Richard J. Ptachcinski

Clinical Pharmacokinetics of Cyclosporin

SummaryCyclosporin (cyclosporin A) is a unique immunosuppressant used to prevent the rejection of transplanted organs and to treat diseases of autoimmune origin. Therapeutic drug monitoring of cyclosporin is essential for several reasons: (a) wide variability in cyclosporin pharmacokinetics has been observed after the oral or intravenous administration of the drug. The variability in the kinetics of cyclosporin is related to a patient’s disease state, the type of organ transplant, the age of the patient and therapy with other drugs that interact with cyclosporin; (b) maintaining a blood concentration of cyclosporin required to prevent rejection of the transplanted organ; (c) minimising drug toxicity by maintaining trough concentrations below that which toxicity is most likely to occur; and (d) monitoring for compliance since patient non-compliance with drug regimens is a significant reason for graft loss after 60 days. Clinical monitoring and pharmacokinetic studies of cyclosporin can be performed using different biological fluids (plasma, serum or whole blood) and different analytical techniques (radioimmunoassay or high pressure liquid chromatography). The available analytical methods provide different results when using blood, plasma, or serum. Comparison of therapeutic ranges and pharmacokinetic parameters should be made with careful attention given to the method of cyclosporin analysis.Following oral administration, the absorption of cyclosporin is slow and incomplete. Peak concentrations in blood or plasma are reached in 1 to 8 hours after dosing. The bioavailability of cyclosporin ranges from less than 5% to 89% in transplant patients; poor absorption has frequently been observed in liver and kidney transplant patients and in bone marrow recipients. Factors that affect the oral absorption of cyclosporin include the elapsed time after surgery, the dose administered, gastrointestinal dysfunction, external bile drainage, liver disease, and food.Cyclosporin is widely distributed throughout the body. Following intravenous administration, the drug exhibits multicompartmental behaviour. The volume of distribution (whole blood; HPLC) ranges from 0.9 to 4.8 L/kg. Cyclosporin is highly bound to erythrocytes and plasma proteins and has a blood to plasma ratio of approximately 2. In plasma, approximately 80% of the drug is bound to lipoproteins. The distribution of cyclosporin in blood can be affected by a patient’s haematocrit and lipoprotein profile.Cyclosporin is extensively metabolised, primarily by mono- and dihydroxylation as well as N-demethylation, and is considered a low-to-intermediate clearance drug. The clearance of cyclosporin (whole blood; HPLC) ranges from 2.0 ml/min/kg in children with congestive heart failure to 11.8 ml/min/kg in paediatric kidney transplant patients. The terminal elimination half-life is highly variable and ranges from 6.3 hours in healthy volunteers to 20.4 hours in patients with severe liver disease (blood; H PLC). Factors affecting the metabolism of cyclosporin include liver disease, age, and concurrent drug therapy.The major route of elimination of cyclosporin is via the bile, primarily as metabolites of the drug. Renal excretion is a minor elimination pathway. Renal failure and haemodialysis do not alter the pharmacokinetics of cyclosporin.Several drugs are known to interact with cyclosporin, including microsomal enzyme inducing and inhibiting agents. Several drugs including amphotericin B, aminoglycoside antibiotics and co-irimoxazole may potentiate the nephrotoxicity of cyclosporin.The dose of cyclosporin used in a patient should be adjusted after considering factors such as the initial response to therapy, the patient’s age, transplant type, disease state and concurrent drug therapy. Initial doses are usually in the range of 10 to 20 mg/kg/day orally or 2.5 to 5 mg/kg/day as an intravenous infusion, and should be adjusted based on the clinical status of the patient and cyclosporin blood concentrations. Long term oral maintenance doses of less than 3 mg/kg/day have resulted in adequate immunosuppression in some patients.The therapeutic range for cyclosporin is poorly defined and depends on the biological fluid being analysed, the analytical technique and the time after transplant. Cyclosporin concentration monitoring should be used in conjunction with other assessment criteria such as serum biochemical parameters, radiological studies, biopsy results and the clinical status of the patient.Even though our understanding of cyclosporin is incomplete, a thorough knowledge of different factors that affect its kinetics will aid the clinician in optimising immunosuppression with this promising new agent.

A PROSPECTIVE RANDOMIZED TRIAL COMPARING SEQUENTIAL GANCICLOVIR-HIGH DOSE ACYCLOVIR TO HIGH DOSE ACYCLOVIR FOR PREVENTION OF CYTOMEGALOVIRUS DISEASE IN ADULT LIVER TRANSPLANT RECIPIENTS

Cytomegalovirus disease is an important cause of morbidity following liver transplantation. To date there has not been an effective prophylaxis for CMV disease after liver transplantation. One hundred forty-three patients were randomized to receive either high dose oral acyclovir (800 mg 4 times a day) alone for 3 months after transplantation (acyclovir group) or intravenous ganciclovir (5 mg/kg twice a day) for 14 days followed by high dose oral acyclovir to complete a 3-month regimen (ganciclovir group). Of 139 patients available for evaluation, 43 of 71 (61%) patients from the acyclovir group developed CMV infection compared with 16 of 68 (24%) from the ganciclovir group (relative risk, 3.69; 95% confidence interval, 2.07–6.56; P<0.00001). Of those randomized, CMV disease was seen in 20 (28%) of the acyclovir group compared with 6 (9%) of the ganciclovir group (relative risk, 5.11; 95% confidence interval, 2.05–12.75; P=0.0001). The median time to onset of CMV infection was 45 days in the acyclovir group compared with 78 days in the ganciclovir group (P=0.004). The median time to onset of CMV disease was 40 days in the acyclovir group compared with 78 days in the ganciclovir patients (P=0.02). With respect to primary CMV infection, there was no difference in the rates in the 2 groups, but tissue invasive disease and recurrent CMV disease were less frequent in the ganciclovir group. It is concluded that a course of 2 weeks of ganciclovir immediately after transplantation followed by high dose oral acyclovir for 10 weeks is superior to a 12-week course of high dose oral acyclovir alone for prevention of both CMV infection and CMV disease after liver transplantation. However, the lack of significant effect in sero-negative recipients who received grafts from sero-positive donors suggests that other strategies are needed to prevent CMV infection in this high risk population.

Cyclosporine kinetics in renal transplantation

The pharmacokinetics of cyclosporine were evaluated in 41 recipients of a cadaveric renal transplant. Cyclosporine was taken by mouth (mean dose 14 mg/kg) on one study day and was intravenously infused over 2 hours (mean dose 4.7 mg/kg) on the next study day. Cyclosporine was extracted from whole blood and analyzed by HPLC. After intravenous infusion, cyclosporine exhibited multicompartmental behavior. The mean (± SD) terminal disposition rate constant was 0.065 ± 0.036 hours−1 and the harmonic mean t½ was 10.7 hours. The harmonic mean total body clearance of cyclosporine was 5.73 ml/min/kg and the mean apparent volume of distribution was 4.5 ± 3.6 L/kg. The absorption of oral cyclosporine was slow and incomplete. Peak blood cyclosporine concentrations (X̄ = 1,103 ng/ml) were reached between 1 and 8 hours after oral dosing (X̄ = 4 hours). The mean relative bioavailability was 27.6% ± 20%. Oral bioavailability was <10% in 17% of our subjects. The absorption and clearance of cyclosporine were highly variable. We conclude that the variability in the kinetics of cyclosporine makes trough blood level monitoring essential in the management of patients who receive renal transplants.

The effect of food on cyclosporine absorption

The effect of food on the absorption of cyclosporine was evaluated in 18 recipients of cadaveric renal transplants. Cyclosporine was administered orally with a standard hospital breakfast on one study day and without breakfast on the alternate study day. The oral absorption rate as measured by the observed time to peak concentration was not significantly altered by food. The administration of cyclosporine with food resulted in a significant increase in the peak (1465 ng/ml versus 1120 ng/ml) and trough (267 ng/ml versus 228 ng/ml) blood concentrations as well as the area under the blood concentration versus time curve (11430 ng . hr/ml versus 7881 ng . hr/ml). The mean increase in area under the blood concentration versus time curve was 60.6%. The exact mechanism by which food increases the absorption of cyclosporine is not known. Regardless of the mechanism involved, if adequate immunosuppression is achieved with lower doses of cyclosporine taken with food, significant cost savings could be realized.

Cyclosporine Absorption Following Orthotopic Liver Transplantation

Blood concentrations of cyclosporine were determined in adult and pediatric patients following orthotopic liver transplantation to quantitate cyclosporine blood clearance and oral absorption. Seventeen bioavailability studies were performed following transplantation surgery in nine children and seven adults. The intravenous cyclosporine study was performed following an average dose of 2.1 mg/kg. The patients were again studied when they received the same intravenous dose plus an oral dose of cyclosporine of 8.6 mg/kg or an oral dose alone. Blood samples were collected and analyzed for cyclosporine using high‐performance liquid chromatography. Cyclosporine blood clearance ranged from 29 to 203 mL/min (1.9–21.5 mL/min/kg) in children and from 253 to 680 mL/min (3.2–7.6 mL/min/kg) in adults. The mean cyclosporine clearance value was 9.3 mL/min/kg in the pediatric patients and 5.5 mL/min/kg in the adults. Cyclosporine bioavailability was less than 5% in six studies on five pediatric patients in the immediate postoperative period. The bioavailability varied from 8% to 60% in adult liver transplant patients (mean, 27%). We conclude that: (1) cyclosporine clearance is highly variable between patients, (2) pediatric patients clear the drug more rapidly than adults and therefore need a higher cyclosporine dose on a body weight basis, (3) cyclosporine is poorly and variably absorbed in liver transplant patients, and (4) cyclosporine blood concentration monitoring is essential following orthotopic liver transplantation.

Anaphylactoid Reactions Associated with Parenteral Cyclosporine Use: Possible Role of Cremophor EL

Acute anaphylactoid reactions occurred immediately after initiation of intravenous infusions of cyclosporine in three patients post-organ transplantation. Shortness of breath, flushing, tachypnea, chest pain, pruritus, or urticaria were noted; rapid recovery followed cessation of drug infusion. Subsequently, oral cyclosporine has been used in each patient without recurrence of the observed reaction. The presence of Cremophor EL as an emulsifying agent in the parenteral dosage formulation of cyclosporine is a likely etiology for this acute adverse reaction, Slowed rates of drug infusion and antihistamine premedication may permit continued intravenous cyclosporine use in affected patients.

Clinical Pharmacokinetics in Organ Transplant Patients

SummaryDiseases of the liver, kidney and heart influence the pharmacokinetics of several drugs. Organ transplantation is an accepted therapeutic option for the treatment of several disease states associated with these organs. Recently, there has been an increase in both graft and patient survival after transplantation of the liver, heart, kidney and bone marrow. Such patients normally receive a wide range of drugs, and optimisation of drug therapy requires a thorough understanding of the pharmacokinetics and pharmacodynamics of these drugs in transplant patients. However, only limited studies have been carried out to characterise drug kinetics in these situations. Available information indicates that drug kinetics cannot be considered normal in transplant patients. Drug absorption generally appears to be similar to that in healthy subjects. The plasma protein binding of drugs that primarily bind to albumin increases after transplantation, but remains lower than that observed in healthy subjects. While the binding of certain basic drugs may increase after transplantation due to an increase in the concentration of α1-acid glycoprotein, a lower albumin concentration may mask this effect. Oxidative and conjugative metabolism as measured by the kinetics of antipyrine (phenazone) and paracetamol (acetaminophen) is normal, while the metabolism of steroids may be impaired. Serum creatinine does not appear to be a good indicator of the Junctional status of the kidney in transplant patients. It is also important to realise that there will be time-dependent changes in several kinetic parameters of drugs due to improvement in the physiological function from that associated with the disease state to that of the normal state. Individualisation and close monitoring of drug therapy is necessary in transplant patients.

Cyclosporine Kinetics in Healthy Volunteers

The pharmacokinetics of cyclosporine was studied in five healthy male volunteers following intravenous administration. The subjects received 2.1 mg/kg of cyclosporine as a two‐hour intravenous infusion. Blood samples were collected over the subsequent 48 hours. Cyclosporine was extracted from whole blood and analyzed by high‐performance liquid chromatography (HPLC) and radioimmunoassay (RIA). Following the intravenous infusion of cyclosporine, the drug exhibited multicompartmental behavior. The harmonic mean distribution half‐life based on HPLC data was 0.45 hours, and the harmonic mean terminal disposition half‐life was 6.2 hours. The clearance of cyclosporine based on HPLC cyclosporine concentrations was 3.9 mL/min/kg, and the volume of distribution at steady state of cyclosporine was 1.23 L/kg. Cyclosporine has a shorter half‐life, lower clearance, and smaller Vss in healthy persons as compared to patient populations. The differences observed in the pharmacokinetics of cyclosporine in healthy persons as compared to patient populations may be due to differences in hematocrit, lipoprotein profiles, and/or concurrent drug therapy between the groups. Cyclosporine concentrations determined by RIA were consistently higher than those determined by HPLC, resulting in a significantly higher area under the blood concentration versus time curve and lower clearance rate for cyclosporine. We conclude that: (1) kinetic parameter estimates for cyclosporine are different in healthy individuals as compared with organ‐transplant recipients, and (2) the kinetic parameters for cyclosporine are different, depending on the assay technique used.

Cyclosporine Concentration Determinations for Monitoring and Pharmacokinetic Studies

The availability of the immunosuppressant cyclosporine has led to significant improvements in the recent success of clinical organ transplantation. Problems associated with cyclosporine therapy include serious adverse reactions, such as nephrotoxicity, wide variability in the drugs pharmacokinetics, and several complex drug interactions. Monitoring of drug concentrations is accepted as a part of the routine care of patients receiving cyclosporine. However, cyclosporine concentrations can be determined in different biologic fluids by either radioimmunoassay or high‐performance liquid chromatographic techniques. Controversy exists regarding the optimal analytic technique to be used for cyclosporine monitoring and pharmacokinetic studies. This commentary addresses factors including: (1) why the monitoring of cyclosporine concentrations is important, (2) the differences between the biologic fluids and analytic techniques, (3) when monitoring and special pharmacokinetic studies are indicated, (4) what some major transplant centers have established as a “therapeutic range” for cyclosporine concentrations, and (5) provides guidelines for the optimal clinical monitoring of cyclosporine concentrations.

Antipyrine kinetics in liver disease and liver transplantation

Antipyrine kinetics were studied in seven normal subjects, 10 patients with liver disease, and 13 clinically stable patients who received a liver transplant. Five patients were studied both before and after liver transplantation. Antipyrine concentrations in saliva after oral dosing were measured by HPLC. The antipyrine t½was significantly longer (P < 0.05) in patients with liver disease than in patients undergoing liver transplantation and normal subjects. Antipyrine clearance was not significantly different between patients undergoing liver transplantation and normal subjects, but it was significantly reduced (P < 0.05) in patients with liver disease. In five patients who were studied before and after liver transplantation, there was a significant (P < 0.05) increase in the antipyrine clearance and a marked reduction in its t½after liver transplantation. These results indicate that liver transplantation improves the drug metabolizing ability of patients with liver disease and that the oxidative metabolizing capacity of the liver in clinically stable patients after liver transplantation is similar to that of normal subjects.