Pentapeptides for the treatment of small cell lung cancer: Optimisation by N(ind)-alkyl modification of the tryptophan side chain

The pentapeptide, tert-Prenyl(4th)-NH(2) (DMePhe-DTrp-Phe-DTrp(N-tert-prenyl)-Leu-NH(2)), has recently been reported by our group to exhibit properties of substance P (SP) antagonist G against small cell lung cancer (SCLC). In this study, we undertook a systematic structure activity investigation to optimise this lead compound to improve its in vitro anti-tumour activity and biocompatibility. A series of d-tryptophan (D-Trp) derivatives were synthesised, with a range of aliphatic N-alkyl chains (methyl to pentyl) on the indole nitrogen (N(ind)). These were incorporated into the pentapeptide sequence by substitution of the N(ind)-tert-prenylated D-Trp 4th residue with the N(ind)-alkylated D-Trp derivatives. These pentapeptides were significantly more potent than tert-Prenyl(4th)-NH(2), with the N(ind)-butyl modification generating the most cytotoxic peptides. Compared to tert-Prenyl(4th)-NH(2), a single butyl modification on the 4th D-Trp residue (Butyl(4th)-NH(2)) showed a ∼3 fold enhancement in cytotoxicity in either the chemo-naive H69 or the DMS79 (originating from a patient treated with chemotherapeutics and radiation therapy) SCLC cell lines. In addition, the di-butylated sequence on the 2nd and 4th D-Trp residues (Butyl(2nd,4th)-NH(2)) gave ∼4.5 times higher cytotoxicity against the H69 cell line and a ∼2 fold increase against the DMS79 cell line, compared to tert-Prenyl(4th)-NH(2). The favoured position for butyl modification was the 4th D-Trp residue, as the Butyl(2nd)-NH(2) peptide gave lower cytotoxicity on both cell lines. Butylated peptide sequences, when exposed to neat mouse plasma for 24 h at 37 °C, were found to resist degradation with >80% remaining intact compared to ∼58% for tert-Prenyl(4th)-NH(2). The degradation pathway in plasma occurs via de-amidation of the C-terminus, confirmed by mass spectrometry and RP-HPLC analysis. The butyl modification also conferred resistance to metabolism when tested using S9 liver fraction from mouse. The optimum analogue responsive against the DMS79 cell line was the Butyl(4th)-NH(2) pentapeptide, which revealed a concentration dependent increase in apoptosis: the level of late apoptotic cells rose from ∼36% at 2 μM to ∼96% at 6 μM, as determined by flow cytometry, compared to the unmodified peptide that showed no such effect. Concluding, the butyl substitutions offered the best perspective for high cytotoxicity, induction of apoptosis and metabolic compatibility thereby comprising an improved broad spectrum SP antagonist candidate for treatment of SCLC.