Methotrexate

Question 1

Mechanism of action

Answer 1

inhibits dihydrofolate reductase
(Enzyme essential for the synthesis of purines and pyrimidines)

Question 2

Indications for the use of methotrexate

Answer 2

inflammatory arthritis, especially rheumatoid arthritis
psoriasis
some chemotherapy - e.g. acute lymphoblastic leukaemia

Question 3

Adverse effects

Answer 3

mucositis
myelosuppression
pneumonitis - non-productive cough, dyspnoea, malaise, fever
pulmonary fibrosis
liver fibrosis

Question 4

When do patients normally develop methotrexate pneumonitis in relation to starting treatment?

Answer 4

within a year of starting treatment

Question 5

For how long after stopping methotrexate should contraceptives still be used to prevent teratogenic side effects?

Answer 5

6 months after treatment has stopped
(For both women and men)

Question 6

How often is methotrexate taken?

Answer 6

WEEKLY

Question 7

Monitoring required whilst on methotrexate

Answer 7

FBC, U&E and LFTs - before starting and repeated weekly until therapy stabilised, THEN every 2-3 months

Question 8

What should always be co-prescribed with methotrexate

Answer 8

Folic acid (taken on days that patient does NOT take methotrexate)

Question 9

Normal starting dose of methotrexate

Answer 9

7.5 mg weekly

Question 10

Drug interactions with methotrexate

Answer 10

Trimethoprim/ co-trimoxazole - increases risk of marrow aplasia
high-dose aspirin - methotrexate toxicity secondary to reduced excretion

Question 11

Treatment of choice for methotrexate toxicity

Answer 11

Folinic acid