Two linear killer plasmids (pGKLl and pGKL2) from Kluyveromyces lactis stably replicated and expressed the killer phenotype in a neutral petite mutant ([rho°]) of Saccharomyces cerevisiae. However, when cytoplasmic components were introduced by cytoduction from a wild-type ([rho+]) strain of S. cerevisiae, the linear plasmids became unstable and were frequently lost from the cytoductant cells during mitosis, giving rise to nonkiller clones. The phenomenon was ascribed to the incompatibility with the introduced S. cerevisiae mitochondrial DNA (mtDNA), because the plasmid stability was restored by [rho°] mutations in the cytoductant cells. Incompatibility with mtDNA was also apparent for the transmission of plasmids into diploid progeny in crosses between killer cells carrying the pGKL plasmids and [rho'] nonkiller cells lacking the plasmids. High-frequency transmission of the plasmids was observed in crosses lacking mtDNA ([rhoo] by [rhoo] crosses) and in crosses involving mutated mtDNA with large deletions of various regions of mitochondrial genome. In contrast, mutated mtDNA from various mit-mutations also exerted the incompatibility effect on the transmission of plasmids. Double-stranded RNA killer plasmids were stably maintained and transmitted in the presence of wild-type mtDNA and stably coexisted with pGKL killer plasmids in [rhoo] cells of S. cerevisiae.

The objective of this study was to examine the AUC dependency of saturable hepatic clearance (CLh) of liposomes and to postulate a mathematical model to describe the characteristics. The AUC dependency of saturable CLh was examined under intravenous rapid administration at various doses. The CLh increased with increasing blood concentration but decreased with the increase of AUC at each dose. In addition, the relationship between AUC and CLh was consistent with that observed in previously reported infusion studies. These experimental data confirm the AUC dependency of saturable CLh of liposomes. A mathematical model was developed for this AUC dependency. The decrease of CLh was described by the uptake amount (X) as follows: CLh = CLm(1-X/Xm), where CLm and Xm represent the maximum uptake clearance and the maximum uptake amount, respectively. The rate equation for uptake was analytically solved as CLh = X/AUC = Xm/AUC(1-exp(CLm/XmAUC)). Uptake clearance can be described by CLm, Xm, and AUC, and so uptake clearance is constant if AUC is constant. These experimental analyses and theoretical considerations show the validity of the AUC-dependent saturable CLh of liposomes.

The objective of this study was to verify the methodology for measuring uptake clearance of liposomes and to characterize kinetically the saturable hepatic uptake of liposomes-through phagocytosis. The correction of vascular space was important in the evaluation of hepatic uptake. The efflux of liposomes from liver was shown to be negligible, by a repeated dose study, and thus, hepatic clearance can be obtained by the hepatic uptake divided by the area under the blood concentration-time curve (AUC). The determinant parameter which describes the saturability of uptake clearance of liposomes, independent of infusion rate, was investigated, using the data of an in-vivo constant infusion study, where infusion rate-dependent saturable hepatic clearance was observed. The mean blood concentration failed to obtain an infusion rate-independent function. On the other hand, the AUC could explain the saturability of hepatic clearance for every infusion rate by a unique relationship. The hepatic uptake amount could also explain this saturability, independent of infusion rate. These kinetic characteristics are inconsistent with Michaelis-Menten type kinetics, therefore a new model is required to describe the saturable hepatic clearance in the disposition of liposomes.

The aim of this study is to develop a kinetic model for the quantitative evaluation of, and to examine dose dependency in liposome degradation in blood circulation in vivo. Multilamellar liposomes labeled with 3H-inulin were administered intravenously into rats and the time courses of blood concentration and urinary excretion of 3H-inulin were measured. The dosages of liposomes were fixed at 1, 5, and 100 mumolPCkg-1. Remarkable saturation was found in the time courses of both blood concentration and urinary excretion. Then a kinetic model for the degradation of liposomes in blood was developed, assuming that the degradation follows the first order rate process for each dose. The model fitted the observed time courses of excreted 3H-inulin well, and dose dependency could be observed in the rate constants for liposome degradation, which are more sensitive than urinary excretion of 3H-inulin. The degradation rate constant correlated well with the uptake rate constant, which suggests the same underlying mechanism for both uptake and degradation. These results indicate the usefulness of kinetic modeling in the quantitative evaluation of liposome degradation in blood circulation in vivo.

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