It’s MS week and so we can focus on MS
The classic way to talk about mechansisms of action in scientific publications is to say, they are “unknown” as a preamble for the work. However, they are often known but unproven. Whilst this paper does not start this way, it is hunting for the new explanations for the action of cladribine and so shifts the focus onto certain elements of the immune system and moves attention of “what is in my opinion” is more relevant.
A few years ago, we made the case that B cell subsets are probably important targets to explain a mechanism of action of cladribine. However, when cladribine was first licenced in Australia and Russia (around 2010), there was a focus on T cells, as well as B cells, without reference to the B cell subsets. Dogma at the time and later was that T cells where the important mediator and so showing inhibitory action on T cells would go some way to explaining the action of cladribine.
However, T cell depletions with cladribine are modest and generally stays within normal levels. So the mechanism of action appeared unclear. Lack of clarity, creates confusion and confusion breeds fear. If people did not understand how cladrine works, were they going to use it?
Here they look at macrophages and dendritic cells and come to the conclusion that cladribine may have actions relevant to activity that are different for the standard view of an action on lymphocytes. So more confusion. This is fine because we can see depletion of some non-lymphocytic cells in some people but on the whole I would say the level of blood macrophages don’t dip too much. However, when you go “off-piste” away from the main message it has the issue of diluting the message so that people don’t really understand what is going on. People become wary and you waste time not looking for answers in the right place.
However, in my mind it could be clearer and a simple explanation is that cladribine works because it wipes out memory B cells for a along time. This is just like a number of other MS treatments, but some may not want to contemplate this. It is better if here is distance between the actions of different drugs. However, I must say with the “imods” they do similar things as do the Bruton Tyrosine kinase inhibitors. However, I am more interested in the biology than marketing aspects.
In the study they find effects not mediated by DCK (see below) and this has been seen before
Cladribine inhibits cytokine secretion by T cells independently of deoxycytidine kinase activity.Laugel B, Borlat F, Galibert L, Vicari A, Weissert R, Chvatchko Y, Bruniquel D.J Neuroimmunol. 2011; 240-241:52-7
In this study they they looked at 0, 0.01, 0.1, 1 and 10 micromolar doses of cladribine (I will explain this in a moment but it is a measure of concenration). It was shown that 0.01micromolar did very little, 0.1micromolar was inhibitory a bit and 1 and 10 micromolar were very inhibitory and had a statistical significant effect. In lymphocytes the inhibition of proliferation at 0.1micromolar of cladribine was mediated by an enzyme called deoxycytidine kinase. (This is an enzyme that adds a phophate group onto cladribine and is the arte limiting step is created a phophorylated molecule (cladribine with three phophate groups) that causes lymphocyte killing.. However activation of the lymphocytes did not need active deoxycytidine kinase. It was found that 0.1micromolar had litle impact on dendritic cells (these activate T ells for the first time) and had marginal impact on monocycte activation.
The maximum level of cladribine in the blood is about 20-30ng/ml in human studies. To put this in context 286g cladribine/1000ml = 1 molar = 286mg/1ml. So 1 milli molar = 286microgramme/ML and 1 micromolar is 286 nanogramme/ml so 30ng/mL is about 0.1 micromolar or 100 nanomolar. In the study it took about 6 hours to drop to about 3ng/ml which is 0.01micromolar. So the elements of 1 to 10 micromolar probably were not that relevant for human studies. So they focused on 0.1 µM “to reflect the concentration that occurs” during human stidues in vivo situations”. The effects of 0.1 micromolar were small and there are subtle differences to explain what is occurring in the human body.
There it seems to work…by what ever mechanisms it is using
Fissolo N, Calvo-Barreiro L, Eixarch H, Boschert U, Espejo C, Montalban X, Comabella M. Immunomodulatory Effects Associated with Cladribine Treatment. Cells. 2021;10(12):3488.
Cladribine is a synthetic deoxyadenosine analogue with demonstrated efficacy in patients with relapsing-remitting multiple sclerosis (MS). The main mechanism of action described for cladribine is the induction of a cytotoxic effect on lymphocytes, leading to a long-term depletion of peripheral T and B cells. Besides lymphocyte toxicity, the mode of action may include immunomodulatory mechanisms affecting other cells of the immune system. In order to induce its beneficial effects, cladribine is phosphorylated inside the cell by deoxycytidine kinase (DCK) to its active form. However, the mechanism of action of cladribine may also include immunomodulatory pathways independent of DCK activation. This in vitro study was designed to explore the impact of cladribine on peripheral blood mononuclear cells (PBMC) subsets, and to assess whether the immunomodulatory mechanisms induced by cladribine depend on the activation of the molecule. To this end, we obtained PBMCs from healthy donors and MS patients and performed proliferation, apoptosis and activation assays with clinically relevant concentrations of cladribine in DCK-dependent and -independent conditions. We also evaluated the effect of cladribine on…..monocytes and dendritic cells (DCs). Cladribine decreased proliferation and increased apoptosis of lymphocyte subsets after prodrug activation via DCK. In contrast, cladribine induced a decrease in immune cell activation through both DCK-dependent and -independent pathways (not requiring prodrug activation). Regarding monocytes and DCs, cladribine induced cytotoxicity and impaired the activation of classical monocytes, but had no effect on DC maturation. Taken together, these data indicate that cladribine, in addition to its cytotoxic function, can mediate immunomodulation in different immune cell populations, by regulating their proliferation, maturation and activation.
Disclaimer. This is the thoughts of the author