Clinical relevance of pharmacokinetics

New approaches to the use of pharmacokinetics in toxicology and drug development

1. The use of pharmacokinetics in toxicology, clinical pharmacology and in the individualization of dosage has been critically examined. 2. In toxicity studies, doses are given to animals with the aim of achieving substantially higher plasma levels than the therapeutic level in man. However, small animals have faster metabolic rates, shorter life spans and drug clearance is many fold faster than in man, and this difference may not be compensated for by simply mg per kg dosing. Since toxicity still occurs at these lower levels, it begs the question whether small animals require such high doses to produce toxic effects. 3. A literature survey revealed that only 5 to 31% of the papers studied attempt to relate activity with plasma levels. Examples are given of how such relationships can be used, as with D-fenfluramine, where by investigating individual responses using drug plasma levels as a probe, a greater understanding of eating disorders may be obtained. Also, with tertatolol its prolonged pharmacological activity (greater than 24 h) can be explained mathematically despite a plasma half-life of only 3 h. 4. The advantages and disadvantages of population kinetics are discussed in relation to its use in individualizing dosage, particularly in disease, its appreciation by pharmaceutical companies and regulatory authorities and the information which has been obtained so far. 5. It is of interest that one of the youngest of drug development disciplines, pharmacokinetics, is now one of the most important.

Clinical pharmacological and therapeutic considerations in general intensive care. A review

The application of clinical pharmacological concepts and therapeutic standards in intensive care settings presents particularly difficult problems due to the lack of adequately controlled background information and the highly variable and rapidly evolving clinical conditions where drugs must be administered and their impact evaluated. In this review, an attempt has been made to discuss the available knowledge within the framework of a problem-oriented approach, which appears to provide a more clinically useful insight than a drug-centred review. Following a brief discussion of the scanty data and the most interesting models to which reference can be made from a pharmacokinetic point of view (the burn patient being taken as an example), the review concentrates on the main general intervention strategies in intensive care patients. These are based mainly on non-pharmacological measures (correction of fluid and electrolyte balance, total parenteral nutrition, enteral nutrition, oxygenation and ventilatory management) and are discussed with respect to the specific challenge they present in various clinical conditions and organ failure situations. In addition, 4 major selected clinical conditions where general management criteria and careful use of prophylactic and therapeutic drug treatments must interact to cope with the variety of presentations and problems are reviewed. These include: acute cerebral damage; anti-infective prophylaxis and therapy; cardiovascular emergencies; and problems of haemostasis. Each problem is analysed in such a way as to frame the pharmacological intervention in its broader context of the underlying (established or hypothesised) pathophysiology, with special attention being paid to those methodological issues which allow an appreciation of the degree of reliability of the data and the recommendations which appear to be practiced (often haphazardly) in intensive care units. The thorough review of the published literature provided (up to mid-1986) clearly shows that in this field the quality of randomised controlled and epidemiological studies is rather unsatisfactory. It would be highly beneficial to research and to clinical care if larger multicentric protocols and prospective epidemiological comparative investigations could be carried out to investigate more timely and adequately the variables which determine drug action, and the final outcome in the many subgroups of patients which must be considered in a proper stratification of intensive care unit populations.

Clinical pharmacokinetic study of 5-FU in continuous 5-day infusions for head and neck cancer

Twenty-nine previously untreated patients with head and neck carcinoma received a total of 63 cycles of an initial chemotherapy protocol combining cis-platinum (100 mg/m2 on day 1) and continuous 5-day infusion of 5-FU (1000 mg/m2/24 h) from day 2 to day 6. This protocol was repeated on day 16 and day 31. Two daily blood samples obtained from all patients every day during 5-FU administration were analyzed by HPLC to determine the 5-FU concentrations. In the majority of cases a constant elevation was observed in total 5-FU cycle exposure (C X T) from cycle to cycle. A close relationship was demonstrated between elevated 5-FU C X T values (over 30 000 ng h ml-1) and the frequency of cycles in which signs of toxicity (myelosuppression, mucositis, diarrhea) were observed. By contrast, no obvious association was noted between response to treatment and systemic 5-FU exposure.

Quality control in antiepileptic drug monitoring in Italy

Measurement of drug concentrations in physiological fluids is increasingly becoming part of the routine management of drug therapy, thus calling for particular care to ensure reliability and avoid misguided management. As part of a long-term national project to set up a network of hospitals where clinical chemists, pharmacists, and physicians could interact and evaluate their activities in connection with therapeutic drug monitoring, the results of a quality control scheme for antiepileptic drugs, which included 50 national hospital laboratories, are described here.

Pharmacokinetics of cephalosporins in normal and septicemic rabbits

The differences in the pharmacokinetics of cefotaxime, moxalactam, and CPW 86-363, a new expanded-spectrum cephalosporin, were studied in healthy rabbits and in rabbits infected intravenously with Streptococcus pneumoniae. The pharmacokinetic analysis of concentration-time courses in the sera of infected animals according to a two compartment-model evidenced a clear decrease of drug fractions in the central compartment but enhanced drug fractions in the peripheral compartment. The shift was more pronounced in animals which received CPW 86-363 (60%; P less than 0.05) than in those which received cefotaxime (20%) or moxalactam (5%). Corresponding increases in drug concentration were observed in soft tissue interstitial fluid; therefore, the areas under the curve and mean residence times in the soft tissue interstitial fluid of infected rabbits were prolonged. The shift of drug fractions from the central compartment to other body fluid compartments during infection was thought to be due to cardiovascular changes associated with fever. No changes in serum binding of the three drugs were found during the course of the infection. The quantitative differences in the extent of altered distribution properties of the drugs might be due to variations in the physicochemical properties of the drugs.

Therapeutic drug monitoring--the need for audit?

The experience of therapeutic drug monitoring (TDM) of seven drugs as part of the normal biochemistry services over the period April 1982-83 is reported. Reagent costs alone for these assays exceeded pounds 15,000. Of 1,841 digoxin estimations, 56 per cent fell within the therapeutic range whereas 25 per cent were potentially toxic. Fifty per cent of 968 lithium measurements were lower than 4 mg L-1, placing many of these patients at risk of treatment failure. Only 29 per cent and 32 per cent of 369 theophylline and 440 phenytoin concentrations respectively were within their well-established ranges. In only around 20 per cent of requests for theophylline and phenytoin assay was sufficient clinical information supplied to the laboratory. In the majority of patients with theophylline or phenytoin concentrations outwith the therapeutic range, further analysis was not requested and so optimisation of dosage cannot be assumed to have occurred. The wide therapeutic ranges of carbamazepine and phenobarbitone ensured that most patients attained acceptable concentrations. Sodium valproate analysis was requested on 160 occasions despite the poor correlation between the concentration of this drug and its therapeutic and toxic effects. TDM can be an expensive exercise which must be subjected to rigorous cost-benefit analysis. Requests should be made on a customised drug assay form and interpretative advice on individual patient problems made available. Recommendations for the organisation, daily running and clinical supervision of TDM are made.

Clinical pharmacokinetics of cerebrospinal fluid

The distribution of drugs into the cerebrospinal fluid has long been considered a challenging field of investigation in 2 major respects: (a) understanding how the physicochemical properties (molecular weight, pKa, plasma protein binding) of various molecules influence their movements across such a specific structure as the blood-brain barrier; and (b) defining the relationship between cerebrospinal fluid concentrations of various drugs and their central (side) effects. An attempt has been made to review the very dispersed information presently available to offer a clinically orientated picture of this area of pharmacokinetics. Drugs acting on the central nervous system (benzodiazepines, tricyclic antidepressants, anticonvulsants, opioids), antibacterial agents, cardiovascular drugs (beta-adrenoceptor blockers and digoxin), antineoplastic drugs (mainly methotrexate), and other miscellaneous agents (corticosteroids, cimetidine, methylxanthines) are reviewed. The available evidence seems to support the conclusion that only for methotrexate and antibacterial agents does knowledge of cerebrospinal fluid pharmacokinetics have direct therapeutic implications, while the mosaic of information available for other drugs does little more than provide a partially satisfactory picture.