Single-dose pharmacokinetics of the new immunosuppressant RS-61443 in the rabbit

Pharmacokinetics of mycophenolate mofetil following single-dose intravenous and single- and multiple-dose oral administration and clinicopathologic effects of mycophenolate mofetil following long-term oral administration in healthy horses

OBJECTIVE

To characterize the pharmacokinetics of mycophenolate mofetil (MMF) following single-dose IV or PO administration, characterize the pharmacokinetics of MMF following long-term PO administration, and describe the clinicopathologic effects of long-term MMF administration in horses.

ANIMALS

12 healthy adult horses.

PROCEDURES

In phase 1, 6 horses received a single IV (2.5 mg/kg) or PO (5 mg/kg) dose of MMF in a randomized balanced crossover assessment (≥ 2-week interval between administrations). In phase 2, 6 other horses received MMF for 60 days (5 mg/kg, PO, q 24 h for 30 days and then 5 mg/kg, PO, q 48 h for an additional 30 days).

RESULTS

Following IV (single-dose) or PO (single- or multiple-dose) administration, MMF was rapidly converted to mycophenolic acid. For single-dose PO administration, mean ± SD maximum plasma mycophenolic acid concentration was 1,778.3 ± 441.5 ng/mL at 0.71 ± 0.29 hours. For single-dose IV administration, mean systemic clearance and volume of distribution at steady state were 0.689 ± 0.194 L/h/kg and 1.57 ± 0.626 L/kg, respectively. Following single doses, mean terminal half-life was 3.99 ± 0.865 hours for IV administration and 4.02 ± 1.01 hours for PO administration. The accumulation index following long-term PO administration was 1.0 ± 0.002, and the terminal half-life was 4.59 ± 1.25 hours following the final dose on day 60. None of the horses developed abnormal clinical signs or had any consistently abnormal clinicopathologic findings.

CONCLUSIONS AND CLINICAL RELEVANCE

Further investigation of the clinical efficacy of long-term MMF treatment of horses with autoimmune diseases is warranted.

Role of Purine Biosynthesis in Persistent Methicillin-Resistant Staphylococcus aureus Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus endovascular infections. In this study, we investigated potential genetic mechanisms underlying the persistent outcomes. Compared with resolving bacteremia (RB) isolates (defined as isolates associated with negative results of blood cultures 2-4 days after initiation of therapy), PB strains (defined as isolates associated with positive results of blood cultures ≥7 days after initiation of therapy) had significantly earlier onset activation of key virulence regulons and structural genes (eg, sigB, sarA, sae, and cap5), higher expression of purine biosynthesis genes (eg, purF), and faster growth rates, with earlier entrance into stationary phase. Importantly, an isogenic strain set featuring a wild-type MRSA isolate, a purF mutant strain, and a purF-complemented strain and use of strategic purine biosynthesis inhibitors implicated a causal relationship between purine biosynthesis and the in vivo persistent outcomes. These observations suggest that purine biosynthesis plays a key role in the outcome of PB and may represent a new target for enhanced efficacy in treating life-threatening MRSA infections.

Developmental changes of MPA exposure in children

BACKGROUND

Developmental changes (ontogeny) of drug disposition of Mycophenolate mofetil (MMF) have been understudied.

METHODS

The charts of 37 pediatric renal transplant recipients (median age 7.3 years, median follow-up 7.8 (IQR 6.6, 14.3 years) who had regular mycophenolic acid (MPA) trough level monitoring in combination with tacrolimus (n = 31) or sirolimus (n = 6) therapy were analyzed retrospectively for their dose-normalized MPA exposure, steroid dose, albumin, hematocrit, and cystatin C estimated glomerular filtration rate (eGFR). Using appropriate univariate and multivariate methods, we determined whether MPA exposure was age dependent when controlling for the confounders.

RESULTS

Dose-normalized MPA trough levels could be calculated in 2,128 (median 45/patient) instances. Spearman rank correlation analysis revealed that age correlated with dose-normalized MPA trough level for both body weight and body surface area, as well as serum albumin, hematocrit, steroid dose, and eGFR. In the multivariate analysis, serum albumin and steroid dose were not significant, and hematocrit only being significant when the youngest group of patients < 6 years of age was compared. eGFR was the most important confounder, but age dependency remained significant when controlling for all confounders.

CONCLUSIONS

Small children are at a significantly greater risk for low MPA trough levels than adolescents, highlighting the need for pharmacokinetic monitoring of MPA.

Effect of combined immunosuppressive drug therapy on small intestinal nutrient transport in the rat

OBJECTIVE

Prevention of rejection and preservation of graft function remain as obstacles to clinical small intestinal transplantation (SIT). This study evaluated the effects of combined immunosuppressive agents (FK506, Rapamycin, and Mycophenolate Mofetil) on intestinal function and animal well being.

METHODS

Screening for additive toxicity was done in experiment one (D1, n = 10); doses were: FK506 0.3 mg/kg/d, Rapamycin 2 mg/kg/d, and Mycophenolate Mofetil 20 mg/kg/d, orally once daily. Control animals (C1, n = 10) received equivalent vehicle. In the second phase of the experiment, the effect of an additional parenteral treatment phase was investigated, with drug treated animals (D2, n = 6) received FK506 0.3 mg/kg, Rapamycin 1 mg/kg, and Mycophenolate Mofetil 10 mg/kg sq q12h for 1 week followed by FK506 3 mg/kg, Rapamycin 1 mg/kg, and Mycophenolate Mofetil 10 mg/kg p.o. q12h for 4 weeks. Control animals (C2, n = 6) received equivalent vehicle. Parameters followed were weight gain, nutrient absorption, drug levels and nutrient transport in vitro.

RESULTS

Controls grew normally, while weight gain was significantly reduced in drug treated animals: This was paralleled by a reduction in dietary fat absorption. Drug levels were low to therapeutic for all drugs in both experiments; FK506 appeared to affect Rapamycin and Mycophenolate Mofetil metabolism, increasing levels of both as FK506 doses increased. Nutrient transport was either not effected (D1) or increased (D2).

CONCLUSIONS

We conclude that low dose combination immunosuppressive therapy inhibits weight gain, without affecting absorption of dietary energy, or adversely affecting glucose transport. We postulate a systemic metabolic cause, which requires additional investigation at the cellular level; additional studies are also required to determine if the additive immunosuppression outweigh the side effects for SIT.

Simple method for the quantitation of mycophenolic acid in human plasma

A simple and rapid isocratic reversed-phase high-performance liquid chromatographic method using UV detection was developed for the quantitation of mycophenolic acid (MPA) in human plasma. The assay was sufficiently robust to allow the analysis of up to 100 samples in a single analytical run.

Pharmacodynamic monitoring of mycophenolic acid in rabbit heterotopic heart transplant model

Pharmacodynamic (PD) monitoring of immunosuppressive drugs provides a novel approach to optimization of drug therapy in transplant recipients. We chose to investigate this using mycophenolic acid (MPA), an immunosuppressive drug that mediates its effect by the inhibition of inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the de novo biosynthesis of purines. A comparison of the relationship between PD versus drug level monitoring was performed using a heterotopic cardiac transplant in New Zealand white rabbits. The animals were divided into four different treatment groups. Control animals were administered the drug vehicle, the treatment groups were administered mycophenolate mofetil (MMF) at doses of 40, 80, and 160 mg/kg/day. Statistically significant (p < 0.05) prolongation of graft survival was obtained at the 160 mg/kg/day dose group. The mean MPA concentration at this dose was approximately 2.5 mg/l, suggesting that this concentration may provide adequate immunosuppression. An increase in IMPDH activity appeared a few days prior to rejection, suggesting that measurement of enzyme activity may have potential for use as a marker of graft rejection. A significant (p < 0.05) relationship exists between MPA concentration and graft survival and the former with dose of MMF. There was a negative correlation (p = 0.17) between MPA concentration and IMPDH activity, while a trend (p = 0.37) to inverse relationship between graft survival and IMPDH activity was found. The data suggests that the measurement of the biological response may provide a useful adjunct to traditional therapeutic drug monitoring (TDM) for optimization of dosing of immunosuppressive drugs.

RS-61443 prevents microvascular rejection of pancreatic islet xenografts

Inadequate vascularization and microvascular rejection are major limitations for successful free pancreatic islet xenotransplantation. Commonly used immunosuppressive regimens may alter the process of vascularization, and are ineffective at preventing graft rejection. In this study, we investigated, in vivo, the action of the new immunosuppressive agent RS-61443 on angiogenesis and microvascular rejection of rat pancreatic islets after xenogeneic transplantation into the dorsal skinfold of Syrian golden hamsters. In nontreated xenografts, intravital fluorescence microscopy demonstrated a regular process of vascularization during the first 6 days after transplantation. On days 10, 14, and 20, graft rejection was observed, characterized by microvascular leukocyte accumulation (244+/-59 mm(-2)), loss of endothelial integrity, and capillary perfusion failure. Islet xenografts of animals treated with RS-61443 (40 mg/kg per day) demonstrated inhibition of vascularization with the consequence of a markedly reduced size of the grafts' microvascular network (0.05+/-0.007 mm2), when compared with that of nontreated xenografts (0.09+/-0.015 mm2; P< 0.05). However, treatment with RS-61443 effectively prevented microvascular graft rejection, as indicated by the absence of leukocyte accumulation (24+/-9 mm(-2); P<0.01), endothelial damage, and nutritive perfusion failure. Thus, RS-61443 treatment may represent an interesting approach for improving the outcome of pancreatic islet xenotransplantation.

Pharmacology of immunosuppressive medications used in renal diseases and transplantation

As understanding of the molecular basis for the immune response has expanded rapidly, so have the possibilities for designing therapeutic interventions that are more effective, more specific, and safer than current treatment options. The promise of therapeutic advances in the future is based on the rapidly expanding insights into the pathogenesis of abnormal immunologic reactions. Nowhere is the understanding of molecular mechanisms, pathophysiology, and targeted therapy more relevant than in the field of renal transplantation, which makes up much of the clinical database for the use of immunosuppressive therapy for renal disease. Despite the recent advances in basic immunology, clinical validation of new agents and approaches is lacking for most drugs at present. This review will focus in the pharmacology of agents used in the therapy of immunologic renal disease and in renal transplantation. It should be recognized that clinical pharmacology and experience with newer agents is limited, and potential utility is based largely on experimental data.

Pharmacodynamic assessment of mycophenolic acid-induced immunosuppression in renal transplant recipients

The combination of pharmacokinetic and pharmacodynamic monitoring of immunosuppressive drugs provides a novel method for the optimization of drug dosing. We chose to investigate this with the use of mycophenolic acid (MPA), an immunosuppressive drug that mediates its effect by the inhibition of inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the de novo biosynthesis of purines. The relationship between MPA concentration in plasma, IMPDH activity in whole blood, and nucleotide concentration in lymphocytes was investigated in renal-transplant recipients, who were randomized to receive either mycophenolate mofetil (MMF) (n = 5) or azathioprine (AZA) (n = 7), in combination with cyclosporine and prednisone. Blood samples were collected throughout the dosing interval. Pharmacokinetic analysis revealed substantial variability among the patients in the absorption and clearance of MPA. An inverse relationship was found between the MPA concentration of IMPDH activity in whole blood. The peak concentration of MPA achieved at 1 hr after dosing resulted in approximately 40% inhibition of IMPDH activity. As the MPA concentration decreased throughout the dosing interval, there was a gradual restoration of IMPDH activity. The inhibition of IMPDH activity (P < 0.05) in MMF-treated patients as compared with the AZA-treated controls was maintained for approximately 8 hr after dosing. No statistically significant (P > 0.05) difference between the predose and the 12 hr postdose activity was observed. The concentrations of guanine nucleotides, GDP and GMP, were significantly lower than in the AZA-treated group at most of the time points after dosing; however, considerable variability was observed. The measurement of the pharmacodynamic response to immunosuppressive drugs may provide not only a mechanism to predict the most appropriate dosing regimen, but also a viable alternative to traditional therapeutic drug monitoring, by assessing the overall state of immunosuppression.

Pharmacokinetics of mycophenolate mofetil, a new immunosuppressant, in rats

Mycophenolate mofetil (MPM), a new immunosuppressant, is the morpholinoethyl ester of mycophenolic acid (MPA). The distribution in blood and pharmacokinetics of MPA after administration of MPM were examined. The plasma to erythrocyte concentration ratio was low (0.10-0.15). MPA existed in rat plasma as the highly bound form (bound fraction was 9.79 +/- 0.57%). MPA disappeared from the systemic circulation with biexponential decay. After i.v. administration of MPM at the doses of 8.3, 16.7, and 33.3 mg/kg, the total clearance of MPA was 0.241 +/- 0.056, 0.321 +/- 0.126, and 0.317 +/- 0.092 L/h/kg, respectively. The terminal elimination half-live were 5.17 +/- 1.44, 8.89 +/- 2.76, and 7.94 +/- 2.94 h, respectively. After i.d. administration of MPM at the doses of 8.3, 16.7, 33.3, and 50.0 mg/kg, the terminal elimination half-live were 6.41 +/- 4.16, 4.49 +/- 2.20, 7.58 +/- 3.72, and 8.18 +/- 1.32 h, respectively. The mean peak times were within 30 min. The systemic availability of MPA after i.d. administration of MPM (at 8.3, 16.7, and 33.3 mg/kg) was calculated using the corresponding mean AUCiv, and the values were 84.3 +/- 35.0%, 69.9 +/- 25.7%, and 63.6 +/- 8.8%, respectively.

Pharmacodynamic assessment of mycophenolic acid-induced immunosuppression by measurement of inosine monophosphate dehydrogenase activity in a canine model

The combination of pharmacokinetic and pharmacodynamic (measurement of the biological effect) monitoring of immunosuppressive drugs provides a method for the optimization of drug dosing. We chose to investigate this using mycophenolic acid (MPA), an immunosuppressive drug that mediates its effect by the inhibition of inosine monophosphate dehydrogenase (IMPDH), a key enzyme in the de novo biosynthesis of purines. Using an assay developed for measurement of IMPDH activity in whole blood, the concentration required for 50% inhibition of IMPDH activity was approximately 200 mg/L (58 +/- 8.3% for whole blood [n = 6] and 55 +/- 10.0% for isolated lymphocytes). To ascertain the relationship between MPA concentration and IMPDH inhibition in vivo, dogs were administered a single dose of mycophenolate mofetil, the pro-drug of MPA, at 20 or 40 mg/kg orally. Pharmacokinetic analysis revealed that the Cmax of the 40-mg/kg group was statistically greater than that of the 20-mg/kg group (P < 0.05). There were no statistical differences in the other parameters investigated (area under the curve, beta half-life, mean residence time, volume of distribution at steady state, and clearance) between the two treatment groups. The half-life was calculated at approximately 8 hr for both dose groups. There was also substantial variability among the dogs in the absorption and clearance of MPA. An inverse relationship was found between the MPA concentration and IMPDH. Maximal inhibition of IMPDH activity of 30-40% occurs approximately 2-4 hr after dosing, followed by a gradual restoration in enzyme activity. After 24 hr, there is an increase in IMPDH activity that exceeds the pre-dosing levels in some cases by 3-fold. Evaluation of the pharmacokinetic and the pharmacodynamic responses to MPA in the canine model suggests that the drug should be administered ever 8 hr to optimize its immunosuppressive efficacy. This combined approach can be used for optimization of doses of this and other immunosuppressive drugs.