Cancer drugs are a unique class of therapeutic agents. Like all therapeutics, they interfere with pathophysiological processes. Unlike other therapeutics, however, interference with these processes in cancer patients is intended to produce death of malignant cells without killing or injuring the non-malignant cells in normal host tissues. This is a daunting engineering challenge. Remarkably, modest successes, and even cures, are achieved through the use of non-selective, proliferation dependent chemotherapeutic poisons in a limited number of cancer types due to their higher proliferative rate and lower apoptotic threshold compared to normal tissues [1]. The price for such non-selectivity, however, is dose-limiting host toxicities, particularly myelosuppression, which greatly limits therapeutic efficacy against the more common solid malignancies. These limitations have lead to a profound re-thinking about the rationale for cancer drug development.

A strategy to overcome such tumor cell heterogeneity based therapeutic resistance is to develop a drug that acts like a “molecular grenade” in that it is designed to “detonate” upon release of a non-selective toxin restrictively within the extracellular microenvironment of metastatic sites of cancer. The chemical engineering requirements for optimizing the non-selective toxin are that it should be: 1) highly cell penetrant (i.e., lipophilic), 2) a potent inhibitor of an essential intracellular process required for survival by all cell types, and 3) capable of peptide bond formation with a specifically engineered carrier peptide. The amino-acid sequence of the carrier peptide is optimized to allow for coupling to the non-selective toxin via a peptide bond to produce a water soluble non-cell penetrant drug which is efficiently hydrolyzed releasing the cell penetrant toxin only by a defined protease whose expression is restricted to metastatic sites of cancer. Thus, when the drug is infused, it distributes systemically throughout the body but can only be activated (i.e. detonated) within the extracellular fluid by a plasma membrane protease expressed by at least a subset of cells within cancer sites, but not by cells in normal tissue. Within tumor sites, the protease “pulls the pin” on the grenade by proteolytically releasing the cell penetrant toxin. Once liberated, the toxin rapidly penetrates cells in its immediate vicinity due to its lipophilicity and does not re-enter the circulation, thus restricting its non-selective toxicity to the cancer microenvironment. The advantage of such selective extracellular hydrolysis is that only a fraction of the cells need to express the enzyme since its continuous activity amplifies the level of cell penetrant toxin liberated into the extracellular fluid shared by all cells within the metastatic site. This amplification minimizes the problem of tumor cell heterogeneity by inducing a substantial “bystander effect” in which, like a detonated grenade, all cells within the tumor site including both malignant and infiltrating host supportive cells are killed, even those that do not express the activating enzyme. Thus, development of resistance is retarded without simultaneously producing non-selective host toxicity.