Drug interactions avoided-a useful indicator of good prescribing practice

Adherence to guidelines for avoiding drug interactions associated with warfarin--a Nationwide Swedish Register Study

PURPOSE

To investigate the extent to which clinicians avoid well-established drug-drug interactions associated with warfarin. We hypothesised that clinicians would avoid combining non-steroidal anti-inflammatory drugs (NSAIDs), tramadol and sulfamethoxazole with warfarin.

METHODS

A cross-sectional analysis of nationwide dispensing data was performed in Swedish individuals 18 years or older (n =  7,563,649). Odds ratios of interacting NSAIDs, tramadol and sulfamethoxazole versus respective prevalence of comparator drugs codeine, and ciprofloxacin in patients co-dispensed interacting warfarin versus patients unexposed was calculated.

RESULTS

The odds of receiving an interacting NSAID versus the comparator codeine was markedly lower in patients with warfarin than in the remaining population (adjusted OR 0.21; 95% CI 0.20 - 0.22). Also, the interacting drugs tramadol and sulfamethoxazole were less common among patients dispensed warfarin as compared to the remaining population, although the decrease was much more modest (adjusted OR 0.83; CI 0.80-0.87 and 0.81; CI 0.73 - 0.90).

CONCLUSIONS

In conclusion, Swedish doctors in the vast majority of cases refrain from prescribing NSAIDs to patients already on warfarin. Tramadol and sulfamethoxazole are however rarely avoided.

Occurrence of potential drug-drug interactions in nursing home residents

Objective

It has been suggested that elderly people are at increased risk of drug-related problems such as drug-induced adverse effects, drug-drug interactions and drug-disease interactions. This is particularly the case for nursing home residents because of the often complicated and multiple co-morbidity that occurs in these people. The aim of this study was to develop prescribing indicators to assess systematically the occurrence and nature of potential drug-drug interactions (DDIs) in a cohort of Dutch nursing home residents.

Method

The study was conducted in residents aged 65 years and over in six nursing homes (n=2,355, two-year study period). Computerised medication data for the residents were evaluated with respect to co-prescribing of potentially interacting drugs. All DDIs that were classified as clinically relevant according to the Dutch National Drug Interaction Database were studied. DDIs were classified into three categories according to their pharmacological mechanism: 1 — pharmacokinetic interactions at the level of gastrointestinal (GI) absorption; 2 — pharmacokinetic interactions at the level of metabolism and excretion; and 3 — pharmacodynamic interactions.

Key findings

Thirty-two per cent (n=748) of all residents were exposed to one or more combinations of drugs that could lead to clinically adverse outcomes. The numbers of residents who received drug combinations with a mechanism of interaction from category 1, 2 or 3 were 73 (3 per cent), 164 (7 per cent) and 612 (26 per cent) respectively. The number of medications prescribed was significantly associated with the occurrence of a potential DDI (P<0.05). Drugs most frequently involved were oral anticoagulants, antibiotics and theophylline.

Conclusion

During the two-year study period, about one-third of the residents were exposed to at least one drug interaction considered clinically relevant. Adequate surveillance systems are needed to enable better identification of these interactions with a view to preventing potential clinical problems. Using the prescribing indicators developed in this study, such surveillance could focus on detection and clinical aspects of potential DDIs and possible alternative treatments.

Factor analysis improves the selection of prescribing indicators

OBJECTIVE

To test a method for improving the selection of indicators of general practitioners' prescribing.

METHODS

We conducted a prescription database study including all 180 general practices in the County of Funen, Denmark, approximately 472,000 inhabitants. Principal factor analysis was used to model correlation between 19 register-based indicators for the quality of non-steroidal anti-inflammatory drug (NSAID) prescribing.

RESULTS

The correlation between indicators ranged widely from 0 to 0.93. Factor analysis revealed three dimensions of quality: (1) "Coxib preference", comprising indicators directly quantifying choice of coxibs, indicators measuring expenditure per Defined Daily Dose, and indicators taking risk aspects into account, (2) "Frequent NSAID prescribing", comprising indicators quantifying prevalence or amount of NSAID prescribing, and (3) "Diverse NSAID choice", comprising indicators focusing on the width of GPs' formularies. The number of indicators for measuring the important aspects of quality in prescribing of NSAIDs could be reduced substantially by selecting the indicator in each dimension with the highest factor loading. A high preference for coxibs indicated both appropriate and inappropriate prescribing, as revealed by the correlation of the indicators in the first factor.

CONCLUSION

Correlation and factor analysis is a feasible method that assists the selection of indicators and gives better insight into prescribing patterns.

Drug metabolism and drug interactions in the elderly

In the elderly concomitant use of several drugs (polypharmacy) is very common. Thus, the risk for drug interactions might be increased in this population. Since most drugs are hepatically eliminated by various metabolic pathways, liver function has to be considered as an additional factor modifying drug response. This chapter focuses on the hepatic mechanisms of interactions, especially on various inhibitors and inducers of the most important cytochrome P450 isoenzymes involved in drug metabolism. In addition, age-dependent changes in liver function are addressed. Based on pharmacokinetic results with different probe drugs, some inconsistencies in this area are discussed. The most important metabolic drug-drug interactions are independent of the age of the patients. However, since elderly patients consume a greater proportional share of drugs, they represent a population at risk for interactions. Awareness of this clinical problem may help to diminish those risks.

Gas6 regulates mesangial cell proliferation through Axl in experimental glomerulonephritis

Proliferation of mesangial cells is a hallmark of glomerular disease, and understanding its regulatory mechanism is clinically important. Previously, we demonstrated that the product of growth arrest-specific gene 6 (Gas6) stimulates mesangial cell proliferation through binding to its cell-surface receptor Axl in vitro. We also showed that warfarin and the extracellular domain of Axl conjugated with Fc portion of human IgG1 (Axl-Fc) inhibit mesangial cell proliferation by interfering the Gas6/Axl pathway in vitro. In the present study, therefore, we examined in vivo roles of Gas6 and Axl in an experimental model of mesangial proliferative glomerulonephritis induced by the injection of anti-Thy1.1 antibody (Thy1 GN). In Thy1 GN, expression of Gas6 and Axl was markedly increased in glomeruli, and paralleled the progression of mesangial cell proliferation. Administration of warfarin or daily injection of Axl-Fc inhibited mesangial cell proliferation, and abolished the induction of platelet-derived growth factor-B mRNA and protein in Thy1 GN. Moreover, the anti-proliferative effect of warfarin was achieved at lower concentrations than those in routine clinical use. These findings indicate that the Gas6/Axl pathway plays a key role in mesangial cell proliferation in vivo, and could be a potentially important therapeutic target for the treatment of renal disease.