Background The elderly are a high risk population, especially because they are often receiving polytherapy. The use of two or more drugs increases the risk of drug-drug interactions that can easily cause an adverse drug reaction. Pharmacokinetic interactions concern absorption, distribution, metabolism and elimination of drugs: the most common site of interaction is hepatic metabolism and the various subtypes of cytochrome P450.

Purpose To analyse the number of prescriptions containing possible pharmacokinetic interactions. The prescriptions were verified for both hepatic metabolism and P-glycoprotein (PgP, or MDR1) interactions.

Material and methods We evaluated discharge prescriptions from the medical area (cardiology, rehabilitation, neurology and medicine) from 1 January 2014 to 30 June 2014. We used two websites to check the cytochrome P450 isozymes responsible for drug metabolism and its possible induction/inhibition. The same websites gave us information about possible interactions mediated by the PgP.

Results We analysed 833 discharge prescriptions, 176 of which contained theoretical drug-drug interactions (21.13%). 55.68% of these prescriptions came from the cardiology unit (98 of 176). This unit prescribed 45 times (15.05% of 299 cardiology prescriptions) clopidogrel with pantoprazole: this proton pump inhibitor reduces the concentration of the active metabolite of clopidogrel by 20% through inhibition of CYP2C19. Digoxin and warfarin are drugs with a low therapeutic index. Physicians showed good prescribing behaviour: 11.93% (21) of 176 prescriptions contained possible interactions, respectively, 14 and 7. A risky interaction occurred between warfarin and prednisone that enhanced CYP3A4 action, reducing warfarin blood levels. 29 prescriptions (3.48%) included possible interactions at the PgP level with various active principles, such as spironolactone, atorvastatin, prednisone and amiodarone. 19 (65.52%, 19 of 29) interactions concerned atorvastatin. Analysing the prescriptions of the medicine unit (20), 11 (55%) contained quinolones and glucocorticosteroids:

co-administration of these two drugs may increase the risk of tendon rupture.

Conclusion This study focused on the pharmacokinetic interactions evaluable at discharge of patients. It intended to check if physicians are aware of the pharmacokinetic interactions by analysing their discharge prescriptions, and by evaluating the most common interactions.

References and/or Acknowledgements

1. http://reference.medscape.com/drug-interactionchecker

2. http://www.codifa.it/cont/codifa-interazioni-multiple/74/trova-interazioni-multiple.asp

References and/or AcknowledgementsNo conflict of interest.