Cladribine tablets are an oral treatment for adults with relapsing forms of multiple sclerosis (MS), which was approved by the European Medicines Agency in 2017 and the US Food and Drug Administration in 2019, and has now received worldwide approval in over 80 countries (1–3). Cladribine treatment is a therapy that induces transient lymphopenia when administered over a very short period, followed by longer treatment-free periods (2, 4, 5). The approved dose of cladribine tablets 10 mg (3.5 mg/kg cumulative dose over 2 years; referred to here as cladribine tablets 3.5 mg/kg) has demonstrated efficacy in patients with relapsing forms of MS, including relapsing-remitting MS (RRMS) (6–10). Cladribine (2-chlorodeoxyadenosine [2-CdA]) is a synthetic, deoxyadenosine analog; it is a prodrug that is preferentially activated in lymphocytes due to their constitutively high deoxycytidine kinase and relatively low 5’-nucleotidase levels (3). Cladribine selectively reduces circulating T and B lymphocytes, which play a central role in the pathogenesis of MS (2, 11). Consistent with the mechanism of action of cladribine, lymphopenia (Grade ≥1) has been reported in ~90% of patients treated with cladribine tablets 3.5 mg/kg over 2 years (7). Analysis of pooled data from the Phase 3 CLARITY and CLARITY Extension trials and the PREMIERE registry showed that treatment with cladribine tablets 3.5 mg/kg resulted in transient decreases in absolute lymphocyte counts (ALC), associated with a reduction in the number of CD19+ B, CD4+ T and CD8+ T lymphocyte subsets (4). This was followed by a recovery phase within weeks of nadir, wherein lymphocyte levels gradually returned to normal levels (4).

The incidence and prevalence of MS have increased over the past decade, with the peak prevalence shifting from the age of 40 years to ~60 years, due to improvements in MS diagnosis and life-expectancy (12, 13). Biological aging of the immune system, known as immunosenescence, is influenced by both genetic and environmental factors and is associated with a reduced ability to fight infections and develop immunological memory (14–17). As immunosenescence occurs with age, total lymphocyte levels are generally lower in older people compared to younger people, therefore use of disease-modifying therapies (DMTs) that further reduce lymphocyte function could potentially put older patients with MS at greater risk of adverse events (12, 15, 18). In addition, age is an important modifier of DMT efficacy; for some DMTs, slowing of MS disability progression decreases with increasing patient age (19). Opportunistic infections such as cryptococcal meningitis are common with some DMTs and this risk increases with age (12). The risk of varicella zoster virus (VZV) reactivation is higher among the elderly (20), and use of DMTs may further increase the risk of viral reactivation among older patients with MS. Low lymphocyte levels, especially in the central nervous system (CNS), are also associated with an increased risk for progressive multifocal leukoencephalopathy (PML) and aging appears to contribute to this risk (18). Given these observations, it is important to understand the immunological impact of DMTs on older versus younger patients with MS.

This post hoc analysis aimed to further explore and characterize the impact of age (≤50 years vs. >50 years) on the nature of lymphopenia experienced by patients treated with cladribine tablets 3.5 mg/kg in an integrated safety analysis.

The immune system undergoes significant remodeling during aging due to immunosenescence. Studies characterizing the impact and efficacy of DMTs in older patients with MS are limited. Results from this post hoc exploratory analysis demonstrated that cladribine tablets 3.5 mg/kg had a similar effect on ALC and lymphocyte subsets in both older and younger patients over 2 years of treatment. In both groups, ALC and levels of lymphocyte subsets decreased in the weeks following cladribine tablets 3.5 mg/kg dosing in both treatment years, and then gradually increased back to normal levels. Additionally, the incidence and duration of Gr≥3L with cladribine tablets 3.5 mg/kg was similar in both younger and older patients with MS.

Decreases in ALC and lymphocyte subsets have been observed in MS patients treated with DMTs such as interferons, dimethyl fumarate (DMF), and alemtuzumab (22). In an integrated analysis of 2470 patients with MS treated with DMF, Gr≥3L lasting ≥6 months was observed in 2.2% of patients (23). Marked reductions of lymphocyte subsets have been observed following infusion of alemtuzumab, an anti-CD52 monoclonal antibody, with a near-complete depletion of ALC, CD19+ B, CD4+ T, and CD8+ T lymphocytes observed (24, 25). Recovery to normal range after alemtuzumab infusion may take from 8 months (B lymphocytes) to nearly 3 years (T lymphocytes) (26). Lymphopenia is an anticipated effect of cladribine tablets due to its mechanism of action; however, recovery of ALC and lymphocyte subsets (CD19+ B and CD4+ T) following reduction due to cladribine tablets starts soon after nadir, reaching normal levels within 36–48 weeks of the start of Year 2 treatment; CD8+ T lymphocytes did not fall below LLN (4). The results from this post hoc analysis showed similar trends in both ALC and levels of lymphocyte subsets with recovery to normal ranges occurring by the end of the treatment year in both age groups.

A potential concern associated with lymphocyte depletion for some DMTs, especially in older patients, is an increased risk of opportunistic infections such as PML caused by the John Cunningham virus (JCV). In a multinational cohort of patients with MS, the seroprevalence of JCV increased from 49.5% in patients <30 years to 66.5% in patients over 60 years (27). It has been reported that patients with MS over 50 years of age are at greater risk for developing PML following fingolimod and DMF treatment (18). Older age at the start of natalizumab treatment may lead to a shorter time to onset of PML (28). The additive effects of immunosenescence, as well as natalizumab-induced narrowing of the T cell receptor repertoire and reduction of lymphocyte subsets, have been attributed to this shorter time to PML onset with increasing age (18). No cases of PML have been reported with cladribine tablets to the present date (3). In a prior post hoc analysis of patients treated with cladribine tablets 3.5 mg/kg in the monotherapy oral cohort (median age ~36 years), an increased frequency of infections was observed during periods of Gr≥3L; the type of infection events in patients with Gr≥3L did not differ from those occurring outside these episodes (4). This post hoc analysis explored the incidence and nature of TEAEs of viral and bacterial infections by age among patients treated with cladribine tablets 3.5 mg/kg who reported Gr≥3L. In this analysis, approximately a quarter of patients treated with cladribine tablets 3.5 mg/kg from each age group experienced at least one episode of transient Gr≥3L. Among patients with Gr≥3L, the rate of certain infection-related TEAEs was numerically higher in older versus younger patients: influenza (4.83 vs. 3.72 Adj-TEAE/100PY), upper RTI (4.60 vs. 3.36), and herpes zoster (3.37 vs. 0.76). The noticeably higher rate of herpes zoster reactivation observed in the Age >50 group is consistent with previous studies, in which viral reactivations were observed in older patients (12, 20).

The current analysis had some limitations. First, it is a post hoc analysis of data from previous Phase 3 trials that were not powered to evaluate differences between the younger and older patient groups. Second, most patients (~80%) in this study were treatment naïve prior to study enrollment, which is an unlikely scenario in older patients in the real world. As older patients with MS in the real world are likely to have received prior DMTs, it is unclear how immunosenescence and prior DMT use might impact the effect of cladribine tablets in these patients; this is a subject that requires further research. Recently published real-world studies of cladribine tablets patients (mostly non-elderly) who switched from another DMT showed that cladribine’s effect on lymphocyte changes and clinical and magnetic resonance imaging outcomes were broadly similar in those who were previously untreated compared with those previously treated with DMF, fingolimod, beta-interferons, glatiramer acetate, or teriflunomide (29, 30). Adverse events were also shown to be similar in a small group of patients treated with cladribine, ocrelizumab or rituximab after immediate prior natalizumab use. The effect of cladribine tablets after natalizumab is the subject of ongoing Phase 4 CLADRINA trial (NCT04178005). Third, the Age >50 group was notably smaller than the Age ≤50 group (185 vs. 1379); as Adj-TEAE/100PY is based on exposure years, a low sample size will lead to reduced exposure time, which will impact TEAE adjustment. Additionally, TEAE data were not adjusted for duration of a Gr≥3L episode and should be interpreted with caution. Fourth, our analysis of older patients was limited by the fact that the CLARITY study included patients only up to 65 years of age at baseline. In the general population more notable declines in immune response have been reported in people over 65 years of age and these have been associated with a higher vulnerability to infections (31, 32). Fifth, it is important to note that the term ‘immunosenescence’ refers to a quantitative and qualitative decline in immune function with age (33). With aging, there is a marked decline in the production of lymphocytes in the thymus and bone marrow (34). Additionally, qualitative impairments in lymphocytes such as a restricted receptor diversity in T and B cells, and reduced humoral responses against new antigens also occur with aging; these changes contribute to weakened immune response with age (33, 34). Lymphocytes from younger versus older individuals also show distinct gene expression signatures, such as an altered expression of chemokine and cytokine receptors (35). While this current analysis focused on the quantitative differences in lymphocytes among patients treated with cladribine tablets or placebo, qualitative differences in these lymphocytes were not measured. Finally, circulating lymphocytes constitute only ~2% of the total lymphocyte population and, therefore, may not accurately reflect changes that occur within the CNS (22).

The findings of this post hoc analysis of data from Phase 3 studies of cladribine tablets 3.5 mg/kg (CLARITY, CLARITY Extension and ORACLE-MS) demonstrate that cladribine tablets 3.5 mg/kg had a similar effect on ALC, and lymphocyte subsets (CD19+ B, CD4+ T and CD8+ T) in younger and older patients during 2 years of treatment; steady recoveries following nadir were noted in both age groups. The incidence of transient Gr≥3L following treatment with cladribine tablets 3.5 mg/kg was similar between age groups, with ~25% of patients in either group experiencing at least one episode of Gr≥3L. Among patients treated with cladribine tablets 3.5 mg/kg who experienced at least one episode of Gr≥3L, the rate of certain infection-related TEAEs was numerically higher in the older versus younger patient group.

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CLARITY (NCT00213135), CLARITY Extension (NCT00641537), and ORACLE-MS (NCT00725985) were conducted in accordance with the ethical principles of Declaration of Helsinki and the International Conference on Harmonization (ICH) Harmonized Tripartite Guidelines for Good Clinical Practice (GCP). The study protocols were approved by the institutional review boards and relevant ethics committees of participating centers. The patients/participants provided their written informed consent to participate in this study.

Conception, design, and methodology: JA, AN, AG, and CL. Acquisition of data: GG, PKC, PV, BW, and TPL. Analysis of data: JA, AN, AG, and CL. Interpretation of data: GG, PKC, PV, BW, JA, AN, AG, CL, and TPL. Writing, review, and/or revision of the manuscript: GG, PKC, PV, BW, JA, AN, AG, CL, and TPL. All authors contributed to the article and approved the submitted version.

This study and manuscript were sponsored by EMD Serono, Inc., Rockland, MA, USA, an affiliate of Merck KGaA (CrossRef Funder ID: 10.13039/100004755).

The authors declare that this study received funding from EMD Serono, Inc., Rockland, MA, USA, an affiliate of Merck KGaA (CrossRef Funder ID: 10.13039/100004755). The funder had the following involvement with the study: designing the study, collecting and analyzing the data. The authors had full control of the manuscript and provided their final approval of all content. GG has received speaker honoraria and consulting fees from Abbvie, Actelion, Atara Bio, Almirall, Bayer Schering Pharma, Biogen Idec, Celgene-BMS, FivePrime, GlaxoSmithKline, GW Pharma, Janssen, Merck & Co., Merck Healthcare KGaA, Darmstadt, Germany, Pfizer Inc, Protein Discovery Laboratories, Teva Pharmaceutical Industries Ltd, Sanofi-Genzyme, UCB, Vertex Pharmaceuticals, Ironwood, and Novartis; and research support unrelated to this study from Biogen Idec, Merck & Co., Novartis, and Ironwood. PKC declares being an advisor or consultant for Accordant, Biogen, Bristol Myers Squibb, Celgene, Genentech/Roche, Genzyme/Sanofi, GlaxoSmithKline, Janssen, Mylan, Novartis, and Viela Bio, and receiving grants for clinical research from Actelion, Alkermes, Celgene, Corrona LLC, Genentech/Roche, MedDay, NINDS, and Novartis. PV has received honoraria or consulting fees from Biogen, Sanofi-Genzyme, Novartis, Merck Healthcare KGaA, Darmstadt, Germany, Celgene, Roche, AB Science, and Imcyse; and research support from Biogen, Sanofi-Genzyme, and Merck Healthcare KGaA, Darmstadt, Germany. BW has received consulting fees from Biogen, Celgene, EMD Serono, Inc., Rockland, MA, USA, an affiliate of Merck KGaA, Novartis, and Sanofi-Genzyme. JA is an employee of EMD Serono Research & Development Institute, Inc., Billerica, MA, USA, an affiliate of Merck KGaA. AN is an employee of Merck Healthcare KGaA, Darmstadt, Germany. AG was employed by Ares Trading SA, Eysins, Switzerland, an affiliate of Merck KGaA at the time of the study. He is now a consultant for Merck Healthcare KGaA, Darmstadt, Germany. CL is an employee of EMD Inc. Mississauga, Ontario, Canada, an affiliate of Merck KGaA. TPL received consultancy fees or clinical research grants from Acorda, Bayer, Biogen, Daiichi, EMD Serono, Inc., Rockland, MA, USA, an affiliate of Merck KGaA, Darmstadt, Germany, Novartis, ONO, Pfizer and Teva Neuroscience.

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