Emerging Insights into Targeted Therapy-Tolerant Persister Cells in Cancer

Abstract

Drug resistance is perhaps the greatest challenge in improving outcomes for cancer patients undergoing treatment with targeted therapies. It is becoming clear that "persisters," a subpopulation of drug-tolerant cells found in cancer populations, play a critical role in the development of drug resistance. Persisters are able to maintain viability under therapy but are typically slow cycling or dormant. These cells do not harbor classic drug resistance driver alterations, and their partial resistance phenotype is transient and reversible upon removal of the drug. In the clinic, the persister state most closely corresponds to minimal residual disease from which relapse can occur if treatment is discontinued or if acquired drug resistance develops in response to continuous therapy. Thus, eliminating persister cells will be crucial to improve outcomes for cancer patients. Using lung cancer targeted therapies as a primary paradigm, this review will give an overview of the characteristics of drug-tolerant persister cells, mechanisms associated with drug tolerance, and potential therapeutic opportunities to target this persister cell population in tumors.

Keywords: acquired drug resistance; drug-tolerant persisters; targeted therapy.

Figure 1

Resistance to targeted cancer therapies. Drug resistance can occur at the time of initial therapy (primary resistance) or develop after initial therapy response (acquired resistance). (A) Primary resistance may be due to intrinsic resistance resulting from ineffective targeting of the oncogenic driver such that oncogenic signaling is not suppressed (for instance, activating KRAS mutations confer resistance to EGFR targeted therapy). Alternatively, in adaptive resistance, cells rapidly rewire oncogenic signaling after initial suppression such that therapy does not induce death in the bulk of tumor cells. (B) In acquired drug resistance, disease relapse after initial therapy response may be driven by Darwinian selection of resistant clones that exist before treatment and expand under the selective pressure of therapy (for instance, rare T790M+ clones in some EGFR-mutant lung cancers), or evolution of drug-tolerant “persister” cells that acquire resistance mechanisms during the course of therapy.

Figure 2

Conceptual similarities between bacterial and cancer persisters. (A) Survival curves depict bacterial state trajectories during exposure to antibiotics. Sensitive bacterial populations (blue) are eliminated by antibiotic treatment, whereas resistant bacteria (dark gray) continue to proliferate in the presence of antibiotic. Persister bacteria (orange) can survive during antibiotic exposure, but upon drug withdrawal, repopulate rapidly, proliferating sensitive bacterial populations. (B) Prototypical survival curve illustrating the emergence and reversibility of drug-tolerant cancer cells from a primarily drug-sensitive population. Treatment of heterogenous tumor cell populations with targeted therapy eliminates sensitive clones, revealing a subpopulation of so-called “drug-tolerant persister cells” (DTPs). A hallmark of the persister state is reversibility upon withdrawal of drug, resulting in re-establishment of a mixed population of sensitive and drug-tolerant cells.

Figure 3

Studying persisters in cancer. (A) Modeling persisters using in vitro cell culture systems. Persisters are modeled by a sub-population of cells that remain viable in culture upon extended drug treatment after the majority of sensitive parental cells are eliminated. PC9 EGFR-mutant lung cancer cells are shown before (left) and during gefitinib treatment. Over time, the population of drug-tolerant persisters (middle images) gives rise to a fully resistant T790M+ clone (right image). (B) Modeling persisters in vivo using mouse xenograft tumors. Drug treatment leads to tumor regression followed by a period of stable “minimal residual disease” (MRD) in which a small population of cancer cells exist within tumors that are no longer regressing. Shown are H&E images of EGFR-mutant lung cancer patient-derived xenograft tumors before or after treatment with osimertinib, revealing small residual clusters of surviving tumor cells. (C) Studying persisters in the clinic. As illustrated by a typical waterfall plot of best tumor response, most patients do not experience complete responses to targeted therapies. Residual disease lesions harbor surviving cancer cells but may remain stable for months or even years. Sequential pre/on-treatment and progression biopsies enable study of persister phenotypes and the evolutionary trajectories of resistant cancer cells. H&E images depict sequential biopsies from an EGFR lung cancer patient treated with EGFR targeted therapy in a clinical trial. In the on-treatment biopsy, small clusters of tumors cells are surrounded by extensive fibroblastic stroma.