Methotrexate and its Mode of Action

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Methotrexate and its Mode of

Action

Written by Bruce N. Cronstein, M.D.

Methotrexate remains the most commonly used

second line agent for the treatment of rheumatoid arthritis and other

inflammatory arthritides. Its use has also been

advocated for the treatment of vasculitis and other

associated inflammatory diseases as well. Despite its wide clinical use, the

mechanism of action of methotrexate remains

somewhat unclear.

Originally developed as an analogue of folic acid to compete with and

diminish the utilization of this vitamin in the de novo synthesis of the purines

and pyrimidines required for cellular

proliferation, methotrexate was first used in the

treatment of malignancies. It was assumed that methotrexate

might also suppress the proliferation of the lymphocytes involved in the

pathogenesis of RA by virtue of its folic acid antagonism; however, this

hypothesis has relatively little support.

In the treatment of inflammatory arthritis, methotrexate

is given in low doses on a weekly basis and, most commonly in the United States, with either folic or folinic acid supplementation to prevent toxicity.

Although recent studies suggest that there is a modest (20-25%) reduction in

serum purine levels in patients given a single dose

of methotrexate, this effect is only transient,

lasting less than 48 hours. [1]

The transient reduction in serum purines (and

presumed de novo purine synthesis) correlates well with the transient

antigen-stimulated lymphocyte apoptosis observed in patients treated with methotrexate. [2]

Both the methotrexate-mediated inhibition of purine synthesis and the antigen-induced apoptosis are

completely reversible by folic acid, a phenomenon clearly not observed in

patients. Moreover, it is hard to understand how one or two days of methotrexate-induced apoptosis per week might lead to the

profound antiinflammatory effects of methotrexate therapy that rapidly reverse upon

discontinuation of the drug. Moreover, other authors have reported that

intermediates in the synthesis of purines, past the

point of putative methotrexate-induced blockade,

are increased in the urine of patients taking the drug. Thus, there is little

support for the hypothesis that inhibition of de novo purine biosynthesis is

primarily responsible for the therapeutic effects of methotrexate

in the treatment of RA.

A second hypothesis regarding its mechanism of action involves the inhibition

of polyamine synthesis by methotrexate. S-Adenosyl-methionine acts as the proximal methyl donor for

many different intracellular reactions including the formation of spermine and spermidine. The

levels of these polyamines are elevated in the synovium

of patients with RA. [3]

Although not directly toxic, macrophage/monocytes

metabolize the polyamines and, in the process of metabolizing them, release

large quantities of H2O2 and other toxic oxygen

metabolites. Against this hypothesis is the presence of a large number of

cells in the rheumatoid synovium and synovial tissue that already release large quantities of

toxic oxygen metabolites (neutrophils and monocyte/macrophages). Others have suggested that the

inhibition of polyamine accumulation in lymphocytes by methotrexate

might lead to diminished rheumatoid factor secretion [4].

Moreover, an inhibitor of transmethylation

reactions was tested for its capacity to diminish inflammation in RA, and

this agent was totally ineffective. The inhibition of polyamine accumulation

by methotrexate may contribute to the antiinflammatory effects of this drug in the treatment of

RA, although the foregoing results suggest that the contribution is small.

It has clearly been established that methotrexate

is a " pro-drug. " It is taken up by cells and metabolized to

long-lived active agents, methotrexate-polyglutamates,

intracellularly. The spectrum of enzymatic

inhibition by methotrexate-polyglutamates differs

from the native compound, and one of the late enzymatic steps in purine synthesis is inhibited more efficiently (AICAR transformylation, see figure 1). The accumulation of

AICAR, observed indirectly in patients taking therapeutic doses of methotrexate [5]

and directly in animals taking intermittent, low-dose methotrexate

[6,

7], has previously been associated with an increase in cellular release

of the antiinflammatory autocoid

adenosine. Experiments in both in vitro

and in vivo systems demonstrate

that methotrexate promotes adenosine release from

cells and into inflammatory exudates [7,

8]. Moreover, these studies demonstrate that the antiinflammatory

effects of methotrexate are reversed by elimination

of adenosine. Adenosine mediates its antiinflammatory

effects via occupancy of one or more receptors; specific adenosine A2

receptor antagonists reverse the antiinflammatory

effects of methotrexate in the animal model of

acute inflammation as well. More recent studies in an animal model of

arthritis suggest greater complexity. Agents that block multiple adenosine

receptors, caffeine and theophylline, completely reverse the antiinflammatory

effects of low-dose, intermittent methotrexate

therapy in the rat adjuvant arthritis model. It is tempting to speculate that

drinking coffee and soft drinks might antagonize the therapeutic effects of methotrexate in our patients, although further studies

must be performed to prove this hypothesis.

Thus, studies in the dish, in animals, and in humans lend strong support to

the theory that adenosine, released in increased concentrations at inflamed

sites, mediates the antiinflammatory effects of

low-dose, intermittent methotrexate therapy.

Hugs,

Deanna