Feasibility of, and critical paths for mycophenolate mofetil Bayesian dose adjustment: pharmacological re-appraisal of a concentration-controlled versus fixed-dose trial in renal transplant recipients.

The aim of this study was to analyze retrospectively and critically the different steps of the individual dose adjustment procedure employed in the concentration-controlled (CC) versus fixed-dose trial Apomygre, which showed that mycophenolate mofetil (MMF) dose adjustment using a limited sampling strategy significantly reduced the risk of treatment failures and acute rejection in renal transplants at one year posttransplantation. The number of AUCs performed during the study and circumstances of collection, time of blood sampling, Bayesian mycophenolic acid (MPA) area-under-the-curve (AUC) estimation procedures and physicians' compliance with MMF dose recommendations were retrospectively analyzed. 92% of AUCs scheduled over the study were actually performed. Sampling times were very well respected. Bayesian estimation of MPA exposure was done by the pharmacologists locally in accordance with the protocol instructions and the AUC estimates obtained were virtually all confirmed a posteriori. On the other hand, a second AUC estimated by multiple linear regression could only be provided for 84% of the profiles and showed a large overestimation with respect to Bayesian estimates for AUC values between 10 and 55mgh/L. In the CC arm, a very good physicians' compliance was observed (85%) and application of the dose recommendations led to higher values of AUCs (42.1+/-14.6mgh/L versus 36.7+/-16.3mgh/L, p=0.0035) and to more AUCs in the target range (69% versus 56%, p=0.0343) than when dose recommendations were not applied. By analyzing in detail the feasibility criteria of MMF Bayesian dose adjustment, this study highlighted the requirements for successful extrapolation of the Apomygre trial results to routine practice: (i) respect of the PK sampling time-windows; (ii) use of relevant tools for accurate drug exposure estimation and dose adjustment calculation; and (iii) good compliance of the physicians with regard to the recommended doses.

Renal vascular reactivity to vasopressin in rats with diabetes mellitus.

We evaluated how renal vascular reactivity to vasopressin changes when nitric oxide (NO) synthesis varies, as has been reported to occur in the course of insulin-dependent diabetes mellitus. Renal vasoconstrictor responses to vasopressin were obtained in young and older Sprague-Dawley control rats (3 and 10 months old) and in age-matched diabetic rats that had been treated with streptozotocin (60 mg/kg i.v.) at the age of 2 months. In young rats, vasopressin (3-1000 ng/kg/min i.v.) induced in vivo a dose-dependent decrease in renal blood flow, which was diminished in streptozotocin diabetic rats (P<0.05). Similarly, in in vitro perfused kidneys, the concentration-response curve for vasopressin (0.03-10 nM) was shifted 3-fold to the right in kidneys isolated from young diabetic rats (P<0.05). This shift was abolished in the presence of an inhibitor of nitric oxide synthesis, N(G)-nitro-L-arginine (100 microM), in the perfusate. In 10-month-old rats, the in vivo renal vasoconstrictor dose-response curve to vasopressin was shifted 10-fold to the left as compared to that for young rats (P<0.001). This shift was similar in both control and diabetic rats. In conclusion, the present study documented the existence of hyporesponsiveness to vasopressin in the early stage of diabetes, possibly related to nitric oxide overproduction. In contrast, renal vascular hyperreactivity to vasopressin occurs with aging, whether the rats are diabetic or not.

High concentrations of oxytocin cause vasoconstriction by activating vasopressin V1A receptors in the isolated perfused rat kidney.

The aim of this study was to evaluate the renal vascular effects of oxytocin in Sprague-Dawley rats and in Brattleboro heterozygous or homozygous rats, the latter being genetically deficient in vasopressin synthesis. Studies were performed in vitro, in the isolated kidney perfused in an open circuit with a Tyrode's solution. Oxytocin induced a concentration-dependent renal vasoconstriction in Sprague-Dawley rats, at rather high concentrations (EC50=170+/-39 nM, mean +/- SEM, n=6) with a maximum response amounting to 44% of that elicited by vasopressin (increase in renal vascular resistance: 11.5+/-0.9 mmHg min ml(-1) vs. 26.2+/-2.2 mmHg min ml(-1)). Oxytocin-evoked renal vasoconstriction was abolished by SR 49059, a selective vasopressin V1A receptor antagonist (10 nM), but not by d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-(NH2)9] vasotocin, an oxytocin receptor antagonist (10 nM). In the presence of SR 49059, oxytocin did not induce renal vasorelaxation. Oxytocin induced renal vasoconstriction in Brattleboro homozygotes and heterozygotes (EC50=59+/-12 nM and 262+/-110 nM; Emax=7.8+/-1.1 mmHg min ml(-1) and 6.9+/-0.4 mmHg min ml(-1), n=5 respectively) with characteristics similar as observed in Sprague-Dawley rats concerning partial agonist activity, low potency and antagonism by SR 49059. Responsiveness to vasopressin did not differ in Brattleboro homozygotes and heterozygotes (EC50 approximately 0.25 nM) and was similar as we reported in Sprague-Dawley rats. These findings indicate that high concentrations of oxytocin induce renal vasoconstriction in the rat by activating vasopressin V1A receptors. The low agonist activity makes it unlikely that oxytocin can substitute functionally for vasopressin at the renal vascular V1A receptor in Brattleboro homozygous rats which are deficient in endogenous vasopressin.

Nitric oxide, but not vasopressin V2 receptor-mediated vasodilation, modulates vasopressin-induced renal vasoconstriction in rats.

The renal vascular response to vasopressin and its modulation were evaluated in vivo by infusing the peptide directly into the renal artery of anaesthetized rats. The intra-renal artery (i.r.a) infusion of vasopressin induced a dose-dependent decrease in renal blood flow. Vasoconstriction was obvious at a dose of 3 ng/kg per min and reached a maximum at 100 ng/kg per min. The dose required for a half-maximal response (ED50) was 24+/-4 ng/kg per min (mean+/-SEM, n=8), corresponding to an estimated concentration in renal arterial blood required for a half-maximal response (EC50) of 1.9+/-0.6 nM. Thiobutabarbitone anaesthesia markedly increased plasma vasopressin concentration. This increase was prevented partially by hypotonic hydration of the rats without any change in the renal vascular response to exogenous vasopressin. Vasopressin-induced vasoconstriction dose/response curves were similar in homozygous and heterozygous Brattleboro rats. Infusion of desmopressin (1-1000 ng/kg per min, i.r.a.), a vasopressin V2 receptor-selective agonist, failed to induce renal vasodilation or vasoconstriction. In the presence of SR 49059 (1 mg/kg i.v.), a vasopressin V1A receptor antagonist that completely abolished the vasopressin-induced renal vasoconstriction, desmopressin again failed to induce vasodilation. Inhibition of nitric oxide synthase by N(omega)-nitro-L-arginine (L-NNA, 100 microg/kg for 10 min and 7.5 microg/kg per min, i.r.a.) enhanced vasopressin-induced renal vasoconstriction (EC50 0.6+/-0.1 nM, P<0.05). In contrast, cyclooxygenase blockade by indomethacin (5 mg/kg, i.v.) neither modified the vasopressin-induced decrease in renal blood flow nor altered the potentiation of vasoconstriction by L-NNA. These results show that the constrictor response of the rat renal vascular bed in vivo is observed only with high local concentrations of vasopressin. This hyporeactivity in vivo was not explained by an anaesthesia-elicited increase in endogenous vasopressin, nor by a modulatory effect linked to V2 receptor activation or prostanoid release. In contrast, NO release contributed to the attenuation of vasopressin-induced renal vasoconstriction.

[Modulation by nitric oxide of vasopressin induced renal vasoconstriction varies with perfusate viscosity in the isolated rat kidney]. / La modulation par le monoxyde d'azote de la vasoconstriction rénale induite par la vasopressine varie avec la viscosité du perfusat sur le rein isolé perfusé de rat.

Previously we reported that AVP is a potent vasoconstrictor in the TYRODE's perfused rat kidney. In vivo however AVP elicited only minor effects on renal blood flow. We hypothetized that differences in shear stress, particularly related to differences in viscosity could be involved. In this study, we investigate the role of perfusate viscosity in the modulation of AVP-induced renal vasoconstriction by NO. Experiments were performed in kidneys isolated from male Sprague-Dawley rats (220 g). Kidneys were perfused at a constant flow of 8 mL/min, in a recirculating system, with TYRODE's solutions supplemented with 6% bovin serum albumin (BSA) or 4.7% Ficoll 400 (Ficoll). The viscosities relative to water were respectively of 1.33 (BSA), 2.32 (Ficoll) and 1.03 (TYRODE). Concentration-response curves to AVP were constructed in the absence or presence of 100 microM N omega-nitro-L-arginine (L-NA), an inhibitor of NO synthase, and compared to those obtained in kidneys perfused with TYRODE's solution. AVP elicited a concentration-dependent renal vasoconstriction, with a progressive shift of the curves to the right and a small decrease in the maximum response when the kidneys were perfused with perfusates of increasing viscosities: logEC50 = -9.9 +/- 0.1 (TYRODE, n = 14), -9.7 +/- 0.1 (BSA, n = 5), -9.0 +/- 0.1 (Ficoll, n = 5) (m +/- e.s.m. Anova, p < 0.001); Emax = 34 +/- 1, 31 +/- 2 and 26 +/- 3 mmHg/mL/min (Anova, p < 0.001). L-NA abolished the differences between kidneys perfused with solutions of different viscosities in logEC50 for vasopressin (10.3 +/- 0.1, 10.4 +/- 0.1 and 10.5 +/- 0.1, n = 5-11, Anova, NS) but did not affect Emax values. In conclusion, present results show that 1) AVP-induced renal vasoconstriction is modulated according to the viscosity of perfusate and 2) NO is involved in this effect. Viscosity, a major determinant of shear stress, should be considered in hemodynamic studies performed on isolated kidneys.

Vasopressin does not effect hypertension caused by long-term nitric oxide inhibition.

Nitric oxide attenuates both vasopressin-induced vasoconstriction and vasopressin release. We tested whether hypertension and renal dysfunction elicited by chronic inhibition of nitric oxide (NO) synthesis using N(G)-nitro-L-arginine (L-NNA) could be mediated in part by vasopressin V(1A) receptors. Male rats were treated orally for 6 weeks with L-NNA (15 mg/kg per day), a nonpeptide V(1A) receptor antagonist (2S)-1-[(2R,3S)-5-chloro-3-(2-chlorophenyl)-1-(3, 4-dimethoxybenzene-sulfonyl)-3-hydroxy-2, 3-dihydro-1H-indole-2-carbonyl]-pyrrolidine-2-carboxamide (SR 49059, 30 mg/kg per day), or a combination of SR 49059 and L-NNA (same doses), or they received no treatment. Both drugs were added to the food. Measurements were performed in conscious rats (urine collection in metabolic cages, tail-cuff arterial pressure) and at the end of the study in anesthetized rats (clearance measurements). L-NNA produced sustained hypertension, decreased glomerular filtration rate, and increased renal vascular resistance, plasma renin activity, and urinary albumin excretion. SR 49059 had no effect per se on these parameters and also did not attenuate the hypertension and renal dysfunction induced by L-NNA. Surprisingly, SR 49059 potentiated L-NNA-induced hypertension at the end of the 6-week treatment. However, the blood pressure response and the renal and mesenteric vasoconstriction elicited by exogenous vasopressin were attenuated in rats treated with SR 49059. L-NNA did not change plasma vasopressin concentration or 24-hour urinary vasopressin excretion. Our findings suggest that activation of vasopressin V(1A) receptors does not contribute to the hypertension and renal dysfunction induced by chronic NO synthesis inhibition. They also document unchanged plasma vasopressin concentration in NO-deficient hypertension.