Dr Catherine Lucas

Methotrexate at low doses (5¿25¿mg/week) is first-line therapy for rheumatoid arthritis. However, there is inter- and intrapatient variability in response, with contribution of variability in concentrations of active polyglutamate metabolites, associated with clinical efficacy and toxicity. Prescribing remains heterogeneous across population groups, disease states and regimens. This review examines current knowledge of dose¿response of oral methotrexate in the setting of rheumatoid arthritis, and how this could help inform dosage regimens.

Endocannabinoid pharmacology is now relatively well understood with a number of endocannabinoids and endogenous cannabinoid neurotransmitters identified and the pharmacokinetics relatively well ascertained. Further, the cannabinoid receptors are now molecularly and pharmacologically characterised and the cell processes involved in endocannabinoid transcription, synthesis, post-translational modification and protein expression are reported. Endogenous cannabinoids have been shown to have key roles in immune and pain pathways and neuro-behavioural signalling including appetite regulation. Significant recent interest has thus been shown in understanding these pathways to guide the development of agents that inhibit the natural catabolism of endogenous cannabinoids to modify pain and appetite, and to synthesise antagonists for the treatment of disease such as obesity. This research is concurrent with the renewed clinical interest in exogenous cannabinoids and their use in disease. However, the complex pharmacology and physiological effects of exogenous cannabinoids, either as individual components or in combination, as extracts or via administration of the whole plant in humans, are less well known. Yet as with all other therapeutics, including those derived from plants, knowledge of the pharmacokinetics and dynamics of the complete plant, the individual chemical molecules and their synthetic versions, including formulations and excipients is a standard part of drug development. This article covers the key pharmacological knowledge required to guide further exploration of the toxicity and efficacy of different cannabinoids and their formulations in blinded placebo-controlled studies.

Objective: To examine the laxative prescriptions in hospital inpatients with cancer and non-cancer pain on oxycodone compared to oxycodone plus naloxone combination. Design: Retrospective case note review. Setting: A palliative care inpatient unit and a general medical ward in a large tertiary referral hospital. Participants: Eighty-four patients receiving oxycodone or combination oxycodone/naloxone on general medical (45 patients) and palliative care wards (39 patients). Main outcome measures: The primary recorded outcomes were regular opioid dose (milligrams per day) and number of prescribed laxatives (type, doses, and frequency per day). Results: Sixty-three (75%) patients in the study were on at least one laxative. In the general medicine inpatients, those on combined oxycodone/naloxone received on average 3.7 laxative doses per day compared to the oxycodone patients receiving 1.6 doses a day. In the palliative medicine population, both groups received a similar number of laxatives, despite the oxycodone/naloxone patients being on lower opioid doses. Conclusion: This retrospective study of hospital inpatients with cancer and non-cancer pain found that laxative use was not reduced in those on combined oxycodone/naloxone compared to oxycodone alone, suggesting that despite the interpretations of the clinical trials in the phase IV setting, the addition of naloxone had no effect on reducing laxative use.

It is not uncommon to be treating people with addiction who also have significant other health problems, including heart, renal or liver failure, diabetes and vascular disease. These conditions require regular medications to be taken. This can be a problem for people living with addiction and difficult social circumstances affecting compliance, among other issues. Our perspective provides a summary of general pharmacological factors affecting medicine taking in people with addiction problems, to provide a guide for hospital doctors in this setting.

Generally, licensed drug-dosing recommendations for chemotherapy are based on results from clinical trials in which subjects are usually of relatively normal body size, middle-aged, and are relatively racially homogeneous, with minimal comorbidity and specific tumor characteristics. Very few nontrial patients meet these characteristics, resulting in clinical practice having to extrapolate dosing recommendations to the specific patient. There is insufficient research on the impact of obesity-associated physiological changes prevalent in patients with common cancers on standard pharmacokinetic and pharmacodynamic parameters. Yet quantifying the influence of obesity on the pharmacology of chemotherapy is vital, as dosing inappropriate for body composition (ie, flat dosing or mg/kg based on total body weight) may increase the risk of adverse events and reduce clinical effectiveness. Unfortunately, there are few cancer guidelines to aid clinicians in selecting the optimal dose in the obese¿even recent guidelines are based predominantly on clinical opinion/current practice in treating obese patients, rather than evidence. Data in many other vulnerable groups, for example, those with significant comorbidity and older patients, are also scarce. Because of the known limitations of body surface area¿guided dosing, therapeutic drug monitoring or pharmacokinetic-guided dosing, which predicts an individual's exposure, has increasingly been shown to be a powerful tool in cancer therapy. Used appropriately, it can adjust for differences in pharmacokinetic parameters not considered when body size¿based dosing or ¿one dose fits all¿ is used. This review will focus predominantly on the rationale for pharmacokinetic-guided dosing of the newer oral molecularly targeted antineoplastics in people whose drug exposure is not predicted by their physiology or body composition.

There are a number of complex and seemingly ignored issues around prescribing safely and effectively for older people, particularly for very old women. These issues include polypharmacy, possible compliance issues and communication barriers between patient, specialists and general practitioners (GPs). There are specific pharmacokinetic (PK) and pharmacodynamic (PD) parameters that change in older age generally, and in women more specifically, which if ignored are likely to cause symptoms and to impair quality of life when drug dosage is unchanged. These changed PK and PD parameters are not all-or-nothing processes, but a continuum across age, sex and comorbidity. Very old people also have less ¿reserve¿ when drugs are used in ¿standard' doses, are more likely to have multiple concurrent therapies, and the risk of adverse effects of drugs in this group is very high. Doctors need to consider these issues when providing therapy for this group, or when trying to unravel the complex prescribing cascade here. This review outlines general principles to consider when prescribing for older people, focusing on age- and sex-related changes in both PK and PD processes.

When patients enter hospital they may have to stop smoking abruptly if the hospital has a 'no smoking' policy. Abrupt smoking cessation can affect the metabolism of drugs. Cigarette smoking induces the activity of human cytochromes P450 (CYP) 1A2 and 2B6. These enzymes metabolise several clinically important drugs, including clozapine, olanzapine and methadone. Decreased CYP1A2 activity after smoking cessation increases the risk of adverse drug reactions, with reports of increased toxicity from clozapine and olanzapine. Predicting the required dose reduction of drugs metabolised by CYP1A2 after smoking cessation is challenging. Therapeutic drug monitoring should be used when possible. Nicotine replacement therapy does not influence CYP1A2 activity.