Coumarin derivatives combine 3 unfavorable properties which make them prone to potentially life threatening drug-drug interactions: (i) high protein binding; (ii) cytochrome P450 dependent metabolism; and (iii) a narrow therapeutic range. An entire list of drugs which are supposed to interact with coumarins (mostly with warfarin) comprises about 250 different compounds. Noteworthy are the interactions with cardiovascular or antilipidaemic drugs which are often coadministered with coumarins: amiodarone, propafenone and fibrates. Cardiovascular drugs which are obviously devoid or proven to be devoid of an interaction are angiotensin converting enzyme (ACE) inhibitors, calcium antagonists, beta-blockers and cardiac glycosides. There are several other drugs which enhance the hypoprothrombinaemic response to coumarins by various mechanisms: inhibitors of the elimination of the eutomer S-(-)-warfarin (e.g. miconazole, phenylbutazone), combined with protein binding displacement (e.g., sulfinpyrazone, phenylbutazone), synergistic hypoprothrombinaemia (e.g. cefazoline). Furthermore, bleeding complications may occur with drugs affecting platelet function [aspirin (acetylsalicylic acid) and several nonsteroidal anti- inflammatories (NSAIDs)]. Strong inducers of coumarin metabolism are rifampicin (rifampin) and carbamazepine. Biphasic interactions may occur where a drug first enhances the hypoprothrombinaemic response to a coumarin but has a sustained inducing effect on coumarin metabolism (e.g. phenytoin or sulfinpyrazone). The complex response of coumarins to concomitant drug therapy makes it difficult to predict the occurrence and degree of a deterioration of anticoagulant control in individual patients. For clinical practice, it seems advisable that one should monitor for changes in prothrombin time when adding or deleting any newly approved drug or any drug suspected (e.g. on the basis of this review) to cause an interaction to patients on coumarin therapy. The onset of the adverse prothrombin time response might be from between 1 to 2 days up to 3 weeks (in case of phenprocoumon) after starting a concomitant drug regimen. With amiodarone, an adverse prothrombin time response might occur up to 2 months after initiating therapy. For heparins, only a drug interaction with aspirin or nitroglycerin seems clinically relevant due to the possibility of coadministration during acute cardiac events. Both drugs are shown to enhance the activated partial thromboplastin time response to heparin

Rheumatic diseases are mostly chronic in nature and often require long term drug treatment. An increasing number of disease-modifying antirheumatic drugs (DMARDs) are used earlier in the course of disease and increasingly in combination with each other. Often there is comorbidity with ensuing pharmacotherapy, especially in the elderly, and therefore the risk of unwanted drug interactions increases. Awareness of these interactions is important in order to either avoid or manage them. Antimalarials, gold, penicillamine (D-penicillamine), sulfasalazine and azathioprine have few clinically important drug interactions. The renal excretion of the antifolate methotrexate is affected by drugs that influence kidney function. This is of particular importance in patients with compromised renal function, e.g. the elderly. Other drugs with influence on folate metabolism, such as trimethoprim, should not be given concomitantly. Cyclosporin is an agent recently introduced in rheumatological practice, and shows a myriad of clinically significant drug interactions mainly based on interference with its metabolic degradation by cytochrome P450 3A, leading to increased or decreased blood concentrations and toxicity or lack of effect. Although most of these interactions with cyclosporin are described in organ transplant patients, they may apply to rheumatological practice as well

We report three patients who had serious disturbance of anticoagulant control while receiving ifosfamide/mesna-containing cytotoxic chemotherapy in association with warfarin. In view of the likely biological potentiation of warfarin by ifosfamide/mesna, close monitoring of anticoagulant control should be performed during the co- administration of these drugs

With the use of combination chemotherapy as well as a wide range of symptomatic therapies (e.g. analgesics and antiemetics) for the treatment of patients with cancer, the field of oncology practises polypharmacy to an extreme degree. The risk for a drug interaction under these conditions is high, and the pharmacological characteristics of the anti-cancer drugs, such as steep dose-response curves, low therapeutic indices and severe toxicities, suggest that even small changes in the pharmacokinetic profile of the affected drug could significantly alter its toxicity or efficacy. In this review, drug interactions which quantitatively affect the absorption, distribution, biotransformation or excretion of the commonly used anticancer drugs are described. Most of the significant drug interactions involving this class of drugs occur at the level of biotransformation and excretion. For example, the renal excretion of methotrexate by glomerular filtration and tubular secretion is affected by a number of weak organic acids, such as probenecid, salicylates and penicillin, which compete for tubular secretion, resulting in delayed clearance of methotrexate. The best described example of an interaction at the level of biotransformation is the effect of allopurinol on the catabolism of 6-mercaptopurine. By inhibiting xanthine oxidase, allopurinol blocks the first-pass metabolism of 6-mercaptopurine following its oral administration, leading to a 4- to 5-fold increase in plasma concentrations. Known drug interactions may potentially be used to enhance the antitumour activity of a drug--for instance, the administration of tetrahydrouridine (a cytidine deaminase inhibitor) with cytarabine in an attempt to block its rapid inactivation to uridine arabinoside. Overall, little information is available concerning the pharmacokinetic interactions of anticancer drugs with each other and with other classes of drugs in man, in part because the high incidence of toxicity and treatment failure, and empirical dosing methods, obscure the recognition of possible interactions. Awareness on the part of the clinician and more extensive pharmacokinetic investigation will be needed to recognise, document and avoid potentially harmful pharmacokinetic drug interactions involving this class of drugs

Our cases confirms an interaction between warfarin and antiplastic drugs with PT prolungation. Furthermore this clinical observation underlines how this adverse effect may appear after a long period, suggesting that this hemostatic alterations is more common than previously reported.