The objectives were: (1) to test the association of
methotrexate (MTX) efficacy in rat
adjuvant arthritis (rat AA) with interference of
purine biosynthesis and
adenosine metabolism and (2) to test the efficacy of erythro-9-(2-hydroxynon-3-yl)
adenine (
EHNA), an inhibitor of
adenosine deaminase, and the efficacy of aminoimidazolecarboxamide (
AICA) riboside plus MTX in rat AA. Radiographic and histologic examinations of the hind limbs were measures of efficacy. Urinary excretions of AICA and
adenosine were markers of AICA ribotide
transformylase inhibition (i.e., blockage of
purine biosynthesis) and interference with
adenosine metabolism, respectively. AICA and
adenosine excretions increased during the day of MTX dosing (treatment day) compared to the previous baseline day in animals responding well to MTX (i.e., low radiographic and histologic scores). Based on radiographic and histologic scores, adjuvant injected rats were separated into two disease categories (i.e., no/mild and moderate/severe). Only AICA excretion was significantly elevated on the treatment day in rat AA with no/mild disease (i.e., those responding well to MTX
therapy). AICA (not
adenosine) excretion was significantly correlated with the above scores.
EHNA was not efficacious, even at toxic levels, while
AICA riboside potentiated the efficacy of MTX. The data suggests that efficacious MTX
therapy in rat AA (1) blocks
purine biosynthesis; (2) increases in in vivo AICA levels. Also
adenosine accumulation and blockage of
adenosine deaminase (i.e., by
EHNA) appear to be less critical to MTX efficacy. Increased levels of AICA metabolites may suppress the immune response in rat AA.