Biological activity of selected tyrosine-containing 2,5-diketopiperazines.

The study investigates two cyclic dipeptides, cyclo(Tyr-Tyr) (cTT) and cyclo(Phe-Tyr) (cPT) with respect to their biological activity. Investigations using the whole-cell patch-clamp technique testing the effects of 100 microM cyclic dipeptide on ion channels, revealed reversible voltage-dependant blockade for cTT, while cPT exhibited irreversible time-dependant blockade of L-type calcium channels. The isolated retrogradely-perfused rat heart was used to determine the effects of 100 microM of either cTT or cPT on heart rate (HR), coronary flow rate (CFR), left ventricular systolic pressure (LVSP) and cardiac conduction speed. Results indicated opposing effects for the two compounds, where cTT increased HR and CF while cPT decreased HR, CF, LVSP as well as conduction speed. Other biological investigations included opioid binding and anti-neoplastic assays. Competitive binding curves, using tritiated DAMGO, revealed significant binding to micro-opioid receptors with IC50 values for cTT and cPT being 0.82 microM and 69.7 microM respectively. Anti-neoplastic activity was tested using three cultured cell lines: HT-29, MCF-7 and HeLa which were exposed to 2.5 mM cyclic dipeptide or 0.1 mM melphalan as a positive control. While cTT showed little activity against these lines, cPT resulted in as much as a 75.6% growth inhibition of MCF-7 cells, while also being active against HeLa (73.4% inhibition) and HT-29 (60.6%). The results indicate potential biological activity, showing a need for more investigation into tyrosine containing cyclic dipeptides and their analogues as potential bioactive compounds.

An investigation into the biological activity of the selected histidine-containing diketopiperazines cyclo(His-Phe) and cyclo(His-Tyr).

Although cyclic diketopiperazines have been known since the beginning of the century, only now have they attracted considerable interest with respect to their biological activity. The aim of this study was to determine if the diketopiperazines cyclo(L-histidyl-L-phenylalanyl) (cyclo(His-Phe)) and cyclo(L-histidyl-L-tyrosyl) (cyclo(His-Tyr)) have significant biological activity relevant to the treatment of cardiovascular-related disease states, cancer and infectious diseases. Haematological studies were performed, including thrombin substrate binding, blood clotting time, platelet adhesion, platelet aggregation and fibrinolysis assays. A cytotoxicity screening utilizing a tetrazolium-based assay on the cell lines HeLa, WHCO3, and MCF-7 was performed. The whole-cell patch-clamp technique was used to investigate ion-channel activity in ventricular myocytes of rats, and isolated rat heart studies were performed to investigate the cardiac effects involving heart rate and coronary flow rate. Cyclo(His-Tyr) produced a significant prolongation of blood clotting time, slowing of clot lysis and inhibition of ADP-induced platelet adhesion and aggregation (P < 0.05). Cyclo(His-Phe) showed significant (P < 0.05) anti-tumour activity, causing greatest reduction of cell viability in cervical carcinoma cells. Preliminary results from patch-clamp studies indicate that both diketopiperazines caused blocking of sodium and calcium ion channels, but opening of inward rectifying potassium ion channels. In the rat isolated heart studies, cyclo(His-Phe) caused a gradual reduction in heart rate (P = 0.0027) and a decrease in coronary flow rate (P = 0.0017). Cyclo(His-Tyr) significantly increased the heart rate (P = 0.0016) but did not cause any significant change of coronary flow rate (P > 0.05). Cyclo(His-Tyr) showed notable (P < 0.05) antibacterial activity and both diketopiperazines showed excellent antifungal activity (P < 0.05). These observations reveal diketopiperazines to be ideal lead compounds for the rational design of an agent capable of preventing metastasis, inhibiting tumour growth, and as potential chemotherapeutic, antiarrhythmic and antihypertensive agents, as well as potential antibacterial and antifungal agents.

Cyclic dipeptides in the induction of maturation for cancer therapy.

Studies have suggested a possible form of therapy based on the use of maturation-inducing compounds to induce differentiation of neoplastic cells and stimulate faster recovery of the normal cell population. The study of the effects of nine cyclic dipeptides on biochemical markers of differentiation implicated their potential to induce differentiation. Studies were undertaken to determine the specificity of these agents for HT-29 cell cultures as well as the identification of the signal transduction pathways affected by these agents inducing the differential gene expression observed in the cells. The cyclic dipeptides studied showed a high degree of specificity, having no significant effect on Caco-2 cells (P > 0.05), representing the normal gastrointestinal mucosa. All inducers administered were shown to affect the total energy state of HT-29 cells, an effect which increased the probability of HT-29 cell differentiation. Results indicated that those agents which induced differential gene expression acted at different steps in the isolated signal transduction pathway. Cyclo(Trp-Trp) and cyclo(Phe-Pro) induced a high degree of acetylation of histones (P < 0.05), while the remaining cyclic dipeptides induced a high degree of phosphorylation of histones (P < 0.05) (cyclo(Trp-Trp) induced a moderate degree of histone phosphorylation). The results from histone phosphorylation and acetylation and cyclic AMP responsive element binding protein phosphorylation studies suggest that the cyclic dipeptides activate a chromatin switch, which leads to the increase in accessibility of lineage-specific genes for transcription.