Collateral sensitivity of natural products in drug-resistant cancer cells

Cancer chemotherapy is frequently hampered by drug resistance. Concepts to combine anticancer drugs with different modes of action to avoid the development of resistance did not provide the expected success in the past, because tumors can be simultaneously non-responsive to many drugs (e.g. the multidrug resistance phenotype). However, tumors may be specifically hypersensitive to other drugs - a phenomenon also termed collateral sensitivity. This seems to be a general biological mechanism, since it also occurs in drug-resistant Escherichia coli and Saccharomyces cerevisiae. Here, we give a timely and comprehensive overview on hypersensitivity in resistant cancer cells towards natural products and their derivatives. Since the majority of clinically established anticancer drugs are natural products or are in one way or another derived from them, it is worth hypothesizing that natural products may deliver promising lead compounds for the development of collateral sensitive anticancer drugs. Hypersensitivity occurs not only in classical ABC transporter-mediated multidrug resistance, but also in many other resistance phenotypes. Resistant cancers can be hypersensitive to natural compounds from diverse classes and origins (i.e. mitotic spindle poisons, DNA topoisomerase 1 and 2 inhibitors, diverse phytochemicals isolated from medicinal plants, (semi)synthetic derivatives of phytochemicals, antibiotics, marine drugs, recombinant therapeutic proteins and others). Molecular mechanisms of collateral sensitivity include (1) increased ATP hydrolysis and reactive oxygen species production by futile cycling during ABC transporter-mediated drug efflux, (2) inhibition of ATP production, and (3) alterations of drug target proteins (e.g. increased expression of topoisomerases and heat shock proteins, inhibition of Wnt/β-catenin pathway, mutations in β-tubulin). The phenomenon of hypersensitivity needs to be exploited for clinical oncology by the development of (1) novel combination protocols that include collateral sensitive drugs and (2) novel drugs that specifically exhibit high degrees of hypersensitivity in resistant tumors.