CLINICAL PRACTICE: Prescribing series Which drug class and why? Sepehr Shakib, MBBS, FRACP, is Clinical Lecturer, Department of Clinical Pharmacology, Royal Adelaide Hospital, and Clinical Lecturer, Clinical and Experimental Pharmacology, University of Adelaide, South Australia. Alison George, MBBS, FRACGP, DipObs, is a general practitioner, Glenunga, South Australia. This is the fourth article in the series on general practice prescribing. This article focusses on the choice of drug class. BACKGROUND Having made the diagnosis, decided on therapeutic goals and the therapeutic approach, the next step is choice of drug class. OBJECTIVE This article discusses the four factors to be considered when choosing an appropriate drug class: efficacy, safety, suitability and cost. DISCUSSION The first consideration in choosing a drug class is the relative efficacy compared with other agents for the particular therapeutic goal. Safety is a broad issue involving adverse reactions, toxicity, tolerance and dependence, teratogenicity, and consideration of special at risk populations. Suitability involves consideration of contraindications to the medication as well as other factors such as the requirements for monitoring, drug formulation and the number of daily doses. Cost includes consideration of drug acquisition as well as the total cost of therapy, including by whom the cost is borne. o far in this prescribing series we have hitting a cyclist is okay, but when you Sdiscussed the issues of therapeutic have children, you go for the Volvo 4WD Case history goals and therapeutic approaches. Now station wagon with extra heavy duty side we get to the juicy bit of how to choose impact protection and 22 front, side, rear, Harold is an active 79 year old man between different drugs to prescribe. In top, and bottom air bags! You then have who lives with his wife. He has had hypertension for years and has been this issue we will cover the choice to think about budgetary constraints, taking a beta blocker. He presents between different drug classes, and in the obviously. There is also another issue to you complaining of transient next issue of Australian Family Physician, though when choosing cars, which I can weakness of his left leg, which you different drugs within a class. The choice only describe as personality: certain cars diagnose as a transient ischemic in both cases is based on four factors: effi- are made for some and obviously not attack (TIA); you also find him to be in slow atrial fibrillation. You organise a cacy, safety, suitability and cost, and we others. Some people I know just wouldn’t CT head scan that shows he has had will be using different examples in each drive that Volvo even if it was given to previous undiagnosed strokes. issue to illustrate the case. them for free! Imagine yourself buying a car: it has to Well, the issue is much the same for get you from point A to B, and it can do drugs. Look at the case of Harold. The alone helps prevent strokes). What would that with varying degrees of efficacy in therapeutic goal in treating Harold’s you prescribe to prevent a stroke: aspirin, terms of the reliability of the car, the atrial fibrillation would be to improve the warfarin or other? amount of acceleration it has, its respon- symptoms it is giving him, and to prevent siveness, handling and fuel efficiency. complications such as stroke. The thera- Efficacy You also want the car to be safe. At dif- peutic approach would be to either aim First let’s consider the issue of efficacy: in ferent stages in your life, the importance for rate control and anticoagulation or metaanalysis of clinical trials, warfarin is of safety will differ: when you are young a rhythm control and anticoagulation associated with an approximate 70% risk sports car that comes off second best (there is no evidence that rhythm control reduction of the incidence of stroke, com- Reprinted from Australian Family Physician Vol. 32, No. 5, May 2003 • 325 n Which drug class and why? pared to only approximately 20% with cious, it cannot be said that they are all farin, and does not have a history of falls, aspirin.1 In Harold’s case his yearly risk of safe (despite the claims made for some warfarin would be suitable for him. Other a stroke without treatment would be pharmaceuticals by their manufacturers). pertinent issues to consider would be approximately 8%,2,3 hence with warfarin All drugs have some element of toxicity. regarding transport, support from his wife, his risk of stroke would go down to Most prescribers would be familiar with and his ability to alter warfarin doses approximately 2.4% (5.6% yearly predictable or idiosyncratic adverse drug according to telephone advice. absolute risk reduction) and with aspirin reactions. Other aspects of safety to con- The suitability of a medication is his risk would still be approximately 6.4% sider are: usually thought of as contraindications (only 1.6% absolute risk reduction). • the potential for chronic toxicity that a patient has to it, eg. history of falls For drugs to be approved by the (eg. pulmonary fibrosis with amiodarone, with warfarin. It is not just about con- Therapeutics Goods Administration nitrofurantoin or methotrexate) traindications, however, and is a much (TGA), they have to have proven effi- • acute toxicity (eg. digoxin, lithium, broader issue when selecting between cacy. The important issue is really relative theophylline) different drug classes. It may also have efficacy compared to the use of other • issues of dependence and withdrawal, to do with the requirements for addi- agents. In the example of Harold, antico- and tional investigations associated with the agulation with warfarin is clearly more • the possibility of teratogenicity. use of a particular drug such as elec- efficacious in preventing strokes than One also needs to consider whether trolyte monitoring with loop diuretics, aspirin, but this is not the only issue. there are particular at risk populations serum concentrations with digoxin, who are more likely to suffer from the phenytoin, perhexiline (especially in a Safety drug’s toxicity (Table 2). patient with needle phobia). Other issues A common reason why clinicians do not are the formulation of the medication, anticoagulate patients such as Harold is Suitability the number of daily doses and the pack- concerns about bleeding.4–6 The bleeding Next let’s consider the suitability of differ- aging of the medication, such as in the risk associated with aspirin use is approxi- ent forms of anticoagulation for Harold. case of paediatric antibiotic syrups mately 0.5–1.0% per year. Metaanalysis Aspirin comes as an easy to administer where the taste can be an important of trials demonstrate there is an approxi- once per day tablet, and even if he forgets issue! Suitability may also have to do mate 0.45% gastrointestinal,7 and 0.04% to take it for a day, because of its irre- with whether the medication comes in a intracerebral8 risk of bleeding per year. versible platelet inhibition, there is no loss ‘one size fits all’ dose, or whether it has There is evidence from cohort studies to of efficacy. Warfarin on the other hand, to be carefully titrated to effect. suggest the absolute rate of bleeding is requires regular monitoring as well as Suitability also has a psychological com- greater in elderly patients.9 There is some greater patient education regarding the ponent such as in the case of cancer evidence of a higher incidence of bleeding importance of compliance and drug and patients who refuse to take morphine at higher doses,10,11 but it is clear that com- food interactions. Given that Harold is because they feel that it represents plications do occur at a substantial rate at active, has no contraindications to war- giving up, or decisions regarding drug doses of 100–150 mg per day. The annual bleeding rate with war- Table 1. Five point warfarin bleeding index14 farin is considerably higher than with aspirin. The average risk of major haem- Score 1 point each for orrhage is estimated to be approximately • age >65 3% per year,12 but this varies with age, • history of gastrointestinal bleeding blood pressure, likelihood of falls, and • history of stroke excessive anticoagulation or factors pre- • any of: diabetes, recent myocardial infarction, haematocrit <30%, creatinine >.12 mmol/L disposing to it such as confusion, dementia, etc.13 A number of scoring tools Risk of bleeding in patients on warfarin treatment have been developed to help predict the Time after Low risk Intermediate risk High risk risk of haemorrhage in patients taking commencement (0 points) (1–2 points) (3–4 points) warfarin in whom absolute contraindica- 6 months 3% 8% 16% tions such as falls or alcohol abuse do not 12 months 3% 8% 30% apply (Table 1).14,15 48 months 3% 12% 53% Although it can be said that all drugs now approved by the TGA are effica- 326 • Reprinted from Australian Family Physician Vol. 32, No. 5, May 2003 Which drug class and why? n therapy being influenced by previous Table 2. At risk populations for particular medications favourable or unfavourable experiences with drugs or diseases, eg. family Medication Adverse reaction At risk group members who have had intracerebral ACE inhibitor, Acute renal impairment Elderly angiotensin II Dehydration or high diuretics dose haemorrhage from being prescribed war- antagonists Na+<130 mmol/L farin, or strokes from not being Pre-existing renal disease anticoagulated for atrial fibrillation. Taking NSAIDs or COX-II inhibitors Sulphonylureas, Hypoglycaemia Living alone Cost insulin Cognitive impairment The cost of a medication warrants an Metformin Lactic acidosis Cardiac failure, hepatic failure, entire series of articles to itself.