Modeling of the time-dependency of in vitro drug cytotoxicity and resistance.

For potential clinical extrapolation of in vitro findings, it is of interest to relate the measured effect of an anticancer agent to concentration and exposure time. The Hill model (A. V. Hill, J. Physiol., 40: iv-vii, 1910) is commonly used to describe pharmacodynamic (PD) effects, including drug-induced growth inhibition of cancer cells in vitro. The IC(X)n x T = k relationship, in which IC(X) is the concentration of agent required to reduce cell growth by X%, T is the exposure time, and n and k are estimable parameters, was first applied to bacterial disinfectant action and then was successfully used to model anticancer drug potency as a function of exposure time (D. J. Adams, Cancer Res., 49: 6615-6620, 1989). Our goal was to create a new global PD modeling paradigm to facilitate the quantitative assessment of the growth-inhibitory effect of anticancer agents as a function of concentration and exposure time. Wild-type human ovarian A2780 and ileocecal HCT-8 carcinoma cells and sublines that were resistant to cisplatin (A2780/CP3), doxorubicin (A2780/DX5B), and raltitrexed (RTX) (HCT-8/DW2) were exposed to various anticancer agents, cisplatin, doxorubicin, paclitaxel, trimetrexate, RTX, methotrexate, and AG2034, for periods ranging from 1 to 96 h. Cell growth inhibition was measured with the sulforhodamine B protein dye assay. Patterns of time-dependency of drug potency, slope of the concentration-effect curves, and relative degree of resistance were characterized. Empirical mathematical expressions were built into a global concentration-time-effect model. The global PD model was then fit to the concentration-time-effect data with iteratively reweighted nonlinear regression. Under specific treatment conditions, the examination of the slope and the shape of the concentration-effect curves revealed a large heterogeneity in drug response, e.g., shallow concentration-effect curve or double or triple Hill "roller coaster" concentration-effect curve. These patterns, which were observed at intermediate exposure times in parental and resistant cells for paclitaxel and trimetrexate or only in resistant HCT-8/DW2 cells for RTX, methotrexate, and AG2034, revealed mechanistic insights for the former cases but possible methodological artifacts for the latter cases. The comprehensive PD modeling of the cytotoxic effect of anticancer agents showed that it was possible to modulate drug effect, response heterogeneity, and drug resistance by altering the time of exposure to the agents. This approach will be useful for: (a) describing complex concentration-time-effect surfaces; (b) refining biological interpretations of data; (c) providing insights on mechanisms of drug action and resistance; and (d) generating leads for clinical use of anticancer drugs.

Effect of alosetron on the pharmacokinetics of alprazolam.

Lotronex (alosetron hydrochloride) is a 5-HT3 receptor antagonist indicated for the treatment of irritable bowel syndrome (IBS) in females whose predominant bowel habit is diarrhea. Alosetron is extensively metabolized by multiple cytochrome P450 (CYP) enzymes, including CYP 2C9 and 3A4. Alprazolam is a short-acting benzodiazepine commonly prescribed for the treatment of anxiety disorders and a potential comedication in patients with IBS. Alprazolam is extensively metabolized by CYP3A4. This clinical study was conducted to assess the potential for a metabolic drug interaction between these two CYP3A4 substrates. This was an open-label, randomized, two-period, crossover study in 12 healthy female and male volunteers to determine the effect of concomitant administration of alosetron at the recommended dose of 1 mg p.o. bid on the pharmacokinetics of alprazolam following a single oral 1 mg dose. The results showed no effect of alosetron on the pharmacokinetics of alprazolam. Mean alprazolam AUC was 210 and 202 ng.h/mL in the absence and the presence of alosetron, respectively. Therefore, alprazolam may be safely coadministered with alosetron without the need for dosage adjustment.

Combined action of paclitaxel and cisplatin against wildtype and resistant human ovarian carcinoma cells.

PURPOSE: The combination of paclitaxel (PTX) and cisplatin (DDP) shows good clinical efficacy against ovarian cancer. In order to examine the potential cellular basis for this, and provide leads as to how to optimize the combination, we examined the role of sequence of exposure to PTX and DDP on cell growth in vitro. METHODS: Four human ovarian carcinoma cell lines, A121, A2780/WT, A2780/DX5B and A2780/CP3, two human head and neck carcinoma cell lines, A253 and FaDu, and the human ileocecal carcinoma cell line, HCT-8, were treated with PTX + DDP with seven schedules: (A) 96 h exposure to PTX + DDP; (B) 24 h PTX alone, then 72 h PTX + DDP; (C) 4 h DDP alone, then 92 h PTX + DDP; (D) 24 h PTX alone, 4 h DDP alone, then 68 h drug-free; (E) 4 h DDP alone, 24 h PTX alone, then 68 h drug-free; (F) 3 h PTX alone, 1 h DDP alone, then 92 h drug-free; and (G) 1 h DDP alone, 3 h PTX alone, then 92 h drug-free. Each of 66 two-drug experiments included five plates (440 randomly treated wells per experiment). Cell growth was measured by the sulforhodamine B assay. The nature and the intensity of the drug interactions were assessed by fitting a seven-parameter model to data with weighted nonlinear regression, enabling the estimation of an interaction parameter, alpha, with its standard error. RESULTS: Overall there was very little departure from Loewe additivity: 43 experiments showed Loewe additivity, 10 showed Loewe antagonism, and 13 showed slight Loewe synergy. In vitro Loewe synergy was rare, was small when present, and reproducible only for the A121 and HCT-8 cells exposed to schedule D (24 h PTX prior to 4 h DDP). Isobolographic analysis showed complex combined-action surfaces with regions of local Loewe synergy and antagonism. CONCLUSION: It appears unlikely that the good clinical efficacy of the combination is primarily caused by a synergistic interaction at the cellular level.

Parabolic growth patterns in 96-well plate cell growth experiments.

In preparing for the routine use of the ubiquitous in vitro cell growth inhibition assay for the study of anticancer agents, we characterized the statistical properties of the assay and found some surprising results. Parabolic well-to-well cell growth patterns were discovered, which could profoundly affect the results of routine growth inhibition studies of anticancer and other agents. Four human ovarian cell lines, A2780/WT, A2780/DX5, A2780/DX5B, and A121, and one human ileocecal adenocarcinoma cell line, HCT-8, were seeded into plastic 96-well plates with a 12-channel pipette, without drugs, and grown from 1-5 d. The wells were washed with a plate washer, cells stained with sulforhodamine B (SRB), and dye absorbance measured with a plate reader. Variance models were fit to the data from replicates to determine the nature of the heteroscedastic error structure. Exponential growth models were fit to data to estimate doubling times for each cell line. Polynomial models were fit to data from 10-plate stacks of 96-well plates to explore nonuniformity of cell growth in wells in different regions of the stacks. Each separate step in the assay was examined for precision, patterns, and underlying causes of variation. Differential evaporation of water from wells is likely a major, but not exclusive, contributor to the systematic well-to-well cell growth patterns. Because the fundamental underlying causes of the parabolic growth patterns were not conclusively found, a randomization step for the growth assay was developed.

Implications for clinical pharmacodynamic studies of the statistical characterization of an in vitro antiproliferation assay.

Modeling of nonlinear pharmacodynamic (PD) relationships necessitates the utilization of a weighting function in order to compensate for the heteroscedasticity. The structure of the variance was studied for concentration-effect data generated in an in vitro 96-well plate cell growth inhibition assay, where data are numerous (480 data points per experiment) and replication is easy. From the five candidate models that were considered, the power function S2Y = phi 2Y phi 3, where Y is the sample mean and S2Y is the sample variance, was shown to be the most appropriate to describe the nonuniformity of the variance along the range of measured effect for 253 sets of (Y; S2Y) data. The Hill model was fit to the concentration-effect data with weighted nonlinear regression, where the weights were equal to the reciprocal of the predicted variance. The examination of the distribution of the 253 sets of parameters of the PD model showed that IC50 was lognormally distributed whereas the distribution of gamma was normal. The characterization of the appropriate variance function and concentration-effect function in a simple in vitro experimental setting with a large number of experiments, with each experiment including a large number of data points, will be useful for guiding similar in vitro concentration-effect studies where data are plentiful and for guiding PD modeling in complex clinical settings in which extensive data for model characterization is impossible to obtain.

Development of a new quantitative approach for the isobolographic assessment of the convulsant interaction between pefloxacin and theophylline in rats.

PURPOSE: A new mathematical approach was developed to quantify convulsant interaction between pefloxacin and theophylline in rats. METHODS: Animals received each compound separately or in different combination ratios. Infusion was stopped at the onset of maximal seizures. Cerebrospinal fluid (CSF) and plasma samples were collected for HPLC drug determination. The nature and intensity of the pharmacodynamic (PD) interaction between drugs was assessed with a new modeling approach which includes (a) data transformation to create an essentially error-free X-variable and (b) estimation of an interaction parameter a by fitting a nonlinear hyperbolic model to the combination data with unweighted nonlinear regression. RESULTS: Drug disposition to the biophase was linear within the range of administered doses. The estimates of a suggested a Loewe antagonistic interaction between pefloxacin and theophylline at the induction of maximal seizures in rats. Similar intensity of PD interaction was observed at the dose and biophase level (alpha was -0.415 +/- 0.069 and -0.567 +/- 0.079, respectively). CONCLUSIONS: The suitability of the proposed model was assessed by Monte Carlo simulation. This new mathematical approach enabled the characterization of the Loewe antagonistic nature of the PD (convulsant) interaction between pefloxacin and theophylline, whereas previously used methodologies failed to do so.