Carnosine phenyl derivatives as specific and efficient sequestering agents of cytotoxic Reactive Carbonyl Species (RCS)

Reactive carbonyl species (RCS) are important cytotoxic mediators generated by lipidoxidation of PUFAs, leading to alteration of the cellular function and inducing irreversible structural modifications to biomolecules.1 RCS belong to different chemical classes such as alfa,beta-unsaturated aldehydes [4-hydroxy-trans-2-nonenal (HNE), acrolein (ACR)], dialdehydes [malondialdehyde (MDA), glyoxal (GO)] and levuglandines.2 RCS and the corresponding adducts with proteins (that is, carbonylated proteins) are widely used as biomarkers of lipidperoxidation and, in general, of oxidative stress. Moreover, there are several convincing evidences supporting a pathogenic role for RCS, such as in the case of diabetic-related diseases, age-dependent tissue dysfunction, and metabolic distress syndrome. Consequently, RCS, in addition to being a predictive biomarker, also represents a biological target for drug discovery. The most promising strategy to neutralize/reduce these pathogenetic factors is based on nucleophilic compounds capable to form covalent and unreactive adducts with RCS (RCS sequestering agents) such as pyridoxamine (PYR), hydralazine (HY), dihydralazine (di-HY), aminoguanidine (AG), and metformin (MF). However these compounds are characterized by a severe aspecificty since they react also with physiological aldehydes such as pyridoxal. We recently found that the endogenous dipeptide carnosine (beta-alanyl-L-histidine) is a specific quencher of alfa,beta-unsaturated aldehydes due to its peculiar mechanism involving the Schiff base formation between the beta-alanine amino group and the RCS aldehyde followed by the Michael adduction between the C3 of the aldehyde and the Ntau of the histidine group3. However, the therapeutic use of carnosine is limited since it is unstable in human plasma due to the serum carnosinase activity.4 Moreover the reactivity of carnosine towards RCS is significant lower in respect to that of AG, HY and PYR. Hence, aim of the work was to derive carnosine analogues characterized by (i) carnosinase stability and (ii) a grater reactivity towards RCS even maintaining the same specificity. The stability was reached by the isomerization of L- to D-histidine aminoacid, leading to beta-alanyl-D-histidine (D-carnosine) which is not recognized by carnosinase but conserves the same quenching activity of L-carnosine. Although the simplest approach to increase the reactivity would be to enhance the nucleophilicity of the amino group, this is not largely exploitable since it would mine the specificity and favor the protonated amino form. Hence we focused our attention to the specific Michael adduction, by modulating the conformational profile of the Schiff base intermediate in order to favor a close conformation in which the imidazole ring approaches enough the C3 of the Schiff base to form the corresponding Michael adduct. A series of D-carnosine derivatives was analyzed by in silico approaches to find out those characterized by a favorable folded conformational profile. The most promising were synthetized and the stability and quenching ability evaluated. By this way a set of phenyl derivatives was identified, characterized by high stability in human plasma and by a quenching activity towards HNE increased by almost 90-100% in respect to D-carnosine