ATC Group: L01BB Purine analogues

Cladribine is a nucleoside analogue of deoxyadenosine. In resting cells cladribine causes DNA single-strand breaks, rapid nicotinamide adenine dinucleotide consumption, ATP depletion and cell death.

Clofarabine is a purine nucleoside anti-metabolite. Its antitumour activity is believed to be due to 3 mechanisms. Clofarabine produces DNA polymerase inhibition resulting in termination of DNA chain elongation and/or DNA synthesis/repair and ribonucleotide reductase inhibition with reduction of cellular deoxynucleotide triphosphate (dNTP) pools. Clofarabine also produces disruption of mitochondrial membrane integrity with the release of cytochrome C and other proapoptotic factors leading to programmed cell death even in non-dividing lymphocytes.

Fludarabine is a water-soluble fluorinated nucleotide analogue of the antiviral agent vidarabine 9-β-D-arabinofuranosyladenine (ara-A) that is relatively resistant to deamination by adenosine deaminase. Fludarabine phosphate is rapidly dephosphorylated to 2F-ara-A which is taken up by cells and then phosphorylated intracellularly by deoxycytidine kinase to the active triphosphate, 2F-ara-ATP. This metabolite has been shown to inhibit ribonucleotide reductase, DNA polymerase α/δ and ε, DNA primase and DNA ligase thereby inhibiting DNA synthesis.

Mercaptopurine is an inactive pro-drug which acts as a purine antagonist but requires cellular uptake and intracellular anabolism to thioguanine nucleotides for cytotoxicity. The mercaptopurine metabolites inhibit de novo purine synthesis and purine nucleotide interconversions. The thioguanine nucleotides are also incorporated into nucleic acids and this contributes to the cytotoxic effects of the active substance.

Nelarabine is a pro-drug of the deoxyguanosine analogue ara-G. Accumulation of ara-GTP in leukaemic blasts allows for preferential incorporation of ara-GTP into deoxyribonucleic acid (DNA) leading to inhibition of DNA synthesis. This results in cell death. Other mechanisms may contribute to the cytotoxic effects of nelarabine. In vitro, T-cells are more sensitive than B-cells to the cytotoxic effects of nelarabine.

Tioguanine is a sulphydryl analogue of guanine and behaves as a purine antimetabolite. It is activated to its nucleotide, thioguanylic acid. Tioguanine metabolites inhibit de novo purine synthesis and purine nucleotide interconversions. Tioguanine is also incorporated into nucleic acids and DNA (deoxyribonucleic acid) incorporation is claimed to contribute to the agent’s cytotoxicity.