Purine nucleoside analogs in the treatment of rarer chronic lymphoid leukemias

Purine nucleoside analogues (PNA) are the cytotoxic agents highly active in the treatment of indolent lymphoid malignancies. These drugs have chemical structure similar to adenosine or deoxyadenosine. PNAs are characterized by a similar mechanism of cytotoxicity both in proliferating and quiescent cells, such as inhibition of DNA synthesis, inhibition of DNA repair and accumulation of DNA strand breaks. In addition, PNAs induce apoptosis which is the end-point of their action. Older PNAs, pentostatin (DCF; 2'- deoxycoformycin), cladribine (2-CdA; 2-chloro-2'-deoxyadenosine) and fludarabine (2-fluoro-9-(-D-arabinosyl)-adenine) were approved by Food and Drug Administration (FDA) for the treatment of hematological malignancies. In addition three novel PNAs: clofarabine (CAFdA), nelarabine (ara-G) and forodesine (immucillin H, BCX-1777) have been synthesized and introduced into preclinical studies and clinical trials. This review summarizes current knowledge on the mechanism of action and pharmacokinetic properties of older and new PNAs. Clinical activity and toxicity of PNAs, especially in hairy cell leukemia (HCL), hairy cell leukemia variant (HCL-V), prolymphocytic leukemia (PLL) and other rarer chronic lymphoid leukemias, are also presented. 2-CdA and DCF, introduced in the 1980s, changed radically the treatment modality, inducing complete and durable responses in the majority of patients with HCL. In contrast, the results of the treatment of HCL-V with PNA are rather poor. There are also several reports indicating activity of PNAs in PLL and large granular lymphocyte leukemia. Clofarabine, nelarabine and forodesine need further investigation in rarer lymphid leukemias, to better define their status in the treatment of these disorders.