Immunosuppressive effect of cyclosporine in the hyperlipidemic rat model

Effects of experimental hyperlipidaemia on the pharmacokinetics of docetaxel in rats

Hyperlipidaemia correlates with an increased risk of occurrence of various cancers. In this study, the effects of hyperlipidaemia on the pharmacokinetics of docetaxel, a member of the taxane class of anti-cancer drugs, were investigated in rats with experimental hyperlipidaemia; we focused on the alterations in docetaxel metabolism and plasma distribution. Docetaxel (5 mg/kg intravenously (i.v.) and 40 mg/kg per oral (p.o.)) was administered to control rats and rats with poloxamer-407 (P-407)-induced hyperlipidaemia (1 g/kg, intraperitoneally). In vitro studies were conducted on hepatic metabolism in S9 fractions and plasma protein binding using the ultrafiltration method. Hyperlipidaemia dramatically increased the area under the plasma concentration-time curve from time 0 to infinity (AUC(0-∞)) of docetaxel after i.v. (1.86-fold) or p.o. (10.8-fold) administration and decreased total body clearance (0.574-fold) and apparent volume of distribution at steady state (0.615-fold) of docetaxel after i.v. administration. Compared with the control rats, the metabolism of docetaxel by hepatic S9 fractions and the unbound fraction in the plasma in the hyperlipidaemic rats were decreased, i.e., by 20.1 and 79.8%, respectively. In conclusion, the alterations in docetaxel pharmacokinetics in rats with P-407-induced hyperlipidaemia may be due, at least in part, to a decrease in hepatic metabolism and the unbound fraction of docetaxel in the plasma. These findings have potential therapeutic implications for predicting human pharmacokinetic responses to hyperlipidaemia.

Modifications in low-density lipoprotein receptor expression affects Cyclosporin A cellular uptake and cytotoxicity

The purpose of this study was to test the effect of modulating the expression of the human low-density lipoprotein receptor (LDLr) in human embryonic kidney (293T) cells on Cyclosporin A (CsA) cellular uptake and CsA-mediated cytotoxicity. LDLr expression was modulated using RNA interference (RNAi) and an LDLr overexpression plasmid. One of the small-interfering RNA (siRNA) constructs, LDLr-792, showed a 60% decrease in LDLr protein expression. The downregulation effect was specific as transfection with an annexin V (AxV) siRNA construct did not decrease LDLr expression levels. AxV and ABCA1 expression levels were not affected in the cells transfected with LDLr-792 (LDLr(LOW) cells) compared to the controls. At a functional level, fluorescent low-density lipoprotein (LDL) (DiI-LDL) internalization in the LDLr(LOW) cells was decreased (30%) compared to control cells. We tested the dose-dependent cytotoxicity induced by CsA using a respiration assay. We found a decrease in CsA-mediated cytotoxicity in the range of CsA doses studied (1-10 microg/mL) in the LDLr(LOW) cells compared to the pSHAG-transfected cells, reaching a statistical significance at 10 microg/mL CsA. At higher CsA doses we found a significant decrease in LDLr expression. When the control and LDLr(LOW) cells were treated with another cytotoxic drug, gentamycin, there was no difference in the cell viability, suggesting that this effect is specific for CsA. We confirmed the association of LDLr expression levels with CsA uptake by overexpressing the LDLr. The LDLr overexpressing cells showed an enhanced uptake of radiolabelled CsA. Taken together these results suggest that CsA internalization and cytotoxicity are affected by the LDL receptor expression levels.

Distribution of cyclosporin in organ transplant recipients

Cyclosporin is an immunosuppressive agent with a narrow therapeutic index. The total concentration of cyclosporin in blood is usually monitored to guide dosage adjustment and to compensate for substantial interindividual and intraindividual variability in cyclosporin pharmacokinetics. Cyclosporin is a highly lipophilic molecule and widely distributes into blood, plasma and tissue components. It mainly accumulates in fat-rich organs, including adipose tissue and liver. In blood, it binds to erythrocytes in a saturable fashion that is dependent on haematocrit, temperature and the concentration of plasma proteins. In plasma, it binds primarily to lipoproteins, including high-density, low-density and very-low-density lipoprotein, and, to a lesser extent, albumin. The unbound fraction of cyclosporin in plasma (CsA(fu)) expressed as a percentage is approximately 2%. It has been shown that both the pharmacokinetic and pharmacodynamic properties of cyclosporin are related to its binding characteristics in plasma. Furthermore, there is some evidence to indicate that the unbound concentration of cyclosporin (CsA(U)) has a closer association with both kidney and heart allograft rejection than the total (bound + unbound) concentration. However, the measurement of CsA(fu) is inherently complex and cannot easily be performed in a clinical setting. Mathematical models that calculate CsA(fu), and hence CsA(U), from the concentration of plasma lipoproteins may be a more practical option, and should provide a more accurate correlate of effectiveness and toxicity of this drug in transplant recipients than do conventional monitoring procedures. In conclusion, the distribution characteristics of cyclosporin in blood, plasma and various tissues are clinically important. Further investigations are needed to verify whether determination of CsA(U) improves the clinical management of transplant recipients.

Effects of hyperlipidemia on the pharmacokinetics of nifedipine in the rat

PURPOSE

The effect of hyperlipidemia on nifedipine pharmacokinetics was studied. The mechanisms by which hyperlipidemia affects pharmacokinetics of drugs are mainly undetermined. Hyperlipidemia may decrease the fraction of unbound drug in plasma and/or decrease intrinsic ability of the cytochrome P-450 systems due to excess membrane cholesterol. Hyperlipidemia is a primary risk factor for coronary artery disease leading to hypertension and ischemic heart disease, for which nifedipine, a calcium channel blocker, is used.

METHODS

Poloxamer 407 (P407)-induced hyperlipidemic rat model was used to study the effects of hyperlipidemia on the pharmacokinetics of nifedipine (6 mg kg-1 given i.v., i.p. and p.o.). Total plasma cholesterol levels increased from 0.82-2.02 to 5.27-11.05 mmol L-1 48 h post P407 administration (1 g kg-1, i.p.). Protein binding studies were conducted by an ultrafiltration method.

RESULTS

Hyperlipidemia significantly decreased CLTB by 38% and CLTB/F by 45 and 42% following po and i.p. doses, respectively, thereby increasing AUC0-infinity, Cmax and half-life. Absolute bioavailability and Vdss remained unchanged. AUC0-infinity was affected to the same extent in each route of administration, therefore, the effect was mainly systemic rather than presystemic. Hyperlipidemia significantly lowered the fraction unbound in plasma by approximately 31%.

CONCLUSIONS

The altered pharmacokinetics of nifedipine by P407-induced HYPERLIPIDEMIA may be, at least in part, due to the decrease in fraction unbound in plasma. A decrease in intrinsic clearance, however, cannot be ruled out.

Changes in trough levels of whole blood cyclosporine and graft function of a kidney transplant recipient with onset of hypothyroidism after transplantation

The immunosuppressive and toxic effects of cyclosporine (CsA) are affected by many factors. We present the first case of a kidney transplant patient who had an onset of hypothyroidism about two months after the transplantation. In this case, trough levels of whole blood CsA and total serum cholesterol levels increased at the same time. But the decrease in renal function was not as severe as expected from the very high trough levels of whole blood CsA. Elevation of the trough level of whole blood CsA may be due to a decrease in CsA clearance resulting from the decreased cytochrome P-450 activity in hypothyroidism. Furthermore, a decrease in thyroid hormone level and increase in plasma lipoprotein level might have affected the distribution of CsA, and this change might have influenced the toxic effect of CsA In conclusion, our case suggests that thyroid function and plasma lipoprotein level should be considered important factors that affect the pharmacokinetics and action of CsA.

Modifications in plasma lipoprotein concentration and lipid composition regulate the biological activity of hydrophobic drugs

The maximum tolerated dose and pharmacokinetics of a drug is usually determined in healthy human volunteers and animals. This data is then used to define the dosing recommendation for the diseased patient population. However, in the case of some hydrophobic drugs, the dose which is deemed nontoxic becomes ineffective and/or toxic when administered to the diseased patient. This observation might be explained by several lines of evidence which indicate that binding of drugs such as amphotericin B (AmpB) and cyclosporine (CSA) to plasma low-density lipoprotein- (LDL) cholesterol is involved in the development of kidney toxicity. Our preliminary studies have suggested that this phenomena might be due to increase lipid transfer protein (LTP 1) activity which promotes the transfer of AmpB from high-density lipoproteins to LDL. In addition, since LTP 1 function is regulated by the lipid content of plasma lipoproteins, we suggest that changes in lipoprotein composition that occur in dyslipidemia regulate the distribution of these and other hydrophobic drugs (i.e., annamycin and nystatin). The impact of these studies on hydrophobic drug therapy could have broad implications on how we evaluate and determine dosing of hydrophobic drugs in dyslipidemic patients. By understanding the mechanism(s) responsible for the distribution of hydrophobic compounds in the bloodstream, we are trying to define the effect of dyslipidemias on the plasma clearance and therapeutic index of hydrophobic compounds.

Liposomal formulations of cyclosporin A: influence of lipid type and dose on pharmacokinetics

PURPOSE

Liposomal formulations of Cyclosporin A (CyA)3 have been described in more than 30 publications to substitute Cremophor EL (CrEL), a triricinoleate ester of ethoxylated glycerol, as drug carrier. However, conflicting reports did not allow to draw consistent conclusions about the influence of liposomes on CyA pharmacokinetics (PK) and pharmacodynamics.

METHODS

A series of liposomal CyA-formulations with varying liposome composition and lipid dose but constant CyA dose was compared in rats. Data were analysed with a PK-model taking into account the varying volume of distribution with the varying lipid concentration in blood.

RESULTS

Surface properties and lipid type of liposomes are not important PK predictors of liposomal CyA, at least for small dosages of liposomes. Rather, the absolute lipid amount and the lipophilicity of cyclosporins are critical factors influencing the PK of liposomal CyA. The higher the concentration of lipid in blood and the greater the lipophilicity of cyclosporin is, the higher are the concentrations of CyA in blood.

CONCLUSIONS

These relations may explain the inconsistent literature results. Together with earlier observations from our group the above findings indicate, that CyA is not caged in the liposomal membranes. Reports in literature, which claim lower clearance and a lower volume of distribution of CyA in obese rats compared to lean rats, support our assumption about the involved mechanisms. A semi-quantitative model of CyA distribution is presented, which points to the variable free fraction of CyA in plasma as the crucial factor for all previously reported phenomena in liposomal CyA formulations.