Perspectives of vitamin C upregulating regulatory T cells in the era of thrombopoietin receptor agonists for immune thrombocytopenia

Abstract

Background Immune thrombocytopenia (ITP) is a rare disorder involving excessive peripheral destruction of platelets and inadequate bone marrow platelet production due to autoimmune reactions against megakaryocytes. Diagnosis is usually by the exclusion of primary causes; its rarity would only permit trial and error therapy experience. Historically, for ITP, oral ascorbic acid (AA) gave inconsistent results. However, unlike other nutrients, AA is found to have an exceptionally tight intestinal absorption restriction mandating intravenous delivery for any therapeutic purposes. Recently, as there is more pre-clinical evidence of AA restoring regulatory T cells (Tregs) by robust demethylation, the historical oral route was likely to be the “bottle neck” restricting effective blood levels. During this era of thrombopoietin receptor agonists (TPO-RAs), planned tapering is often required upon discontinuation but unplanned discontinuations, for example, due to side effects, are more problematic. Moreover, combinations with TPO-RAs are currently being tried using steroids and rituximab except rituximab is inappropriate during pandemics. Notably, with the indirect (but prognostically relevant) action of TPO-RAs on Tregs, boosting Treg functions by adding other Treg active agents, for example, steroids or AA, seems appropriate, especially if without added toxicity upon combining. Conclusions With updated pharmacokinetic concepts, it is timely to repeat AA clinical trials for ITP but using intravenous administration. Initially, this may be on compassionate grounds for its probable role as an adjunct to TPO-RAs for unsuccessful tapering and, especially, for all unplanned discontinuations of TPO-RAs. Such off-label usage of AA is justified because of AA's robust pre-clinical evidence on demethylation and initial clinical experience (despite the inappropriate administrative route). Moreover, moderate intravenous AA dosages have one of the best safety profiles, let alone its eminent affordability. Admittedly, after the initial clinical experience, more systematic trials on AA are required, especially for the most appropriate dosage range and its efficacy as monotherapy.