyclic peptide and its linear precursor differ in exposed hydrophobic surface and so differ in retention. Resolving the two is the point of the method rather than an incidental result of it.

Mass spectrometry supplies the sequence-consistent mass. Electrospray ionisation produces a multiply charged envelope that deconvolutes to a neutral mass. Tandem fragmentation can be used to confirm the ring, because a cyclic region fragments differently from an open chain and yields a distinguishable series.

Lot analysis is carried out by Janoshik Analytical as an independent third party. The Certificate of Analysis records identity, purity as area percent and net peptide content, and carries the underlying chromatogram and spectrum with it. Each certificate belongs to one lot and to no other.

Handling and storage

The lyophilised solid tolerates ambient temperature during shipment. Transit at room temperature is expected behaviour for this class of material and is not a defect. Place the vial at minus 20 degrees Celsius on arrival, in the dark, and leave it sealed until it is required.

Bring vials to room temperature before breaking the seal. Condensation on cold glass drives water into the cake, and a wet cake is a degraded one. In solution the peptide is held at 2 to 8 degrees Celsius. Solutions are working stock, not storage.

Freeze-thaw cycling costs more material than time in the freezer does. Split reconstituted peptide into single-use volumes and freeze those, so that no portion is thawed more than once. Label each aliquot with lot number and date. For laboratory research use only, not for human or veterinary use.

References

1. Potent and prolonged acting cyclic lactam analogues of alpha-melanotropin: design based on molecular dynamics
2. Cyclic lactam analogues of Ac-[Nle4]alpha-MSH4-11-NH2
3. Cyclic lactam alpha-melanotropin analogues of Ac-Nle4-cyclo[Asp5, D-Phe7,Lys10] alpha-melanocyte-stimulating hormone-(4-10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors
4. NMR and quenched molecular dynamics studies of superpotent linear and cyclic alpha-melanotropins
5. Design, synthesis, and biological evaluation of new cyclic melanotropin peptide analogues selective for the human melanocortin-4 receptor
6. Conformational study on cyclic melanocortin ligands and new insight into their binding mode at the MC4 receptor

Related compounds