Comparative Study of Chemical Stability of a PK20 Opioid–Neurotensin Hybrid Peptide and Its Analogue [Ile(9)]PK20—The Effect of Isomerism of a Single Amino Acid

Chemical stability is one of the main problems during the discovery and development of potent drugs. When ignored, it may lead to unreliable biological and pharmacokinetics data, especially regarding the degradation of products’ possible toxicity. Recently, two biologically active drug candidates were presented that combine both opioid and neurotensin pharmacophores in one entity, thus generating a hybrid compound. Importantly, these chimeras are structurally similar except for an amino acid change at position 9 of the peptide chain. In fact, isoleucine (C(6)H(13)NO(2)) was replaced with its isomer tert-leucine. These may further lead to various differences in hybrids’ behavior under specific conditions (temperature, UV, oxidative, acid/base environment). Therefore, the purpose of the study is to assess and compare the chemical stability of two hybrid peptides that differ in nature by way of one amino acid (tert-leucine vs. isoleucine). The obtained results indicate that, opposite to biological activity, the substitution of tert-leucine into isoleucine did not substantially influence the compound’s chemical stability. In fact, neither hydrolysis under alkaline and acidic conditions nor oxidative degradation resulted in spectacular differences between the two compounds—although the number of potential degradation products increased, particularly under acidic pH. However, such a modification significantly reduced the compound’s half-life from 204.4 h (for PK20 exposed to 1M HCl) to 117.7 h for [Ile(9)]PK20.