Converting cancer therapies into cures: lessons from infectious diseases

Abstract

During the past decade, cancer drug development has shifted from a focus on cytotoxic chemotherapies to drugs that target specific molecular alterations in tumors. Although these drugs dramatically shrink tumors, the responses are temporary. Research is now focused on overcoming drug resistance, a frequent cause of treatment failure. Here we reflect on analogous challenges faced by researchers in infectious diseases. We compare and contrast the resistance mechanisms arising in cancer and infectious diseases and discuss how approaches for overcoming viral and bacterial infections, such as HIV and tuberculosis, are instructive for developing a more rational approach for cancer therapy. In particular, maximizing the effect of the initial treatment response, which often requires synergistic combination therapy, is foremost among these approaches. A remaining challenge in both fields is identifying drugs that eliminate drug-tolerant "persister" cells (infectious disease) or tumor-initiating/stem cells (cancer) to prevent late relapse and shorten treatment duration.

Figure 1. Mechanisms of Resistance to Antimicrobials and Targeted Anticancer Agents

(A) Resistance via target mutation. This mechanism has been well described in both antimicrobial resistance and tumor cell resistance. The red drug binds tightly to its target (black square). A mutational event leads to alteration in the binding site for the drug (yellow circle), leading to loss of drug binding. This mechanism governs resistance to β-lactam antimicrobials (and other antimicrobial classes) as well as resistance to kinase inhibitors. (B) Resistance via bypass pathways. Treatment with antimicrobial or anticancer agents (red lines) leads to a block in the pathway converting X to Y. Conceptually, Y can be a metabolite or a phenotypic state (e.g., cell proliferation). Resistance to the effect of the drug can be mediated by upregulation of a parallel pathway that allows Y to be restored. This mechanism of resistance has been documented in anticancer therapy, for example, amplification of MET to bypass a drug-induced block in EGFR signaling. (C) Resistance by drug destruction or modification. Bacterial enzymes, such as β-lactamases or aminoglycoside-modifying enzymes, mediate antimicrobial resistance by drug destruction or modification. This mechanism has not been described for resistance to anticancer agents, although modification of anticancer agents through CYPs can affect efficacy (although this effect is not mediated by the tumor cell). (D) Intrinsic, nonmutational, resistance. Here the population of cells (either tumor or microbial) are genotypically identical. The red cells are drug sensitive and are rapidly killed by antimicrobial or anticancer therapy, but the yellow cells are poorly killed by antimicrobial or cancer therapy (tolerant). These tolerant cells are called microbial persisters or cancer stem cells. Therapy with antimicrobials or anticancer agents leads to substantial killing, but the persister cells are able to resist treatment and can repopulate the infection or tumor with drug-sensitive cells, causing disease relapse.

Figure 2. Relative Efficacy of Mono- versus Combination Therapy for Three Chronic Infections and Effect on Resistance

(A) The three curves schematically indicate the reduction in viral burden during monotherapy of HIV (dashed green line), combination therapy for HIV (solid green line), and monotherapy for hepatitis B (solid red line). Monotherapy of HIV produces a transient reduction in viral load, which becomes more dramatic and sustained with combination therapy. In contrast, monotherapy for hepatitis B (with entecavir or tenofovir) produces sustained virologic suppression. See text for specific references. (B) Monotherapy and combination therapy for tuberculosis. The y axis schematically represents clinical response (reduction in bacterial load, clinical improvement, radiographic improvement). Monotherapy and combination therapy have similar efficacy early in treatment, but the benefit of monotherapy is not sustained. (C) Effect of combination therapy on emergence of resistance. The y axis represents the % of patients with resistant bacteria or viruses according to week of treatment. HIV and TB resistance emerges rapidly during mono-therapy, leading to the loss of therapeutic effect depicted in panel A (HIV) and panel B (TB). Combination therapy for HIV and TB suppresses the emergence of resistant organisms and allows the sustained therapeutic benefit depicted in (A) and (B). In contrast, monotherapy for hepatitis B with entecavir or tenofovir is not associated with the emergence of resistant viruses, allowing sustained therapeutic benefit with monotherapy.