Methotrexate is an antimetabolite of the enzyme dihydrofolate reductase (DHF reductase) [
1,
2]. MTX is taken up into cells by an active transport mechanism. Intracellularly it exerts its effects mainly during the "S-phase" of cell division by competitive inhibition of DHF reductase [
3]. Dihydrofolates are usually reduced by DHF reductase to tetrahydrofolates, which are required for the transfer of methyl groups, particularly in the formation of thymidine and purine bases [
1,
2,
3]. MTX thus inhibits DNA/RNA synthesis, repair, and cell proliferation. The affinity of dihydrofolate reductase for MTX is much greater than the affinity for folic or dihydrofolic acid [
3]. Actively proliferating tissues such as malignant cells, bone marrow, fetal cells, oral and intestinal mucosa, hair matrix, and the cells of the urinary bladder are generally more sensitive to the effects of MTX [
3]. Due to the non-specific mechanism of action of MTX, rapidly dividing healthy cells are usually affected. This is why the specific antidote, folinic acid, is often administered after high dose therapy, whilst folic acid is substituted after 24 hours of the application of the once-weekly low dose therapy [
3]. The primary pharmacologic action of MTX correlates with its ability of exerting cytostatic, apototic and immunosuppressive effects [
1]. However, the exact mechanism of action of low dose MTX in its immunosuppressive effect is not fully elucidated [
1,
2,
3]. It also possesses analgesic, antiphlogistic and anti-inflammatory properties [
2,
3]. It is not entirely clear whether the efficacy of MTX in the treatment of inflammatory diseases is due to its anti-inflammatory or immunosuppressive effects [
3]. Intracellularly, MTX is converted to MTX polyglutamate. MTX polyglutamate accumulates because it gets entrapped and cannot passively diffuse to the extracellular compartment [
4,
5,
6]. Intracellular accumulation of these MTX polyglutamates occurs primarily in the liver, kidney and spleen, which may persist for weeks to months [
3,
6]. MTX can however be transported outside the cell by the drug efflux transporter P-glycoprotein (P-gp) [
7,
8]. P-gp inhibitors can therefore prolong the intracellular retention of MTX [
7]. The plasma protein binding of MTX is approximately 50% [
3]. Approximately 10% of MTX is metabolized in the liver [
3]. The cytochrome P450 (CYP) system has not been shown to be involved in its metabolism. 5-20% of MTX and 1-5% of the major metabolite, 7-hydroxymethotrexate (7-OH-MTX), are eliminated through the biliary tree, with some portion undergoing significant enterohepatic recycling [
3,
4]. The elimination of MTX occurs mainly in unchanged form through the kidneys via glomerular filtration and active tubular secretion [
4,
5]. The metabolite 7-OH-MTX is 3 to 5 times less soluble than the parent compound [
3]. Although this metabolite accounts for only a small proportion of the total drug, significant accumulation may occur at high doses. Elimination is significantly delayed in impaired renal function. The terminal half-life averages 6-7 hours and shows considerable inter-individual variations (about 3-17 hours) [
3]. In patients with a third distribution space (pleural and pericardial effusion or ascites), the half-life may be prolonged up to fourfold [
3]. Very severe degree of dehydration, especially in old patients, can potentially increase the toxicity of MTX [
3,
9]. The Swiss drug information of MTX describes hematotoxicity (leukocytopenia, anemia and thrombocytopenia) as a common ADR with a prevalence of about 1% to 10% and hepatic derangement as a very common ADR with prevalence of up to 70%.
2.1. Pharmacodynamic Interactions of Methotrexate with Other Drugs
The concomitant administration of MTX and metamizole can result in a significant increase in the hematotoxic effects of both drugs in a synergistic way. Under the heading "Precautions and Warnings" of the Swiss drug information, reference is made to agranulocytosis or neutropenia caused by a mono therapy with metamizole. Agranulocytosis under metamizole is described as an allergic immune reaction [
3,
4,
5]. This adverse drug reaction (ADR) is however unrelated to the dose administered, and may occur at any time during treatment in an idiosyncratic process [
3,
4]. According to the American databanks, Micromedex™ and UpToDate™, agranulocytosis occurs very commonly during the administration of metamizole. When left untreated, agranulocytosis under metamizole can sometimes lead to fatal outcomes [
4,
5]. Drug-induced agranulocytosis was most commonly associated with metamizole in a 20-year descriptive study using data from the drug safety unit of the Dutch inspectorate for health care [
10]. In some retrospective analyses, the onset of agranulocytosis was usually unpredictable and fatal cases occurred after short-term, intermittent or long-term administration [
11,
12,
13]. In many instances, a hypersensitivity mechanism is postulated [
14]. Due to the inherent hematotoxicity of both drugs, especially in elderly patients, concomitant use of MTX and metamizole must be avoided and alternative analgesics should be considered.
The concomitant administration of drugs that cause folate deficiency such as sulfonamides, trimethoprim, pyrimethamine, triamterene, aminopterin can result in increased toxicity of MTX [
15]. Cases of severe MTX toxicity have been reported in patients receiving a combination of trimethoprim with MTX [
16,
17,
18]. Although trimethoprim exerts a relatively lower affinity for the human dihydrofolate reductase [
19], it can potentiate the hematotoxic adverse reactions of MTX, particularly in the presence of pre-existing risk factors such as folic acid deficiency, advanced age, hypoalbuminemia, renal impairment, and reduced bone marrow reserves [
3]. Furthermore, trimethoprim can increase the concentration of free MTX by about 30% and decrease its excretion by about 50% [
20]. Pyrimethamine is an antiprotozoal agent from the diaminopyrimidine group indicated for the treatment of toxoplasmosis. It interferes primarily with the folic acid metabolism of protozoan parasites by competitively inhibiting dihydrofolate reductase [
21,
22]. Although the substance has a relatively higher affinity for the parasitic enzyme than the human enzyme, it can significantly contribute in antagonizing the human enzyme when co-administered with MTX [
22].
2.2. Pharmacokinetic Interactions of Methotrexate with Other Drugs
Proton pump inhibitors (PPIs) at particularly high doses can potentially reduce the renal elimination of MTX, which can result in increased effective serum concentration with corresponding increase in the toxicity of MTX [
3]. A competition for organic anion transport proteins (OAT) in the kidney has been shown to underlie this pharmacokinetic interaction [
23]. In vitro pharmacokinetic investigations showed that the inhibition of renal tubular OAT3 by PPIs was directly responsible for the impaired renal elimination of MTX [
24,
25]. In another pharmacokinetic study, the H2-recepter blocker, famotidine demonstrated a relatively weak inhibitory effect on OAT3-mediated MTX uptake compared to that of PPIs [
26]. The clinical significance of this interaction has been shown particularly in patients taking relatively high dosages of MTX and PPIs [
27,
28,
29]. In a patient who was administered a combination of low-dose MTX with pantoprazole 20 mg/d, an inhibition of the renal elimination of the metabolite 7-OH-MTX with severe myalgia has been reported [
30]. Probenecid, nonsteroidal anti-inflammatory drugs, salicylates and some weak organic acids such as loop diuretics can potentially decrease the renal excretion of MTX by competitively inhibiting renal OAT leading to higher serum concentrations and increased hematologic toxicity of MTX [
31,
32,
33,
34,
35,
36,
37,
38]. Previous pharmacokinetic studies have demonstrated significant increase in the serum concentrations of MTX after co-administration with probenecid [
39,
40]. Non-steroidal anti-inflammatory drugs (NSAIDS) can potentially decrease the effective renal excretion of MTX by decreasing renal perfusion at the afferent arterioles and by inhibiting diverse transport proteins in the renal tubules that are directly involved in the secretion of MTX [
41,
42,
43]. In some clinical and pharmacoepidemiologic studies, diverse NSAIDs have been implicated in increasing the toxicity of MTX in patients that were otherwise methotrexate-stable [
44,
45,
46,
47]. The results from these studies need to be further validated with higher patient populations.
Penicillin antibiotics have been shown to compete with MTX for excretion sites via the organic anion transporters in the proximal renal tubules [
3,
41]. This pharmacokinetic interaction can increase the likelihood of MTX retention which can subsequently lead to its toxicity. The importance of this interaction is expected to increase with increasing penicillin or MTX dose [
3]. In pharmacokinetic studies involving laboratory animals, the concomitant administration of piperacillin and MTX resulted in significant increase in the area under the curve of MTX and its metabolite 7-OH-MTX respectively [
42,
43,
44]. In vitro and in vivo pharmacokinetic investigations carried in rhesus and cynomolgus monkeys concluded that penicillin competitively inhibits MTX uptake in the renal tubular cells resulting in reduced secretion and elimination of MTX [
45]. Clinically significant MTX-toxicities due to concomitant administration with penicillin antibiotics have been reported particularly after high dosages of both drugs [
46,
47]. The concomitant administration of cephalosporins and MTX can potentially aggravate the toxic effects of MTX. Since MTX is mainly eliminated through OAT1/3 and cephalosporins primarily through OAT3 [
48,
49], competition at the binding site of OAT3 can lead to reduced elimination and subsequent increased accumulation of MTX [
5,
41]. In a case report, two patients with joint infection developed severe neutropenia after co-administration of low-dose MTX and ceftriaxone [
50]. In relation to fluoroquinolones, several cases of acute renal failure triggered by ciprofloxacin have led to increased plasma exposure of MTX, with potential risk of increased MTX toxicity [
51,
52,
53,
54]. However, a clinically relevant interaction with MTX is not to be expected in the absence of significant renal insufficiency [
5]. In another case report, the co-administration of MTX and levofloxacin resulted in delayed MTX elimination due to competition for tubular secretion between MTX and levofloxacin [
55].