DNA synthesis machinery in lymphocytes

Class

Purine nucleoside analog (synthetic deoxyadenosine analog); lymphocyte-depleting immune reconstitution therapy

Pathway

Cytotoxic impairment of DNA synthesis causing selective B- and T-lymphocyte depletion

Notes

original target text: DNA synthesis machinery in lymphocytes (via intracellular phosphorylation/deoxycytidine kinase pathway)

Cladribine is a synthetic analog of the DNA building block deoxyadenosine, engineered to resist the enzyme (adenosine deaminase) that would normally break it down. Once inside a cell, it must be activated by phosphorylation, a chemical modification carried out mainly by an enzyme called deoxycytidine kinase. Lymphocytes—the B cells and T cells of the immune system—have unusually high levels of this activating enzyme relative to the enzymes that would deactivate the drug, so they accumulate the active form far more than most other cell types. The active metabolite is incorporated into DNA during synthesis and repair, causing breaks that the cell cannot fix, which triggers programmed cell death. Because resting and dividing lymphocytes alike depend on ongoing DNA repair, both populations are affected, producing a sustained reduction in circulating B and T cells. This selective depletion is the basis for using cladribine as an 'immune reconstitution' approach in autoimmune conditions such as multiple sclerosis, where pathogenic lymphocyte activity drives disease: rather than continuously suppressing immunity, the drug reduces the aberrant lymphocyte pool and allows the immune system to repopulate in a less self-reactive configuration over time.