What possible interactions are there between methotrexate (MTX) and different antibiotics (pivmecil

Fråga: What possible interactions are there between methotrexate (MTX) and different antibiotics (pivmecillinam, doxycycline, norfloxacin, trimethoprim). A female patient with psoriasis is treated with methotrexate once a week. She has now developed cystitis and also a suspected chlamydia infection. What antibiotic is most suitable in this case?

Sammanfattning: Considering the possible risk of interaction between MTX and trimethoprim, norfloxacin and pivmecillinam and the possibility of a coexisting chlamydia infection, doxycycline is probably the best alternative.

Svar: This question has in part been answered in Drugline (1,2).

Trimethoprim: Several case reports suggest an increased risk of pancytopenia in patients treated with both MTX and trimethoprim (3). This can be mediated via pharmacokinetic mechanisms (trimethoprim reduces the tubular secretion of MTX) as well as through effects on folate metabolism (1,3). However, no causal relationship has been established.

Norfloxacin and pivmecillinam: No reports on possible interactions between MTX and either antibiotic could be found. However, both norflaxacin and mecillinam are weak organic acids and, as such, have the potential to inhibit the tubular secretion of MTX (3).

Doxycycline: No data on a possible interaction with MTX could be retrieved. In one study (4), doxycycline had no influence on the the pharmacokinetics of MTX in rabbits. 1 Drugline nr 11323 (year 1994) (enclosed) 2 Drugline nr 08869 (year 1992) (enclosed) 3 FASS 1996; page 1179 4 Iven H, Brasch H: Influence of the antibiotics piperacillin, doxycycline, and tobramycin on the pharmacokinetics of methotrexate in rabbits. Cancer Chemother Pharmacol 1986; 17: 218-222