Preformulation studies with melanotan-II: a potential skin cancer chemopreventive peptide

Advances in Automated Piston Liquid-Liquid Microextraction Technique

A new automated micro liquid-liquid extraction technique was successfully developed. This novel syringe-based technique capitalizes on the advantages of vigorous fluid agitation and the shearing effect of two fluids with different properties to achieve high extraction efficiency. The technique is at least 20 times faster than mechanical shaking or sonication in achieving a similar recovery even with a hydrophilic probe molecule such as 1,4-dioxane in an aqueous medium. Excellent repeatability with a relative standard deviation as low as 0.56% over a five-day test, n = 2 per day, was demonstrated with 1,4-dioxane. Other model compounds in aqueous matrices evaluated, including phenolics and extraction solvents like chloroform and hexane, showed similar performance in repeatability. An added advantage of this technique involves performing multiple extractions. Its capabilities in conducting complicated extraction steps and minimizing the use of organic solvents as low as 200 µL to achieve a preconcentration effect were demonstrated. The technique is suitable for use with emulsion-forming samples without further sample manipulation by incorporating a demulsifier such as acetone during the extraction process. The technique was found to be efficient and environmentally friendly with low solvent waste. This technique is ideal for implementation in automated high throughput and cost-effective quality assurance laboratory environments.

Incorporation of Agouti-Related Protein (AgRP) Human Single Nucleotide Polymorphisms (SNPs) in the AgRP-Derived Macrocyclic Scaffold c[Pro-Arg-Phe-Phe-Asn-Ala-Phe-dPro] Decreases Melanocortin-4 Receptor Antagonist Potency and Results in the Discovery of Melanocortin-5 Receptor Antagonists

While the melanocortin receptors (MCRs) are known to be involved in numerous biological pathways, the potential roles of the MC5R have not been clearly elucidated in humans. Agouti-related protein (AgRP), an MC3R/MC4R antagonist and MC4R inverse agonist, contains an exposed β-hairpin loop composed of six residues (Arg-Phe-Phe-Asn-Ala-Phe) that is imperative for binding and function. Within this active loop of AgRP, four human missense polymorphisms were deposited into the NIH Variation Viewer database. These polymorphisms, Arg111Cys, Arg111His, Phe112Tyr, and Ala115Val (AgRP full-length numbering), were incorporated into the peptide macrocycles c[Pro¹-Arg²-Phe³-Phe⁴-Xaa⁵-Ala⁶-Phe⁷-dPro⁸], where Xaa was Dap⁵ or Asn⁵, to explore the functional effects of these naturally occurring substitutions in a simplified AgRP scaffold. All peptides lowered potency at least 10-fold in a cAMP accumulation assay compared to the parent sequences at the MC4Rs. Compounds MDE 6-82-3c, ZMK 2-82, MDE 6-82-1c, ZMK 2-85, and ZMK 2-112 are also the first AgRP-based chemotypes that antagonize the MC5R.

Orally Absorbed Cyclic Peptides

Peptides and proteins are not orally bioavailable in mammals, although a few peptides are intestinally absorbed in small amounts. Polypeptides are generally too large and polar to passively diffuse through lipid membranes, while most known active transport mechanisms facilitate cell uptake of only very small peptides. Systematic evaluations of peptides with molecular weights above 500 Da are needed to identify parameters that influence oral bioavailability. Here we describe 125 cyclic peptides containing four to thirty-seven amino acids that are orally absorbed by mammals. Cyclization minimizes degradation in the gut, blood, and tissues by removing cleavable N- and C-termini and by shielding components from metabolic enzymes. Cyclization also folds peptides into bioactive conformations that determine exposure of polar atoms to solvation by water and lipids and therefore can influence oral bioavailability. Key chemical properties thought to influence oral absorption and bioavailability are analyzed, including molecular weight, octanol-water partitioning, hydrogen bond donors/acceptors, rotatable bonds, and polar surface area. The cyclic peptides violated to different degrees all of the limits traditionally considered to be important for oral bioavailability of drug-like small molecules, although fewer hydrogen bond donors and reduced flexibility generally favored oral absorption.

Correlation of in vitro and in vivo models for the oral absorption of peptide drugs

The aim of this study was to evaluate two in vitro models, Caco-2 monolayer and rat intestinal mucosa, regarding their linear correlation with in vivo bioavailability data of therapeutic peptide drugs after oral administration in rat and human. Furthermore the impact of molecular mass (Mm) of the according peptides on their permeability was evaluated. Transport experiments with commercially available water soluble peptide drugs were conducted using Caco-2 cell monolayer grown on transwell filter membranes and with freshly excised rat intestinal mucosa mounted in Using type chambers. Apparent permeability coefficients (P (app)) were calculated and compared with in vivo data derived from the literature. It was shown that, besides a few exceptions, the Mm of peptides linearly correlates with permeability across rat intestinal mucosa (R (2) = 0.86; y = -196.22x + 1354.24), with rat oral bioavailability (R (2) = 0.64; y = -401.90x + 1268.86) as well as with human oral bioavailability (R (2) = 0.91; y = -359.43x + 1103.83). Furthermore it was shown that P (app) values of investigated hydrophilic peptides across Caco-2 monolayer displayed lower permeability than across rat intestinal mucosa. A correlation between P (app) values across rat intestinal mucosa and in vivo oral bioavailability in human (R (2) = 0.98; y = 2.11x + 0.34) attests the rat in vitro model to be a very useful prediction model for human oral bioavailability of hydrophilic peptide drugs. Presented correlations encourage the use of the rat in vitro model for the prediction of human oral bioavailabilities of hydrophilic peptide drugs.

Melanoma prevention strategy based on using tetrapeptide α‐MSH analogs that protect human melanocytes from UV‐induced DNA damage and cytotoxicity

Melanoma is the deadliest form of skin cancer, with no cure for advanced disease. We propose a strategy for melanoma prevention based on using analogs of alpha-melanocyte stimulating hormone (alpha-MSH) that function as melanocortin 1 receptor (MC1R) agonists. Treatment of human melanocytes with alpha-MSH results in stimulation of eumelanin synthesis, reduction of apoptosis that is attributable to reduced hydrogen peroxide generation and enhanced repair of DNA photoproducts. These effects should contribute to genomic stability of human melanocytes, thus preventing their malignant transformation to melanoma. Based on these findings, we synthesized and tested the effects of 3 tetrapeptide alpha-MSH analogs, Ac-His-D-Phe-Arg-Trp-NH2, n-Pentadecanoyl- and 4-Phenylbutyryl-His-D-Phe-Arg-Trp-NH2, on cultured human melanocytes. The latter two analogs were more potent than the former, or alpha-MSH, in stimulating the activity of tyrosinase, thus melanogenesis, reducing apoptosis and release of hydrogen peroxide and enhancing repair of DNA photoproducts in melanocytes exposed to UV radiation (UVR). The above analogs are MC1R agonists, as their effects were abrogated by an analog of agouti signaling protein, the physiological MC1R antagonist, and were absent in melanocytes expressing loss-of-function MC1R. Analogs, such as 4-Phenylbutyryl-His-D-Phe-Arg-Trp-NH2 with prolonged and reversible effects, can potentially be developed into topical agents to prevent skin photocarcinogenesis, particularly melanoma.

Octanol-water partition of nonzwitterionic peptides: predictive power of a molecular size-based model

A remarkably simple, molecular size-based model developed to predict octanol-water partition coefficients for organic compounds is tested on a set of 188 neutral peptides with available experimental partition data. Despite using only two parameters, it gives a promising correlation (r2 = 0.914; sigma = 0.455, F = 1978.0), and predictions are in a realistic range even for larger peptides (cyclosporin, melanotan, sandostatin) where common, overparametrized fragment methods become quite unreliable. Ion-pair partitioning and the extraction constant formalism is briefly reviewed to describe the sigmoidal lipophilicity profile of ionizable, nonzwitterionic peptides. It seems possible to extend the present model to estimate apparent partition coefficients measured around neutral pH and physiological conditions for monoionic peptides; however, as no standard conditions are yet defined and only relatively small number of experimental data are available, the situation here is more complex.

Evaluation of melanotan-II, a superpotent cyclic melanotropic peptide in a pilot phase-I clinical study

A pilot phase I study was conducted with a cyclic heptapeptide analog of alpha-melanocyte stimulating hormone (alpha-MSH). The lactam-bridged molecule, called Melanotan-II (MT-II), has the structure Ac-Nle4-Asp5-His6-D-Phe7-Arg8-Trp9-Lys10 alpha-MSH4-10-NH2 (MT-II) and has superpotent melanotropic activity in vitro. A single-blind, alternating day (saline or MT-II), placebo-controlled trial was conducted in 3 normal male volunteers at the starting dose of 0.01 mg/kg of MT-II. Subcutaneous injections of MT-II or saline were given daily (Monday-Friday) for 2 consecutive weeks. Two subjects were escalated by 0.005 mg/kg increments to 0.03 mg/kg and one to 0.025 mg/kg. The 0.03 mg/kg dose produced Grade II somnolence and fatigue in one of two subjects (WHO standards). Mild nausea, not requiring antiemetic treatment, was reported at most MT-II dose levels. A stretching and yawning complex appeared to correlate with the onset of spontaneous, penile erections which were intermittently experienced for 1-5 hours after MT-II dosing, depending on the MT-II dose. Two subjects had increased pigmentation in the face, upper body and buttock, as measured by quantitative reflectance and by visual perception 1 week after MT-II dosing ended. These results demonstrate that MT-II has tanning activity in humans given only 5 low doses every other day by subcutaneous injection. The recommended single MT-II dose for future Phase I studies is 0.025 mg/kg/day.