indication

For the treatment of pediatric patients 1 to 21 years old with relapsed or refractory acute lymphocytic (lymphoblastic) leukemia after at least two prior regimens. It is designated as an orphan drug by the FDA for this use.

pharmacology

Clofarabine is a purine nucleoside antimetabolite that differs from other puring nucleoside analogs by the presence of a chlorine in the purine ring and a flourine in the ribose moiety. Clofarabine seems to interfere with the growth of cancer cells, which are eventually destroyed. Since the growth of normal body cells may also be affected by clofarabine, other effects also occur. Clofarabine prevents cells from making DNA and RNA by interfering with the synthesis of nucleic acids, thus stopping the growth of cancer cells.

mechanism of action

Clofarabine is metabolized intracellularly to the active 5'-monophosphate metabolite by deoxycytidine kinase and 5'-triphosphate metabolite by mono- and di-phospho-kinases. This metabolite inhibits DNA synthesis through an inhibitory action on ribonucleotide reductase, and by terminating DNA chain elongation and inhibiting repair through competitive inhibition of DNA polymerases. This leads to the depletion of the intracellular deoxynucleotide triphosphate pool and the self-potentiation of clofarabine triphosphate incorporation into DNA, thereby intensifying the effectiveness of DNA synthesis inhibition. The affinity of clofarabine triphosphate for these enzymes is similar to or greater than that of deoxyadenosine triphosphate. In preclinical models, clofarabine has demonstrated the ability to inhibit DNA repair by incorporation into the DNA chain during the repair process. Clofarabine 5'-triphosphate also disrupts the integrity of mitochondrial membrane, leading to the release of the pro-apoptotic mitochondrial proteins, cytochrome C and apoptosis-inducing factor, leading to programmed cell death.

toxicity

There were no known overdoses of clofarabine. The highest daily dose administered to a human to date (on a mg/m² basis) has been 70 mg/m²/day × 5 days (2 pediatric ALL patients). The toxicities included in these 2 patients included grade 4 hyperbilirubinemia, grade 2 and 3 vomiting, and grade 3 maculopapular rash.

biotransformation

Clofarabine is sequentially metabolized intracellularly to the 5’-monophosphate metabolite by deoxycytidine kinase and mono- and di-phosphokinases to the active 5’-triphosphate metabolite. Clofarabine has high affinity for the activating phosphorylating enzyme, deoxycytidine kinase, equal to or greater than that of the natural substrate, deoxycytidine.

half life

The terminal half-life is estimated to be 5.2 hours.

route of elimination

Based on 24-hour urine collections in the pediatric studies, 49 - 60% of the dose is excreted in the urine unchanged.

drug interactions

Leflunomide: Immunosuppressants such as clofarabine may enhance the adverse/toxic effect of leflunomide. Specifically, the risk for hematologic toxicity such as pancytopenia, agranulocytosis, and/or thrombocytopenia may be increased. Consider eliminating the use of a leflunomide loading dose in patients who are receiving other immunosuppressants in order to reduce the risk for serious adverse events such as hematologic toxicity. Also, patients receiving both leflunomide and another immunosuppressive medication should be monitored for bone marrow suppression at least monthly throughout the duration of concurrent therapy.

Natalizumab: Immunosuppressants such as clofarabine may enhance the adverse/toxic effect of natalizumab. Specifically, the risk of concurrent infection may be increased. Patients receiving natalizumab should not use concurrent immunosuppressants, and patients receiving chronic corticosteroids prior to natalizumab should be tapered off of steroids prior to starting natalizumab.

Pimecrolimus: Pimecrolimus may enhance the adverse/toxic effect of immunosuppressants such as clofarabine. Avoid use of pimecrolimus cream in patients receiving immunosuppressants.

Tacrolimus: Tacrolimus (topical) may enhance the adverse/toxic effect of immunosuppressants such as clofarabine. Avoid use of tacrolimus ointment in patients receiving immunosuppressants.

Trastuzumab: Trastuzumab may increase the risk of neutropenia and anemia. Monitor closely for signs and symptoms of adverse events.