Failure of single-dose kinetics to predict steady state

Evaluation of pharmacokinetic studies: is it useful to take into account concentrations below the limit of quantification?

PURPOSE

Based on real data, to evaluate the usefulness of taking into account samples with values below the limit of quantification (LOQ) for the evaluation of pharmacokinetic studies.

METHODS

To compare for two drugs, after single dose administration the pharmacokinetic parameters obtained by using a poorly sensitive assay (PSA) and a highly sensitive assay (HSA), acting as reference; To evaluate the results of pharmacokinetic studies in the light of different values for the LOQ.

RESULTS

Under certain conditions, such as homogeneous population, sufficient subject number, sufficient sampling times and acceptable accuracy (CV < 20%) for the concentrations, it is possible to get valuable and more reliable kinetic information by using concentrations obtained with a poor precision (CV > 20%). This is especially true for the parameters associated with the terminal phase, such as t1/2 beta and AUC, but also for parameters depending to a lesser extent on the terminal phase, such as t1/2 alpha and AUCm. Moreover, the mean concentration time curve is by far best defined by using all the concentrations.

CONCLUSIONS

In some situations, it is preferable to use concentrations with values below the LOQ to evaluate the results of pharmacokinetic studies. However, this should not be the rule, especially when this does not bring any additional information, or when it is possible to increase the sensitivity of the bioanalytical assay.

Pharmacokinetics of cefepime in patients with respiratory tract infections

The steady-state pharmacokinetics of cefepime were evaluated in 10 middle-aged and elderly patients with acute lower respiratory tract infections who were receiving 1 g intravenously every 12 h. One preinfusion and 15 postinfusion serum samples and total urine output were collected over one dosing interval between days 3 and 8 of therapy. Cefepime concentrations in serum over time exhibited a multicompartmental profile. Peak and trough concentrations in serum determined by a validated high-performance liquid chromatography method were 71.2 +/- 17.2 (mean +/- standard deviation) and 6.0 +/- 4.9 mg/liter, respectively. The steady-state volume of distribution was 0.22 +/- 0.05 liter/kg. Elimination half-lives ranged from 1.93 to 6.04 h (3.92 +/- 1.28 h), and total body clearances ranged from 36.9 to 102 ml/min per 1.73 m2 (73.0 +/- 19.7 ml/min per 1.73 m2). The disposition of cefepime at steady state in patients was comparable to previous observations in healthy elderly volunteers. The predictive performance of regression equations derived from single-dose studies in volunteers relating creatinine clearance with total body and renal clearances of cefepime exhibited slight biases (mean predictive errors, -9.7 and 2.1 ml/min per 1.73 m2, respectively) and similar precisions. Predicted and observed total body clearances (63.3 +/- 25.1 versus 73.0 +/- 19.7 ml/min per 1.73 m2, respectively) and renal clearances (51.3 +/- 24.4 versus 49.3 +/- 19.6 ml/min per 1.73 m2, respectively) were not significantly different. The pharmacokinetics of cefepime in infected patients appeared to be unaltered by illness, and the steady-state disposition of cefepime was predictable from data derived from single-dose studies in volunteers.

Disposition of cefmetazole in healthy volunteers and patients with impaired renal function

The disposition of cefmetazole was studied in 25 subjects with various degrees of renal function after a 1,000-mg, constant-rate, 30-min intravenous infusion of cefmetazole sodium. In six subjects with creatinine clearance (CLCR) of greater than 90 ml/min per 1.73 m2 (group 1), the terminal elimination half-life (t1/2 beta) was 1.31 +/- 0.54 h (mean +/- standard deviation), cefmetazole total body clearance (CLP) was 132.8 +/- 25.1 ml/min per 1.73 m2, and volume of distribution at steady state was 0.165 +/- 0.025 liter/kg. The fraction of dose excreted unchanged in the urine was 84.0% +/- 26.1%. Subjects with CLCRS of 40 to 69 (group 2, n = 6) and 10 to 39 (group 3, n = 6) ml/min per 1.73 m2 demonstrated prolongation of the t1/2 beta (3.62 +/- 1.06 and 5.93 +/- 1.81 h, respectively) and significant reductions in cefmetazole CLP (52.8 +/- 14.3 and 30.2 +/- 10.2 ml/min per 1.73 m2, respectively), compared with group 1. In seven subjects on chronic hemodialysis (group 4) studied during an interdialytic period, the cefmetazole t1/2 beta was increased to 24.10 +/- 8.12 h and the CLP was reduced to 6.8 +/- 2.1 ml/min per 1.73 m2. Cefmetazole CLP correlated positively with CLCR (r = 0.951, P less than 0.001): CLP = (1.181 . CLCR) -- 0.287. The disposition of cefmetazole was also assessed in six group 4 subjects during an intradialytic period. The t1/2 beta during hemodialysis (2.09 +/- 0.69 h) was significantly shorter than that observed during the interdialytic period. The hemodialysis clearance of cefmetazole was 86.1 +/- 20.1 ml/min, and the fraction of cefmetazole removed during hemodialysis was 59.8% +/- 5.9%. It is recommended that patients with renal insufficiency received standard doses of cefmetazole at extended intervals and patients on maintenance hemodialysis received standard doses after hemodialysis.

The influence of assay variability on pharmacokinetic parameter estimation

The impact of assay variability on pharmacokinetic modeling was investigated. Simulated replications (150) of three "individuals" resulted in 450 data sets. A one-compartment model with first-order absorption was simulated. Random assay errors of 10, 20, or 30% were introduced and the ratio of absorption rate (Ka) to elimination rate (Ke) constants was 2, 10, or 20. The analyst was blinded as to the rate constants chosen for the simulations. Parameter estimates from the sequential method (Ke estimated with log-linear regression followed by estimation of Ka) and nonlinear regression with various weighting schemes were compared. NONMEM was run on the 9 data sets as well. Assay error caused a sizable number of curves to have apparent multicompartmental distribution or complex absorption kinetic characteristics. Routinely tabulated parameters (maximum concentration, area under the curve, and, to a lesser extent, mean residence time) were consistently overestimated as assay error increased. When Ka/Ke = 2, all methods except NONMEM underestimated Ke, overestimated Ka, and overestimated apparent volume of distribution. These significant biases increased with the magnitude of assay error. With improper weighting, nonlinear regression significantly overestimated Ke when Ka/Ke = 20. In general, however, the sequential approach was most biased and least precise. Although no interindividual variability was included in the simulations, estimation error caused large standard deviations to be associated with derived parameters, which would be interpreted as interindividual error in a nonsimulation environment. NONMEM, however, acceptably estimated all parameters and variabilities. Routinely applied pharmacokinetic estimation methods do not consistently provide unbiased answers. In the specific case of extended-release drug formulations, there is clearly a possibility that certain estimation methods yield Ka and relative bioavailability estimates that would be imprecise and biased.