Determination of drug absorption rate in time-variant disposition by direct deconvolution using beta clearance correction and end-constrained non-parametric regression

Flip-flop pharmacokinetics--delivering a reversal of disposition: challenges and opportunities during drug development

Flip-flop pharmacokinetics is a phenomenon often encountered with extravascularly administered drugs. Occurrence of flip-flop spans preclinical to human studies. The purpose of this article is to analyze both the pharmacokinetic interpretation errors and opportunities underlying the presence of flip-flop pharmacokinetics during drug development. Flip-flop occurs when the rate of absorption is slower than the rate of elimination. If it is not recognized, it can create difficulties in the acquisition and interpretation of pharmacokinetic parameters. When flip-flop is expected or discovered, a longer duration of sampling may be necessary in order to avoid overestimation of fraction of dose absorbed. Common culprits of flip-flop disposition are modified dosage formulations; however, formulation characteristics such as the drug chemical entities themselves or the incorporated excipients can also cause the phenomenon. Yet another contributing factor is the physiological makeup of the extravascular site of administration. In this article, these causes of flip-flop pharmacokinetics are discussed with incorporation of relevant examples and the implications for drug development outlined.

Erythropoietin pharmacokinetic/pharmacodynamic analysis suggests higher doses in treating neonatal anemia

BACKGROUND

The establishment of effective treatment of neonatal anemia using recombinant human erythropoietin (r-HuEPO) requires a thorough understanding of the physiology and mechanism of EPO's pharmacologic effect. The purpose of the present preclinical study in sheep was to elucidate the stimulatory effect of EPO on erythroid progenitors and their differentiation into reticulocytes useful in predicting optimal r-HuEPO dosing.

METHODS

Five young adult sheep each underwent two phlebotomies spaced 4-6 weeks apart in which their hemoglobin levels were reduced from 12 g/dL to 3-4 g/dL. Endogenous EPO levels and reticulocyte counts produced in response to anemia were sampled throughout the study and analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model.

RESULTS

The phlebotomy-induced drop in hemoglobin resulted in a increase in EPO levels, which reached a maximum of 764 +/- 55 mU/mL (mean +/- %CV) in 0.5-2.6 days. The reticulocyte counts increased from baseline values of 76.9 x 10(3) +/- 67/microL to 619 x 10(3) +/- 30/microL in 8 days. The PK/PD analysis indicated an increased maturation time for the reticulocytes (4.88 +/- 35 days) and demonstrated that the E(max) model for EPO's activation of the progenitors did not show significant effect saturation at the endogenous EPO levels reached.

CONCLUSIONS

In extrapolating from the animal pilot experiment, the present study provides a case for the use of higher r-HuEPO doses in human studies to determine if higher doses are more effective in treatment of neonatal anemia to reduce, and in some less severe cases, eliminate, the need for blood transfusions.

A 'bottom-up' approach for endo-PK/PD analysis

A 'bottom-up' PK/PD analysis approach employing system analysis principles of convolution/deconvolution and special nonparametric estimation procedures is presented to resolve the complex 'endo-PK/PD' of the endogenous form of recombinant drugs using erythropoietin (EPO) as an example. A novel cellular deconvolution algorithm is presented that facilitates the identification of the functional relationship between the variables involved in EPO's complex PK/PD. Five sheep each underwent two phlebotomies spaced 4-6 weeks apart when their hemoglobin levels were reduced from 12 g/dl to 3-4 g/dl. EPO levels and reticulocyte counts were frequently sampled. The data were analysed using end-constrained cubic splines. The rate of reticulocyte production was determined using the novel deconvolution methodology. The erythroid progenitor cells activation rate by EPO was estimated from the reticulocyte production rate using a lag-time parameter which determines the delay in the reticulocyte appearance in the blood relative to the activation of erythroid progenitors. Hysteresis minimization combined with cellular deconvolution was employed to determine the population PK/PD transduction function relating the progenitor activation rate to EPO concentrations in a nonparametric manner without assuming a specific structure. The proposed approach provides a rational informative starting point for developing parametric PK/PD models to resolve the complex endo-PK/PD of recombinant drugs.