Interstrand crosslink inducing agents in pretransplant conditioning therapy for hematologic malignancies

Abstract

Despite successful molecularly targeted, highly specific, therapies for hematologic malignancies, the DNA interstrand crosslinking agents, which are among the oldest and least specific cytotoxic drugs, still have an important role. This is particularly true in stem cell transplantation, where virtually every patient receives conditioning therapy with a DNA-alkylating agent-based program. However, due to concern about serious additive toxicities with combinations of different alkylating drugs, the last several years have seen nucleoside analogs, whose cytotoxic action follows vastly different molecular pathways, introduced in combination with alkylating agents. The mechanistic differences paired with different metabolic pathways for the respective drugs have clinically translated into increased safety without appreciable loss of antileukemic activity. In this report, we review pre-clinical evidence for synergistic antileukemic activity when nucleoside analog(s) and DNA-alkylating agent(s) are combined in the most appropriate manner(s), without a measurable decrease in clinical efficacy compared with the more established alkylating agent combinations. Data from our own laboratory using combinations of fludarabine, clofarabine, and busulfan as prototype representatives for these respective classes of cytotoxic agents are combined with information from other investigators to explain how the observed molecular events will result in greatly enhanced synergistic cytotoxicity. We further present possible mechanistic pathways for such desirable cytotoxic synergism. Finally, we propose how this information-backed hypothesis can be incorporated in the design of the next generation conditioning therapy programs in stem cell transplantation to optimize antileukemic efficacy while still safeguarding patient safety.

Fig. 1

Suggested mechanism of synergistic cytotoxicity of clofarabine (Clo), fludarabine (Flu), and busulfan (Bu). Clo and Flu are taken up by cells and phosphorylated by kinases to their triphosphate forms (Clo-TP and Flu-TP), which are incorporated into growing DNA strands during replication. Clo and Flu inhibit ribonucleotide reductase (RR) and cause a decrease in the dCTP pool. As dCTP is an inhibitor of deoxycytidine kinase (DCK), a decrease in its concentration may result in activation of DCK, preferential phosphorylation of Clo and Flu, and increased inclusion of these analogs into nascent DNA strands. The incorporation of nucleoside analogs into DNA strands triggers DNA damage responses including DNA repair and histone modifications. Modified histones cause chromatin remodeling and better accessibility of genomic DNA to Bu crosslinking, which propagates DNA damage responses and the cycle continues. The inability of cells to repair DNA damage triggers apoptosis.