Chemical formula: C₁₀H₁₂ClN₅O₃ Molecular mass: 285.687 g/mol PubChem compound: 20279
Cladribine is a nucleoside analogue of deoxyadenosine. A chlorine substitution in the purine ring protects cladribine from degradation by adenosine deaminase, increasing the intracellular residence time of the cladribine prodrug. Subsequent phosphorylation of cladribine to its active triphosphate form, 2-chlorodeoxyadenosine triphosphate (Cd-ATP), is particularly efficiently achieved in lymphocytes, due to their constitutively high deoxycytidine kinase (DCK) and relatively low 5'-nucleotidase (5'-NTase) levels. A high DCK to 5'-NTase ratio favours the accumulation of Cd-ATP, making lymphocytes particularly susceptible to cell death. As a result of a lower DCK/5'-NTase ratio other bone marrow derived cells are less affected than lymphocytes. DCK is the rate limiting enzyme for conversion of the cladribine prodrug into its active triphosphate form, leading to selective depletion of dividing and non-dividing T and B cells.
The primary apoptosis-inducing mechanism of action of Cd-ATP has direct and indirect actions on DNA synthesis and mitochondrial function. In dividing cells, Cd-ATP interferes with DNA synthesis via inhibition of ribonucleotide reductase and competes with deoxyadenosine triphosphate for incorporation into DNA by DNA polymerases. In resting cells cladribine causes DNA single-strand breaks, rapid nicotinamide adenine dinucleotide consumption, ATP depletion and cell death. There is evidence that cladribine can also cause direct caspase-dependent and -independent apoptosis via the release of cytochrome c and apoptosis-inducing factor into the cytosol of non-dividing cells.
MS pathology involves a complex chain of events in which different immune cell types, including autoreactive T and B cells play a key role. The mechanism by which cladribine exerts its therapeutic effects in MS is not fully elucidated but its predominant effect on B and T lymphocytes is thought to interrupt the cascade of immune events central to MS.
Variations in the expression levels of DCK and 5'-NTases between immune cell subtypes may explain differences in immune cell sensitivity to cladribine. Because of these expression levels, cells of the innate immune system are less affected than cells of the adaptive immune system.
Cladribine has been shown to exert long-lasting effects by preferentially targeting lymphocytes and the autoimmune processes involved in the pathophysiology of MS.
Across studies, the largest proportion of patients with grade 3 or 4 lymphopenia (<500 to 200 cells/mm³ or <200 cells/mm³) was seen 2 months after the first cladribine dose in each year, indicating a time gap between cladribine plasma concentrations and the maximum haematological effect.
Across clinical studies, data with the proposed cumulative dose of 3.5 mg/kg body weight show a gradual improvement in the median lymphocyte counts back to the normal range at week 84 from the first dose of cladribine (approximately 30 weeks after the last dose of cladribine). The lymphocyte counts of more than 75% of patients returned to the normal range by week 144 from the first dose of cladribine (approximately 90 weeks after the last dose of cladribine).
Treatment with oral cladribine leads to rapid reductions in circulating CD4+ and CD8+ T cells. CD8+ T cells have a less pronounced decrease and a faster recovery than CD4+ T cells, resulting in a temporarily decreased CD4 to CD8 ratio. Cladribine reduces CD19+ B cells and CD16+/CD56+ natural killer cells, which also recover faster than CD4+ T cells.
Cladribine is a prodrug that has to be phosphorylated intracellularly to become biologically active. Cladribine pharmacokinetics were studied following oral and intravenous administration in MS patients and patients with malignancies, and in in vitro systems.
Following oral administration, cladribine is rapidly absorbed. Administration of 10 mg cladribine resulted in a cladribine mean Cmax in the range of 22 to 29 ng/mL and corresponding mean AUC in the range of 80 to 101 ngh/mL (arithmetic means from various studies).
When oral cladribine was given in fasted state, median Tmax was 0.5 h (range 0.5 to 1.5 h). When administered with a high-fat meal, cladribine absorption was delayed (median Tmax 1.5 h, range 1 to 3 h) and Cmax was reduced by 29% (based on geometric mean), while AUC was unchanged. The bioavailability of 10 mg oral cladribine was approximately 40%.
Cladribine shows complete bioavailability after parenteral administration; the mean area under the plasma concentration versus time curve (AUC) is comparable after continuous or intermittent 2-hour intravenous infusion and after subcutaneous injection.
The volume of distribution is large, indicating extensive tissue distribution and intracellular uptake. Studies revealed a mean volume of distribution of cladribine in the range of 480 to 490 L. The plasma protein binding of cladribine is 20%, and independent of plasma concentration.
The distribution of cladribine across biological membranes is facilitated by various transport proteins, including ENT1, CNT3 and BCRP.
In vitro studies indicate that cladribine efflux is only minimally P-gp related. Clinically relevant interactions with inhibitors of P-gp are not expected. The potential consequences of P-gp induction on the bioavailability of cladribine have not been formally studied.
In vitro studies showed negligible transporter-mediated uptake of cladribine into human hepatocytes.
Cladribine has the potential to penetrate the blood brain barrier. A small study in cancer patients has shown a cerebrospinal fluid/plasma concentration ratio of approximately 0.25.
Cladribine and/or its phosphorylated metabolites are substantially accumulated and retained in human lymphocytes. In vitro, intra- versus extracellular accumulation ratios were found to be around 30 to 40 already 1 hour after cladribine exposure.
After subcutaneous bolus injection of a 0.14 mg/kg cladribine dose, a Cmax of 91 ng/ml is reached on average after 20 minutes only. In another study using a dose of 0.10 mg/kg body weight/day, the maximum plasma concentration Cmax after continuous intravenous infusion was 5.1 ng/ml (tmax: 12 hours) compared to 51 ng/ml after subcutaneous bolus injection (tmax: 25 minutes).
Intracellular concentration of cladribine exceeds its plasma concentration by 128 to 375 times.
The mean volume of distribution of cladribine is 9.2 l/kg. Plasma protein binding of cladribine is 25% on average, with a wide interindividual variation (5-50%).
The metabolism of cladribine was studied in MS patients following the administration of a single 10-mg tablet and a single 3-mg intravenous dose. Following both oral and intravenous administration, the parent compound cladribine was the main component present in plasma and urine. The metabolite 2-chloroadenine was a minor metabolite both in plasma and in urine, e.g. accounting only for ≤3% of plasma parent drug exposure after oral administration. Only traces of other metabolites could be found in plasma and in urine.
In hepatic in vitro systems, negligible metabolism of cladribine was observed (at least 90% was unchanged cladribine).
Cladribine is not a relevant substrate to cytochrome P450 enzymes and does not show significant potential to act as inhibitor of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4. Inhibition of these enzymes or genetic polymorphisms (e.g. CYP2D6, CYP2C9 or CYP2C19) are not expected to result in clinically significant effects on cladribine pharmacokinetics or exposure. Cladribine has no clinically meaningful inductive effect on CYP1A2, CYP2B6 and CYP3A4 enzymes.
After entering the target cells, cladribine is phosphorylated to cladribine monophosphate (Cd-AMP) by DCK (and also by deoxyguanosine kinase in the mitochondria). Cd-AMP is further phosphorylated to cladribine diphosphate (Cd-ADP) and cladribine triphosphate (Cd-ATP). The dephosphorylation and deactivation of Cd-AMP is catalysed by cytoplasmic 5'-NTase. In a study of the intracellular pharmacokinetics of Cd-AMP and Cd-ATP in patients with chronic myelogenous leukaemia, the levels of Cd-ATP were approximately half of the Cd-AMP levels.
Intracellular half-life of Cd-AMP was 15 h. Intracellular half-life of Cd-ATP was 10 h.
Based on pooled population pharmacokinetic data from various studies, the median values for elimination were 22.2 L/h for renal clearance and 23.4 L/h for non-renal clearance. Renal clearance exceeded the glomerular filtration rate, indicating active renal tubular secretion of cladribine.
The non-renal part of the elimination of cladribine (approximately 50%) consists of negligible hepatic metabolism and of extensive intracellular distribution and trapping of the active cladribine principle (Cd-ATP) within the targeted intracellular compartment (i.e. the lymphocytes) and subsequent elimination of intracellular Cd-ATP according to the life-cycle and elimination pathways of these cells.
The estimated terminal half-life for a typical patient from the population pharmacokinetic analysis is approximately 1 day. This however does not result in any drug accumulation after once daily dosing as this half-life only accounts for a small portion of the AUC.
After oral administration of cladribine across a dose range from 3 to 20 mg, Cmax and AUC increased in a dose-proportional fashion, suggesting that absorption is not affected by rate- or capacity-limited processes up to a 20 mg oral dose.
No significant accumulation of cladribine concentration in plasma has been observed after repeated dosing. There is no indication that cladribine pharmacokinetics might change in a time-dependent fashion after repeated administration.
No studies have been conducted to evaluate the pharmacokinetics of cladribine in elderly or in paediatric MS patients, or in subjects with renal or hepatic impairment.
A population kinetic analysis did not show any effect of age (range 18 to 65 years) or gender on cladribine pharmacokinetics.
Renal clearance of cladribine was shown to be dependent on creatinine clearance. Based on a population pharmacokinetic analysis including patients with normal renal function and with mild renal impairment, total clearance in patients with mild renal impairment (CLCR = 60 mL/min) is expected to decrease moderately, leading to an increase in exposure of 25%.
The role of hepatic function for the elimination of cladribine is considered negligible.
The use of cladribine in children has not been investigated.
Experience with patients older than 65 years is limited. Elderly patients should be treated by individual assessment and careful monitoring of the blood counts and of the renal and hepatic function.
A drug interaction study in MS patients showed that the bioavailability of 10 mg oral cladribine was not altered when co-administered with pantoprazole.
When cladribine was given by continuous intravenous infusion over 7 days the mean steady-state serum concentration was estimated to be 5.7 ng/ml with an estimated systemic clearance of 663.5 ml/h/kg. Accumulation of LEUSTAT over the seven day treatment period was not noted.
Plasma concentrations are reported to decline multi-exponentially after intravenous infusions with terminal half-lives ranging from approximately 3-22 hours. In general, the apparent volume of distribution of cladribine is very large (mean approximately 9l/kg), indicating an extensive distribution of cladribine in body tissues. The mean half-life of cladribine in leukaemic cells has been reported to be 23 hours.
There is little information available on the metabolism or route of excretion of cladribine in man. An average of 18% of the administered dose has been reported to be excreted in urine of patients with solid tumours during a 5-day continuous intravenous infusion of 3.5-8.1 mg/m²/day of cladribine. The effect of renal and hepatic impairment on the elimination of cladribine has not been investigated in humans.
Cladribine penetrates into cerebrospinal fluid. One report indicates that concentrations are approximately 25% of those in plasma.
Cladribine is bound approximately 20% to plasma proteins.
Non-clinical safety pharmacological and toxicological assessment of cladribine in animal models relevant for the safety assessment of cladribine did not yield significant findings other than those predicted by the pharmacologic mechanism of cladribine. The primary target organs identified in the repeat-dose toxicology studies by parenteral routes (intravenous or subcutaneous) up to 1-year duration in mice and monkeys were the lymphoid and haematopoietic system. Other target organs after longer administration (14 cycles) of cladribine to monkeys by subcutaneous route were the kidneys (karyomegaly of renal tubular epithelium), adrenals (cortex atrophy and decreased vacuolation), gastrointestinal tract (mucosa atrophy) and testes. Effects on the kidneys were also seen in mice.
Cladribine is incorporated into DNA strands and inhibits DNA synthesis and repair. Cladribine did not induce gene mutation in bacteria or mammalian cells, but it was clastogenic causing chromosomal damage in mammalian cells in vitro at a concentration which was 17-fold above the expected clinical Cmax. In vivo clastogenicity in mice was detected at 10 mg/kg, which was the lowest dose tested.
The carcinogenic potential of cladribine was assessed in a long-term 22-month study with subcutaneous administration in mice and in a short-term 26-week study by oral route in transgenic mice.
Cladribine was also assessed in a 1-year monkey study by the subcutaneous route. No increased incidence in lymphoproliferative disorders and no tumours were seen in this study.
Although cladribine may have a potential for genotoxicity, long-term data in mice and monkeys did not provide any evidence of a relevant increased carcinogenicity risk in humans.
While there were no effects on female fertility, reproductive function or general performance of offspring, cladribine was shown to be embryolethal when administered to pregnant mice, and the compound was teratogenic in mice (also following treatment of the males only) and rabbits. The observed embryolethal and teratogenic effects are consistent with the pharmacologic mechanisms of cladribine. In a male mouse fertility study, malformed foetuses with agenesis of portions of appendage(s) distal the humerus and/or femur were seen. The incidence of affected mouse foetuses in this study was in the same range of spontaneous incidence of amelia and phocomelia in this strain of mice. However, considering cladribine genotoxicity, male-mediated effects related to potential genetic alteration of differentiating sperm cells cannot be excluded.
Cladribine did not affect the fertility of male mice, but observed testicular effects were reduced testicular weights and increased numbers of non-motile sperm. Testicular degeneration and reversible decrease in spermatozoa with rapid progressive motility were also seen in the monkey. Histologically, testicular degeneration was only seen in one male monkey in a 1-year subcutaneous toxicity study.
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