David W. Hobson

Stagewise, adaptive dose allocation for quantal response dose-response studies

A principal design objective of many dose-response studies is to estimate extreme percentiles of a dose-response distribution, e.g., the ED95 dose for a particular drug therapy, as precisely as feasible using the smallest number of experimental subjects possible. Such a design requirement necessitates that allocation of subjects to drug doses be carried out in a stagewise fashion to maximize the information obtained from each subsequent experimental observation in light of what has previously been determined. This paper describes and illustrates specialized methods and associated computer programs to evaluate, on a stagewise basis, the anticipated relative sensitivities of alternative experimental plans in the case of dichotomous responses. Following each stage of experimentation, the current estimates of the dose-response distribution parameters, as well as the uncertainties in these estimates, are updated and are used to assign subjects to experimental dose levels for the next stage of testing. Competing dose allocations are compared with respect to anticipated improvement in estimation precision. The adoption of such a stagewise dose allocation strategy is illustrated by example.

Stagewise, group sequential experimental designs for quantal responses. one-sample and two-sample comparisons.

The use of stagewise, group sequential experimental designs with dichotomous responses in toxicity or drug screening programs is discussed. Such designs represent a compromise between the standard, fixed sample size designs and fully sequential designs. Stagewise group sequential designs place specified numbers of animals on test at each stage, up to a maximum number of stages. The greatest increases in sample size efficiency occur with small numbers of stages, particularly when going from one stage to two. Two-stage designs can result in a 15 to 20 percent reduction in average sample size. Five-stage designs can result in a 30 to 40 percent reduction in average sample size, with no appreciable decrease in Type 1 error or power. Examples of the efficiencies that arose in actual screening programs are given. This paper demonstrates that the routine use of stagewise, group sequential designs in standardized screening protocols can result in substantial savings in animal use with virtually no sacrifice of statistical sensitivity.

Statistical Analysis of Dose-Response Experiments by Maximum Likelihood Analysis and Iteratively Reweighted Nonlinear Least Squares Regression Techniques:

Dose-response studies often form integral parts of pharmacological investigations of drug activity and efficacy and of toxicological investigations of drug and chemical safety. Standardized dose-response study protocols, statistical models, model fitting techniques, and computer programs are widely available for such applications. Many studies however, require nonstandard models and model fitting procedures to adequately describe the resulting data. Maximum likelihood analysis can accommodate a wide variety of model structures in a unified manner. This presentation illustrates how general purpose nonlinear regression analysis routines, such as those that are available in SAS or in BMDP, can be used to obtain maximum likelihood model solutions and associated error analyses for nonstandard model fitting situations. This reduces the need for special purpose computer programs for individual modeling applications. Methodological considerations in the application of nonlinear regression modeling procedures to maximum likelihood estimation are discussed. The methodology is illustrated with several modeling situations.

The metabolism of n-octane in Fischer 344 rats

The urinary metabolites of n-octane in Fischer 344 rats given the hydrocarbon by gavage included 2-octanol, 3-octanol, 5-oxohexanoic acid, and 6-oxoheptanoic acid. The sex of the animals influenced the relative amounts of metabolites formed. Analyses were performed by gas-liquid chromatography (GC) and gas-liquid chromatography/mass spectrometry (GC/MS). This is the first reported finding of keto acids in hydrocarbon oxidative metabolism. No kidney damage was found as a result of n-octane dosing although the 2,2,4-trimethylpentane (iso-octane) isomer does cause kidney lesions in male rats.

Evaluation of compounds as barriers to dermal penetration of organophosphates using acetylcholinesterase inhibition.

An efficient, objective method for evaluating the efficacy of barrier compounds in preventing dermal penetration of organophosphates (OP) in rabbits was developed using time-dependent reduction in erythrocyte (RBC) acetylcholinesterase (AChE) activity as an endpoint. Anesthetized rabbits, with or without a dermal application of a mixture of high- and low-molecular-weight polyethylene glycols (mean molecular weight of 540 daltons; PEG 540), were exposed to different percutaneous doses of 3 highly toxic OP compounds. Dose-response curves were generated for RBC AChE inhibition as a function of percutaneous dose for each OP test material over time. From data generated, a single dose of each OP was selected to challenge PEG-540-protected and unprotected animals to validate the method as a means of differentiating effective from ineffective barriers to skin penetration. Data for a complete evaluation of a PEG 540 test barrier application were obtained within 4 h and anesthesia was maintained for the entire period.

Metabolism and nephrotoxicity of tetralin in male Fischer 344 rats

Tetralin, a component of fuels, solvents, and varnishes, is metabolized in male Fischer 344 rats to 1-tetralol, 2-tetralol, 2-hydroxyl-1-tetralone, 4-hydroxyl-1-tetralone, 1,2-tetralindiol, and 1,4-tetralindiol. Rats treated with tetralin demonstrated the classic lesions of hydrocarbon-induced nephropathy.

The metabolism of 2,3,4-trimethylpentane in male fischer-344 rats

The urinary metabolites of the potent nephrotoxic hydrocarbon 2,3,4-trimethylpentane (2,3,4-TMP) given Fischer-344 male rats by gavage included 1-hydroxy-2,3,4-trimethylpentane, 2,3,4-trimethyl-1-pentanoic acid and 2,3,4-trimethyl-5-hydroxy-1-pentanoic acid. Analyses were performed by gas-liquid chromatography (GC) and gas-liquid chromatography/mass spectrometry (GC/MS). A comparison of the urinary metabolites of 2,3,4-TMP with those of 2,2,4-trimethylpentane (2,2,4-TMP) showed that more monocarboxylic acid was produced with 2,3,4-TMP.

Acetylcholinesterase Inhibition Measurements for the Evaluation of Decontaminant Efficacy Following Percutaneous Organophosphorus Compound Exposure

A rapid and sensitive in vivo method for the evaluation of skin decontaminant efficacy following percutaneous exposure to organophosphonates (OPs) was developed using erythrocyte acetylcholinesterase (AChE; EC 3.1.1.7) inhibition in the rabbit as an end point. The level of AChE inhibition was evaluated for use as a more humane means of assessing skin decontaminant efficacy than lethality-based methods. Groups of anesthetized animals were exposed percutaneously to either of two highly toxic OPs [thickened soman (TGD) or VX], and 2 min later were treated with a known effective skin decontaminant or were untreated (negative control). Blood samples were drawn and assayed for AChE activity 5 min before TGD or VX exposure and at 30, 60, and 120 min after exposure. Percent AChE inhibition relative to preexposure levels was calculated at each postexposure time for each animal in control and treatment groups. Efficacy data based on percent AChE inhibition were compared with results from previous efficacy studies p...

Metabolism and nephrotoxicity of indan in male Fischer 344 rats

Indan, a component of fuels, solvents, and varnishes, is metabolized in male Fischer 344 rats to 1-indanol, 2-indanol, 5-indanol, 1-indanone, 2-indanone, 2-hydroxy-1-indanone, cis-1,2-indandiol, and trans-1,2-indandiol. The metabolites were identified using the techniques of gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The rats treated with indan demonstrated the classic lesions of hydrocarbon-induced nephropathy. The kidney damage produced was less than that found for tetralin and other branched-chain acyclic hydrocarbons.