For many decades, the folate antagonist methotrexate (MTX) has served as anchor
drug in the treatment of selected cancer types, e.g., (pediatric) leukemia, and
chronic inflammatory and joint destructive diseases like rheumatoid arthritis
(RA). MTX treatment of leukemia commonly includes high dose MTX therapy (1–10
g/m) [1], whereas RA treatment is based on low-dose MTX therapy (7.5–30
mg/wk) [2]. In both treatment modalities, therapy-induced toxicities, e.g.,
mucositis with high dose MTX and liver toxicities with long-term low dose MTX,
are antagonized by post-supplementation of folates; leucovorin (LV) after HD-MTX
and folic acid with low-dose MTX. Despite longstanding experience with various
MTX/folate supplementation schedules in clinical practice, it is still an
unresolved issue what is the most optimal schedule of MTX and folate
supplementation is in terms of dosing and timing of folate supplementation.
Furthermore, given the large variation in folic acid supplementation dosages
prescribed with MTX treatment of RA patients worldwide, awareness for folate
over-supplementation and concomitant long term adverse effects is called for.
This commentary will address this issue in light of recent insights from
laboratory, nutritional and clinical studies.
The rationale for HD-MTX and leucovorin rescue treatment is based on tumor cell
uptake of MTX via the reduced folate carrier and subsequent intracellular
conversion to MTX-polyglutamates (MTX-PGs) by the enzyme folylpolyglutamate
synthetase (FPGS) [3]. These MTX-PGs are potent inhibitors of key enzymes in
folate metabolism, including dihydrofolate reductase (DHFR), thymidylate synthase
(TS), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase (AICARFT)
and glycinamide ribonucleotide formyltransferase (GARTF), thereby impairing de
novo biosynthesis of purines and thymidylate (for DNA synthesis), amino acids and
methylation reactions. Sustained MTX inhibition of DHFR (99% for 24 h) is
considered necessary to initiate antitumor effects. It is assumed that normal
cells harbor lower FPGS activities than tumor cells and thus accumulate lower
MTX-PG levels will thus impose a less sustained DHFR block. Consequently, upon
administration of the reduced folate cofactor leucovorin the blockade of DHFR
will be reversed more easily in normal cells than in tumor cells thereby reducing
toxic effects. From this perspective, it is evident that optimal dosing of MTX
and timing of leucovorin administration after MTX dosing is crucial to obtain
highest efficacy and minimize toxicity. In most standard protocols, LV
administration is commonly initiated at the time (between 24–48 h) when plasma
levels of MTX had dropped below 1–2 M. However, to allow more
personalized approach during the course of treatment, it would be of added value
to monitor additional MTX cellular pharmacology parameters (cellular transport
capacity, MTX-PG accumulation and FPGS activity) as well as intracellular folate
status. Notably, MTX-PG analyses in leukemic blast cells revealed a large
inter-patient variability of MTX-PG accumulation during HD-MTX therapy [1]. In
addition, a recent study analyzed MTX-PG and folate levels in erythrocytes (RBC)
during the course of HD-MTX/LV treatment of leukemia patients, again
demonstrating a large interpatient variability with the standard dose of MTX [4].
Whether leukemic blast cells also harbor large inter-patient variability in
folate levels needs further exploration and thus advocate the inclusion of a
therapeutic drug monitoring arm in future HD-MTX/LV treatments to identify
whether patients are either under- or over-dosed with MTX and/or LV accounting
for differential therapy efficacies.
Dedicated studies related to MTX toxicity in mucosal tissues may also benefit
from the recent availability of mucosal organoid models [5], which may help to
optimize timing and dosing of LV after MTX administration to minimize toxicity.
In RA treatment, LV (at a dose of 2.5–5 mg/once a week) also proved to be
effective in controlling MTX toxicity without compromising MTX efficacy at MTX
dosages 15 mg and 15 mg, respectively [6]. In the same study, 1–2 mg of
folic acid (daily after MTX) demonstrated almost equal results as for LV. For
cost-effective reasons, folic acid is nowadays the preferred choice to manage MTX
toxicity. Although initially 1 mg daily dosages of FA were commonly used to
manage MTX toxicity in RA treatment, during the past 2 decades variable and
increasingly higher dosages of FA (up to 30 mg weekly [7]) have employed in
clinical practice. In some cases, these changes were indicated because of social
health care reimbursements reason, whereby (e.g., in the Netherlands) 1 mg FA
tablets for daily use were not reimbursed but 5 mg FA for once weekly use were.
Remarkably, this change in policy was not backed up with a proper clinical trial
comparing the 2 dosages for MTX efficacy, MTX toxicity and possible long term
(adverse) effects.
A systematic review by Liu et al. [8] reported that folic acid
supplementation dosages of 10 mg/week and dosages of 25 mg/week were
equally effective in reducing MTX toxicity and sustaining MTX efficacy in RA
patients. Notwithstanding this fact, the wide variation of folic acid
supplementation dosages and scheduling calls for reflection as one hallmark
folate nutritional study has provide compelling evidence that humans can only
metabolize 1 mg of folic acid/day because of the very low DHFR activity in the
liver [9]. Thus, administering dosages of 1 mg folic acid/day will result in
an increase of unmetabolized folic acid (UMFA) in the circulation. This was
clearly shown in elderly persons who were administered 5 mg folic acid daily for
3 weeks after which UMFA reached plasma levels of 15–209 nmol/L [10]. Similarly,
in a clinical RA setting, patients on low dose MTX therapy with either weekly 10
or 30 mg folic acid supplementation, serum UMFA levels were also markedly
increased (up to 300 nmol/L) after 24 weeks of therapy [7]. It is well recognized
that long-term circulating UMFA may impose a diversity of (adverse) effects,
e.g., impairing uptake of circulating 5-methyltetrahydrofolate in vascular
endothelial cells, impairing NK cell function, impairing neurodevelopment,
increasing the risk of gestational hypertension, providing a growth advantage for
premalignant cells, and epigenetic effects [11, 12, 13]. A recent prospective cohort
study of 10,000 subjects also revealed an age-dependent relationship between
UMFA serum levels and an increased risk of mortality [14].
Altogether, these observations call for awareness of potential adverse effects
associated with prolonged folic acid over-supplementation at daily dosages
exceeding 1 mg/day in MTX treatment protocols for RA, even though MTX efficacy
and toxicity may not be compromised. This notion also relates to complementary
folic acid intake via folate food fortification programs and or multi-vitamin
preparations. Like for cancer patients, there is accumulating evidence of a large
interpatient variability of blood cell accumulation of MTX-PGs of RA patients who
all received a standard dose of 15–25 mg MTX [15]. This would argue not only for
personalizing MTX therapy, but also folic acid supplementation. To this end,
emerging therapeutic drug monitoring of folates/UMFA levels in plasma and MTX-PG
and folate levels in blood cells of RA patients during the course of treatment
could further guide optimal MTX therapies, reduction of MTX toxicities and
avoidance of adverse effects initiated by folic acid over-supplementation.
Notwithstanding decades of successful clinical use of MTX in cancer and RA
treatment, deciphering the complexity of regulation of folate metabolism and MTX
cellular pharmacology in cancer cells and immune cells may further assist in
fine-tuning current MTX treatment protocols and designing novel
MTX/antifolate-based protocols. Targeting mitochondrial folate metabolism [16]
deserves future exploration as therapeutic option.
Author contributions
The author confirms sole responsibility for the following: study
conception and design, data collection, and manuscript preparation.
Ethics approval and consent to participate
Not applicable.
Acknowledgment
Not applicable.
Funding
This research received no external funding.
Conflict of interest
The author declares no conflict of interest. Gerrit Jansen is serving as
one of the Editorial Board members of this journal. We declare that
Gerrit Jansen had no involvement in the peer review of this article and
has no access to information regarding its peer review. Full
responsibility for the editorial process for this article was delegated
to Paola Perego.