Natural prenylated
indoles have been proposed as potential
anticancer agents. To exploit this discovery for developing new
peptide therapeutics, we report the first studies whereby incorporation of prenylated
indoles into primary sequences has been achieved. We developed a route to synthesise Nα-Fmoc-protected
tryptophan derivatives in which the
prenyl group is linked to the N-
indole core, using Pd(ii)-mediated C-H functionalisation of 2-methyl-2-butene. Based on the
Substance P antagonist G (SPG), a well-known
Small Cell Lung Cancer (SCLC)
anticancer agent, we designed a new
penta-
peptide sequence to include a
prenyl moiety on one of the
tryptophan residues. The N-tert-prenylated
tryptophan analogue was assembled into the pentameric
peptide using standard solid phase peptide synthesis or liquid phase synthesis by fragment coupling. In vitro screening showed that the N-tert-prenylation of the
indole ring on the
tryptophan residue located near the C-terminal of the
penta-
peptide enhanced the cytotoxicity against H69 (IC50 = 2.84 ± 0.14 μM) and DMS79 (IC50 = 4.37 ± 0.44 μM) SCLC cell lines when compared with the unmodified
penta-
peptide (H69, IC50 = 30.74 ± 0.30 μM and DMS79, IC50 = 23.00 ± 2.07 μM) or the parent SPG sequence (IC50 > 30 μM, both cell lines). SCLC almost invariably relapses with
therapy-resistant disease. The DMS79 cell line was established from a patient following treatment with a number of chemotherapeutics (
cytoxan,
vincristine and
methotrexate) and
radiation therapy. Treating DMS79 tumour-bearing nude mice provided a human xenograft model of drug resistance to test the efficacy of the prenylated
peptide. A low dose (1.5 mg kg-1) of the prenylated
peptide was found to reduce tumour growth by ∼30% (P < 0.05) at day 7, relative to the control group receiving vehicle only. We conclude that the availability of the Fmoc-Trp(N-tert-
prenyl)-
OH amino acid facilitates the synthesis of prenylated-
tryptophan-containing
peptides to explore their therapeutic potential.