INDIVIDUAL VARIATIONS OF DRUG METABOLISM AS A FACTOR IN DRUG TOXICITY

Delayed adverse events in phase I trials of molecularly targeted and cytotoxic agents

BACKGROUND

Grade 3 and 4 adverse events (AEs) during cycle 1 are traditionally used for dose escalation decisions in Phase I oncology trials. With molecularly targeted agents (MTAs), assessment of lower grade AEs and those in later cycles is considered increasingly relevant.

METHODS

We conducted a retrospective analysis of AEs in patients enrolled onto relevant phase I trials of MTAs and cytotoxic combinations (CCs) at our UK centre between 2006 and 2016. All AEs in the first six cycles deemed at least 'possibly related' were recorded.

RESULTS

A total of 912 AEs were identified in 127 patients across 15 trials. Mean AE totals for CCs or MTAs respectively was 4.7 versus 3.0 in cycle 1, 3.8 versus 2.8 in cycles 2-6. Patients on CCs had higher mean AEs in six cycles compared to those on MTAs (8.5 vs. 5.7, p = 0.0005). For patients experiencing grade 3 AEs, 58% (CCs) and 60% (MTAs) occurred for the first time after cycle 1.

CONCLUSION

Overall AE incidence was lower in MTAs than CCs across six cycles. For MTAs, more frequent incidence of first grade 3/4 AEs after cycle 1 supports incorporation of delayed AEs into recommendations for Phase 2 dosing.

Statistical analysis of polymorphic drug metabolism data using the Rosin Rammler Sperling Weibull distribution

The Rosin Rammler Sperling Weibull distribution and its use in the analysis of complex data is explained with reference to metoprolol and acebutolol AUC values and isoniazid plasma concentrations. The technique is then applied to sparteine and debrisoquine data to resolve populations into distinct sub-groups. Goodness of fit is measured by applying the chi 2 test to the untransformed data. The method is simple to use and sub-groups can be identified rapidly. Each sub-group can be characterised by a simple exponential equation.

Absorption, distribution and residue evaluation of active substances used as feed additives

Administration of feed additives is based on the necessity of satisfying a need in human nutrition. Oral feed additive intake results in an equilibration level which is followed by a steady state after absorption. The equilibrium point of the blood (tissue) level can be calculated mathematically. 70 Per cent of the plateau of the steady state is reached within four times the half-life. Some compounds give rise to an "initial peak" with higher concentrations than the expected plateau level (phase of adaptation). For residue evaluation these effects have to be considered. The relevance of different metabolic processes and the impact on a traditional monofactorial evaluation, neglecting the incidence of combination, is mentioned. Commonly used residue assessment is discussed and the safety margin of a toxicological threshold elucidated.

N-acetylation of drugs. Observations on the properties of partially purified N-acetyltransferase from peripheral blood of rabbit

A method is described for the partial purification of N-acetyltransferase (EC 2.3.1.5) from peripheral blood of rabbit. This enzyme purified from both rapid- and slow-ioniazid-acetylator phenotype rabbits was examined with regard to stability, substrate specificity, cation effect, rate of inactivation, temperature and pH optimum. Data show that in the biochemical characteristics examined the enzyme is independent of acetylator phenotype. The possible significance of these findings to the evolutionary origin of drug-acetylating enzymes in rabbit blood and liver is discussed.

Gas chromatographic-mass fragmentographic determation of "steady-state" plasma levels of imipramine and desipramine in chronically treated patients

A sensitive and specific method for the simultaneous determination of imipramine and desipramine in the blood plasma of depressed patients under treatment that involves separation by gas chromatography and detection by mass fragmentography is described. Concentrations were determined by focusing the mass spectrometer upon the ions at m/e 280 and 235 for imipramine and 308, 236 and 114 for desipramine (N-acetyl derivative), while promazine was used as the internal standard (ions at m/e 284 and 238). Determinations are possible at levels as low as 10 ng/ml in plasma.

Metabolism, anticholinergic effects, and therapeutic outcome of desmethylimipramine in depressive illness

In order to test two theories of the mode of action of imipramine 58 depressive patients were treated at two dosage levels with desmethylimipramine (DMI), its active metabolite. Clinical judgments were used to distinguish therapeutic responders from non-responders, who showed clear differences in depressive rating scales and body weight changes. Measurements of urinary output of DMI, of salivation, and of the incidence of side-effects were compared in responders and non-responders: no differences appeared between the two groups. It is concluded that the therapeutic effect of imipramine does not depend on either its anticholinergic or other action revealed by the common side-effects, and that variability of response cannot be accounted for by differences in the rate of metabolism of DMI.

Sulphadimidine acetylation test for classification of patients as slow or rapid inactivators of isoniazid

Sulphadimidine acetylation studies were undertaken in 103 patients, 52 of whom had been classified as slow and 51 as rapid inactivators of isoniazid by a standard microbiological assay method. Each patient received sulphadimidine by mouth in a dose of 44 mg./kg. body weight, and free and total sulphadimidine were estimated in blood and urine collected at six hours. The findings suggest that patients may be classified as slow inactivators of isoniazid if the proportion of acetylated sulphadimidine (total minus free) is (a) less than 25% in blood or (b) less than 70% in urine. The sulphadimidine test is easy to perform and the result is available the same day; urine specimens for the test can be stored at room temperature for over a week without any loss of drug.

Plasma levels of imipramine and desmethylimipramine during therapy

Individual variations in drug metabolism are important factors affecting both toxicity and clinical effectiveness (Brodie, 1964; Kalow, 1965; Price Evans, 1965). In most cases the best available index of the drug concentration at the site of action is the plasma level. An estimate of this will show whether the optimal concentration is present and in some cases may indicate whether new symptoms can be attributed to toxic side reactions.