Design, Synthesis, and Functional Studies on Noscapine and Cotarnine Amino Acid Derivatives as Antitumor Agents

Noscapine, a phthalide isoquinoline alkaloid isolated from the opium poppy, alongside cotarnine, a tetrahydroisoquinoline (THIQ) scaffold produced by the oxidative degradation of noscapine, has exhibited antitumor activities against several types of cancer. Although derivatization with amino acids is regarded as a promising strategy to improve chemotherapeutics’ anticancer properties, amino acid conjugates of noscapine and cotarnine have been the least investigated. In the present study, 20 amino acid conjugated derivatives of noscapine and cotarnine at the 6-position were synthesized and evaluated for anticancer activity in both in vitro and in vivo conditions. Analysis of the antiproliferative activity against 4T1 mammary carcinoma tumor cells showed that compounds 6h (noscapine–phenylalanine), 6i (noscapine–tryptophan), and 10i (cotarnine–tryptophan) with IC50 values of 11.2, 16.3, and 54.5 μM, respectively, were found to be far more potent than noscapine (IC50 = 215.5 μM) and cotarnine (IC50 = 575.3 μM) and were consequently opted for further characterization. Annexin V and propidium iodide staining followed by flow cytometry demonstrated improved apoptotic activity of compounds 6h, 6i, and 10i compared to those of noscapine and cotarnine. In a murine model of 4T1 mammary carcinoma, noscapine–tryptophan inhibited tumor growth more effectively than noscapine and the other amino acid conjugates without adverse effects. Moreover, molecular docking studies conducted on tubulin as the intracellular target of noscapine suggested a good correlation with experimental observations. Based on these results, noscapine–tryptophan could be a promising candidate for further preclinical investigations.