The next disease-modifying therapy (DMT) expected to become available in Canada is cladribine, an oral agent initially developed a decade ago (see The return of cladribine in RRMS, NeuroSens, August 23, 2017). The drug was approved by the European Medicines Agency in June, and is currently in review with Health Canada.
Cladribine is a prodrug (2-chloro-2′-deoxyadenosine) that mimics the nucleoside deoxyadenosine. It enters cells via the purine nucleoside transporter (Liliemark J. Clin Pharmacokinet 1997;32:120-131), where it accumulates intracellularly due to its resistance to adenosine deaminase. Cladribine is phosphorylated by deoxycytidine kinase (DCK) to an active triphosphate deoxynucleotide, which inhibits DNA synthesis and repair; and is metabolized by the phosphatase 5′-nucleotidase. Lymphocytes are preferentially depleted due to high levels of DCK, which activates the prodrug, and low levels of the phosphatase that inactivates the drug (Leist & Weissert. Clin Neuropharmacol 2011;34:28-35). Cell depletion is largely confined to T and B lymphocytes, with lesser effects on natural killer cells and monocytes (Salvat et al. AAN 2009; abstract P09.105).
The kinetics of lymphocyte depletion with cladribine were reported in the CLARITY phase III trial (Giovannoni et al. N Engl J Med 2010;362:416-426; see Supplementary Appendix). Oral cladribine was administered in weeks 1 and 5, and weeks 48 and 52 (total cumulative dose 3.5 mg/kg; a higher dose [5.25 mg/kg] will not be discussed). Median lymphocyte counts reached the nadir at four weeks after the last dose (week 9); the reduction from baseline was 45.8%. There was a modest recovery of lymphocyte counts (35.6% reduction) at the time of the second treatment period (week 48). After the second treatment period, median lymphocyte counts reached the nadir eight weeks after the last dose (week 60). At the end of the study, median lymphocyte counts were 43.3% below baseline. Severe lymphopenia occurred in 25.6% of subjects receiving cladribine 3.5 mg/kg. During the clinical development program, the dosing protocol was amended to initiate the drug only in patients with normal lymphocyte counts, and delay redosing in year 2 until recovery to grades 0-1 lymphopenia.
A more detailed analysis of lymphocyte kinetics with cladribine using data from the CLARITY trial has recently been published (Baker et al. Neurol Neuroimmunol Neuroinflamm 2017;4:e360). CD4+ T cell counts were reduced 45% after the first two weeks of therapy, and were maintained until the second year of treatment. The maximum depletion over the 96-week study was 60%. Depletion of CD8+ counts was less pronounced: 30% depletion after the first two weeks of treatment, and 40% after year 2 dosing. Naïve and memory T cells were similarly affected. There was also a marked depletion of B cell counts (85% reduction at nadir), with some recovery (to 30% depletion) at the time of year 2 dosing.
Baker and colleagues also compared the lymphocyte kinetics of cladribine with those of alemtuzumab, using data from the CARE-MS I trial (Baker 2017). Alemtuzumab had a more pronounced effect on CD4+ T cells (alemtuzumab >95% depletion vs. cladribine 60%). The effect on CD8+ counts was similar with the two drugs. The most significant difference was on B cell counts. The two drugs produced similar levels of B cell depletion. However, B cell recovery was more gradual with cladribine, with B cell counts remaining below baseline at the time of redosing. In contrast, with alemtuzumab, B cell counts returned to baseline within 6 months, and hyper-repopulated by 9 and 12 months.
Additional modes of action
The efficacy of cladribine in MS is believed to be due primarily to lymphocyte depletion. However, in the long-term extension of the CLARITY trial, annualized relapse rates remained low (0.15) even in subjects who initially received cladribine and were then randomized to placebo for two years (Giovannoni et al. Mult Scler 2017; epublished August 1, 2017). This may be due to a durable clinical effect, lymphocyte repopulation by less pathogenic T and B subsets, and/or effects unrelated to lymphocyte depletion.
Several other modes of action of potential usefulness in MS have been proposed for cladribine in addition to its lymphocyte-depleting effects. Cladribine has been reported to reduce the level of soluble adhesion molecules, and to block the migration of immune cells across the blood-brain barrier, which may occur in part through downregulation of matrix metalloproteinases (Mitosek-Szewczyk et al. Acta Neurol Scand 2010;122:409-413. Kopadze et al. Eur J Neurol 2009;16:409-412).
In vitro studies have suggested that cladribine induces a shift in cytokine release to a less inflammatory profile, with enhanced release of IL-4 and IL-10; no effect was seen on pro-inflammatory cytokines, such as interferon-gamma, TNF-alpha or IL-17 (Korsen et al. PLoS One 2015;10:e0129182).
An emerging area of scientific interest is epigenetic mechanisms in MS that may bridge the gap between genetic and environmental factors (for a recent review see Aslani et al. Neuromolecular Med 2017;19:11-23.) Epigenetic mechanisms (methylation, histone modification, microRNA-associated gene expression) influence gene expression without altering nucleotide sequences, which may provide a partial explanation for the discordance of MS in monozygotic twins (Xiang et al. J Autoimmun 2017; epublished April 12, 2017). Recent studies have reported epigenetic alterations at two MHC loci (HLA-DRB1, HLA-DRB5); and distinctive DNA methylation profiles in CD4+ and CD8+ T cells (Maltby et al. Clin Epigenetics 2017;9:71. Maltby et al. Clin Epigenetics 2015;7:118).
Preliminary data suggest that cladribine may alter the expression of genes involved in immune cell signalling and proliferation by indirectly inhibiting DNA methylation (Schreiner & Miravalle. J CNS Dis 2012;4:1-14; free full text at www.ncbi.nlm.nih.gov/pmc/articles/PMC3619698/pdf/jcnsd-4-2012-001.pdf). This may be relevant to efficacy, as noted above, and to safety. Hypermethylation of tumour suppressor genes is known to promote some malignancies through gene silencing. The hypomethylating effects of cladribine have been demonstrated in leukemic cell lines (Wyczechowska et al. Biochem Pharmacol 2003;65:219-225); and in breast cancer cell lines, an effect that was augmented when administered with vitamin D (Stefanska et al. Eur J Pharmacol 2010;638:47-53). Whether these effects have an impact on the drug’s malignancy potential in MS will require further investigation.
The mechanisms of action of cladribine allow for annual dosing – two weeks of treatment (weeks 1 and 5) followed by re-dosing a year later – with modest recovery of T and B cell populations during the intervening period. This is similar to the intermittent immunosuppression seen with alemtuzumab, in contrast to the continuous immunosuppression required for other disease-modifying therapies, such as teriflunomide, fingolimod and ocrelizumab. Ongoing research is investigating the profile of T and B subsets as they repopulate, and whether intermittent immunosuppression alters the pathogenicity of recovering lymphocytes, or shifts the relative proportions of immune cell subsets.
The effect of cladribine on relapse rate, MRI activity and disability progression appears to be durable, with a majority of patients not requiring additional treatment courses at four-year follow-up (Giovannoni 2017). During the CLARITY extension, in the subgroup of patients who received cladribine 3.5 mg/kg in the first two years, 75.6% remained relapse-free at long-term follow-up (up to 6.5 years from study entry).
Intermittent dosing of cladribine results in limited drug exposure, which likely contributes to the safety and tolerability profile of the drug. The terminal half-life of the drug is 5.7-19.7 hours, although clearance of intracellular metabolites may be longer (Liliemark J. Clin Pharmacokinet 1997;32:120-131). The overall incidence of infections was lower with cladribine 3.5 mg/kg than placebo (24.93 vs. 27.05 per 100 patient-years) in the integrated safety analysis (Cook et al. AAN 2017; abstract P5.394). The only infection of note was herpes zoster, which occurred at an incidence of 0.83 per 100 patient-years vs. 0.20/100 PY with placebo.
The most common adverse effects with cladribine 3.5 mg/kg during phase III testing were headache (24.2% vs. 17.2% with placebo), nasopharyngitis (14.4% vs. 12.9% with placebo), and upper respiratory tract infections (12.6% vs. 9.7% with placebo) (Giovannoni 2010). During long-term treatment (cumulative dose 7.0 mg/kg), the incidence of these adverse effects declined (Giovannoni 2017), suggesting there is no cumulative risk with continued exposure.