Pathogenesis of herpes simplex labialis. I. Replication of herpes simplex virus in cultures of epidermal cells from subjects with frequent recurrences.

Primary cultures were established with epidermal cells from the skin of 11 patients with frequent episodes of herpes simplex labialis and 13 control subjects with titers of neutralizing antibody to herpes simplex virus (HSV) type 1 but no history of herpetic disease. Confluent monolayers were exposed to HSV type 1 strain E115, and the infection was monitored by assay of the rate of virus appearance in the culture medium. The mean slope of the virus growth curves ([log10 pfu/ml]/log10 hr) was 9.0 in cultures from patients vs. 9.5 in cultures from controls, and the respective mean titers of virus 53 hr after infection were 10(6.8) and 10(6.5) (differences not statistically significant). Genetically controlled host factors may play some role in the clinical response to HSV infection, but variation in the susceptibility of epidermal cells, the natural target for HSV, is not one of the critical determinations.

Disposition of tacrolimus (FK 506) in rabbits. Role of red blood cell binding in hepatic clearance.

Tacrolimus is a macrolide immunosuppressive drug undergoing clinical trials for organ transplantation. Whole animal studies were undertaken to assess the rabbit as an animal model for tacrolimus pharmacokinetics. The disposition of tacrolimus in rabbits following 0.5 mg/kg iv and 2.0 mg/kg po doses is similar to man. The apparent plasma clearance 1.67 liters/hr/kg is more than 5-fold higher than blood clearance 0.31 liters/hr/kg. The steady-state volume of distribution is 30.7 liters/kg for plasma and 6.3 liters/kg for whole blood. The bioavailability after oral administration calculated from plasma and whole blood is low with a mean value of 9.7%. The in vitro studies and fitting using a nonlinear red blood cell (RBC)-plasma binding/diffusion model showed slow diffusion of drug from RBC to plasma (t1/2 = 7 min, CLD = 0.085 ml/min). In perfused liver studies, the extraction ratio of tacrolimus from blood with hematocrit of 0.1 is low (0.20). However, extraction of drug from plasma is moderate (0.42), and plasma concentrations are elevated by an average of 28% because of redistribution of tacrolimus from RBC. This creates a lower apparent plasma clearance (DO/AUC) for equilibrated (30 min at 37 degrees C) samples (15.4 ml/min) compared with samples centrifuged within 5 min (22.1 ml/min). RBC efflux was accounted for using a comprehensive perfusion/diffusion model. The intrinsic metabolic clearance averaged 29.2 ml/min. Simulations showed that the apparent plasma clearance of tacrolimus is closely correlated with RBC binding capacity (whole blood:plasma ratio). Higher ratios caused greater apparent plasma clearance (lower concentration of tacrolimus in reservoir plasma) because strong binding of drug by erythrocytes prevents tacrolimus from diffusion into plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of phytohaemagglutinin-induced lymphocyte proliferation by immunosuppressive drugs: use of whole blood culture.

A whole blood lymphocyte proliferation assay was compared to a standard method requiring the isolation of lymphocytes from blood. Both methods were used to measure inhibition of proliferative responses of phytohaemagglutinin (PHA1)-stimulated human peripheral blood lymphocytes by tacrolimus (FK 506(1)), cyclosporine A (CsA1), rapamycin (RA1), dexamethasone (DEX1), prednisolone (PR1), and methylprednisolone (MP1). Three of the drugs studied (FK 506, CsA, and DEX) yielded similar IC50 values with both methods. The whole blood proliferation assay produced modestly lower IC50 values for RA, PR and MP. The whole blood lymphocyte proliferation method is simple and can be used when only limited volumes of blood can be obtained or isolation of cells gives unsatisfactory yields.

Effect of hematocrit and albumin concentration on hepatic clearance of tacrolimus (FK506) during rabbit liver perfusion.

Tacrolimus is an immunosuppressive agent used for organ transplantation. Studies were performed to examine the influence of different perfusate hematocrits and albumin concentrations on hepatic extraction of tacrolimus. In vitro binding, efflux and influx between red blood cells (RBCs) and buffer or plasma, and rabbit liver perfusion with use of human erythrocytes were studied. In the range of hematocrits from 0.05 to 0.4, plasma concentrations of tacrolimus were not affected by increased albumin content. Increased hematocrit caused decreases in whole blood:plasma (buffer) concentration ratios. The binding capacity of drug with RBCs was independent of hematocrit, with a value of 440 ng/ml of RBCs; the binding affinity was 0.876 ng/ml using plasma or buffer. Diffusion of tacrolimus from RBCs to buffer was rapid with a clearance of 0.940 ml/min, and equilibration was achieved within 2 min. Diffusion in the opposite direction (buffer-RBCs) was slower with a clearance of 0.576 ml/min. In such diffusion studies, plasma produced a greater difference between efflux (1.70 ml/min) and influx (0.276 ml/min) clearances. During liver perfusion, the major factor regulating elimination of tacrolimus was hematocrit. Both well-stirred and parallel-tube models reflected a low extraction ratio drug with values of 0.15 and 0.17 for the 0.05 and 0.2 hematocrits. Intrinsic clearances were 8.43 and 17.44 ml/min for the well-stirred and parallel-tube models. Albumin had a negligible influence on liver extraction of drug. A model-building process of characterizing nonlinear RBC binding, RBC diffusion rates, and liver perfusion parameters allows the complexities of tacrolimus hepatic clearance to be dissected and shows that strong RBC binding can be artificially perceived as causing a high clearance of the drug.

Pharmacokinetic and pharmacodynamic effects of coadministration of methylprednisolone and tacrolimus in rabbits.

Tacrolimus (FK 506) is used for treatment of various organ transplantations in combination with corticosteroids. Rabbits are a good animal model for pharmacokinetic studies of these drugs. The i.v. administration of methylprednisolone (MP; 1.25 mg/kg) (sodium succinate) at 0.5 hr after tacrolimus (0.15 mg/kg) did not influence the disposition of these drugs in rabbits. Clearances of MP were 0.45 liters/hr/kg after given alone and 0.48 liter/hr/kg after tracrolimus. The plasma and whole blood clearances of tacrolimus were 2.18 and 0.21 and 2.35 and 0.19 liters/hr/kg after separate and joint administration with the steroid. The concentrations of corticosterone were variable in all groups and was not useful as a pharmacodynamic parameter. Whole blood histamine concentrations as a marker for basophils did not change after drug administration, but increased over several weeks of experiments. Helper-T cells in rabbits as in humans show a circadian rhythm with an acrophase at 1:00 A.M. MP and tacrolimus decreased the number of circulating helper-T cells with IC50 values of 23 ng/ml for MP and 6.7 ng/ml for tacrolimus. After coadministration of these drugs, an interaction occurs, but the nature (additive or antagonistic) of this interaction was not clear. The IC50 of tacrolimus for in vitro inhibition of lymphocyte proliferation was 0.39 ng/ml (0.47 nM) and 1.02 ng/ml (2.73 nM) for MP. The maximum percentage of inhibition was 94.6% for tacrolimus and 64.9% for MP. The interaction between low concentrations of tacrolimus (0.1-0.3 ng/ml) and MP was additive, but mild antagonism was observed at higher concentration of tacrolimus (1 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling interactions between adrenal suppression and T-helper lymphocyte trafficking during multiple dosing of methylprednisolone.

A physiologic pharmacodynamic model was developed to jointly characterize the effects of corticosteroid treatment on adrenal suppression and T-helper cell trafficking during single and multiple dosing in asthmatic patients. Methylprednisolone (MP), cortisol, and T-helper cell concentrations obtained from a previously published study during single day and 6 days of multiple dosing MP treatment were examined. The formation and disposition kinetics of MP were described with a compartmental model. The biorhythmic profile of basal cortisol secretion rate was analyzed using a recent Fourier approach based on circadian harmonics. A three-compartment loop model was proposed to represent three major T-helper cell pools: blood, extravascular site, and lymph nodes. T-helper cell synthesis and degradation rate constants were obtained from the literature. The suppressive effects of cortisol and MP on T-helper cell concentrations were described with a joint additive inhibition function altering the cell migration rate from lymph nodes to blood. The model adequately described both plasma cortisol profiles and T-helper cells in blood after single and multiple doses of MP. The potency of MP for suppression of cortisol secretion was estimated as IC50 = 0.8 ng/ml. The biorhythmic nature of the basal T-helper cells in blood was well described as under the influence of basal circadian cortisol concentrations with IC50 = 79 ng/ml. The model fitted potency of MP for suppression of T-helper cells was IC50 = 4.6 ng/ml. The observed rebound of T-helper cells in blood can also be described by the proposed model. The rhythm and suppression of plasma cortisol and T-helper cells before and during single and multiple dose MP treatment were adequately described by these extended indirect response models.

Pharmacokinetics and pharmacodynamics of a humanized monoclonal antibody to factor IX in cynomolgus monkeys.

The pharmacokinetics and pharmacodynamics (PK/PD) of a humanized anti-Factor IX IgG1 monoclonal antibody (SB 249417, FIX mAb) were studied in Cynomolgus monkeys. Single i.v. bolus doses of 1, 3, or 10 mg/kg of FIX mAb were administered. The total FIX mAb concentration, activated partial thromboplastin time (aPTT), and Factor IX activity were monitored for up to 4 weeks after dosing. In the monkey, FIX mAb had a plasma clearance of 0.6 ml/h/kg and a steady-state volume of distribution of approximately 70 ml/kg. The elimination phase half-life (3.8 days) was considerably less than other humanized IgG1 mAbs in the monkey, for which there is no binding to endogenous antigen. The suppression of Factor IX activity and the prolongation of aPTT were rapid and dose dependent. The time for aPTT values to return to basal levels (25-170 h) increased with increasing dose. A mechanism-based PK/PD model consistent with the stoichiometry of binding (2:1) was developed to describe the Factor IX activity and aPTT response time course. The model incorporated Factor IX synthesis and degradation rates that were interrupted by the sequestration of Factor IX by the antibody. aPTT values were related to free Factor IX activity. This model was able to describe the PD profiles from the three dose levels simultaneously. The estimated Factor IX half-life was 11 h and the third-order association rate constant was 3.96 x 10(3) microM(-2) h(-1). The PK/PD modeling was useful in summarizing the major determinants (endogenous and antibody-ligand binding) controlling FIX mAb-related effects.