Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Pharmacokinetics and renal excretion of desmopressin after intravenous administration to healthy subjects and renally impaired patients

OBJECTIVE

To evaluate the influence of renal impairment on the pharmacokinetics of desmopressin.

METHODS

Twenty-four subjects were enrolled in the study, 18 with varying degrees of renal impairment and six healthy volunteers. Each subject received a single intravenous dose of 2 microg desmopressin. Blood and urine samples were collected for 24 h and assayed for desmopressin by radioimmunoassay. Plasma concentrations and the amounts of desmopressin excreted in the urine were analysed simultaneously by use of mixed effects modelling.

RESULTS

Only mild adverse events were observed. Both the renal and the nonrenal clearance of desmopressin were found to vary with the creatinine clearance (CrCL). A decrease of 1.67% in the CrCL (corresponding to 1 ml min(-1) from 60 ml min(-1)) was found to cause a 1.74% decrease in the renal clearance and a 0.93% decrease in the nonrenal clearance. The fall in renal clearance caused the amount of desmopressin excreted in urine to decrease from 47% in healthy subjects to 21% in the patients with severe renal impairment. The mean systemic clearance of desmopressin was 10 litres h(-1) in healthy subjects and 2.9 litres h(-1) in patients with severe renal impairment (difference -7.5 litres h(-1), 95% CI [-11; -4.3] litres h(-1)). Correspondingly, the mean terminal half-life, was 3.7 h in healthy subjects and 10 h in patients with severe renal impairment (difference 6.7 h, 95% CI [4.0; 9.4] h).

CONCLUSION

Although desmopressin appears to be safe and well-tolerated by patients with impaired renal function, great caution should be exercised when titrating towards an efficient dosage regimen if patients with moderately or severely impaired renal function are to be treated with desmopressin at all.

Antidiuretic activity of aqueous bark extract of Sri Lankan Ficus racemosa in rats

The decoction (D) of the bark of Ficus racemosa Linn (Family: Moraceae) is claimed as an antidiuretic by some Sri Lankan traditional practitioners. However, the validity of this claim has not been scientifically proven or refuted. The aim of this study was to evaluate the antidiuretic potential of D of the bark of F. racemosa (made as specified in traditional use) in rats using three doses (250, 500 or 1000 mg/kg) following oral administration. The reference drug used was ADH. The results demonstrated both the low- and high-doses of D and ADH significantly impaired the total urine output. The D-induced antidiuresis had a rapid onset (within 1 h), peaked at 3 h and lasted throughout the study period (5 h). However, antidiuretic potential of D was about 50% lower than that of ADH. The D was well tolerated even with subchronic administration. The D caused a reduction in urinary Na+ level and Na+/K+ ratio, and an increase in urinary osmolarity indicating multiple mechanisms of action. The results provide scientific support for its claimed antidiuretic action and deserve intensive scrutiny.

Characterization of a new selective antagonist for angiotensin-(1-7), D-pro7-angiotensin-(1-7)

Angiotensin-(1-7) [Ang-(1-7)] has biological actions that can often be distinguished from those of angiotensin II (Ang II). Recent studies indicate that the effects of Ang-(1-7) are mediated by specific receptor(s). We now report the partial characterization of a new antagonist selective for Ang-(1-7), D-Pro7-Ang-(1-7). D-Pro7-Ang-(1-7) (50 pmol) inhibited the hypertensive effect induced by microinjection of Ang-(1-7) [4+/-1 vs 21+/-2 mm Hg, 25 pmol Ang-(1-7) alone] into the rostral ventrolateral medulla without changing the effect of Ang II (16+/-2.5 vs 19+/-2.5 mm Hg after 25 pmol Ang II alone). At 10(-7) mol/L concentration, it completely blocked the endothelium-dependent vasorelaxation produced by Ang-(1-7) (10(-10) to 10(-6) mol/L) in the mouse aorta. The antidiuresis produced by Ang-(1-7) (40 pmol/100 g body weight) in water-loaded rats was also blocked by its analog [1 microg/100 g body weight; 3.08+/-0.8 vs 1.27+/-0.33 mL in Ang-(1-7)-treated rats]. D-Pro7-Ang-(1-7) at a molar ratio of 40:1 did not change the hypotensive effect of bradykinin. Moreover, D-Pro7-Ang-(1-7) did not affect the dipsogenic effect produced by intracerebroventricular administration of Ang II (11.4+/-1.15 vs 8.8+/-1.2 mL/h after Ang II) and did not show any demonstrable angiotensin-converting enzyme inhibitory activity in assays with the synthetic substrate Hip-His-Leu and rat plasma as a source of enzyme. Autoradiography studies with 125I-Ang-(1-7) in mouse kidney slices showed that D-Pro7-Ang-(1-7) competed for the binding of Ang-(1-7) to the cortical supramedullary region. In Chinese hamster ovary cells stably transfected with the AT1 receptor subtype, D-Pro7-Ang-(1-7) did not compete for the specific binding of 125I-Ang-II in concentrations up to 10(-6) mol/L. There was also no significant displacement of Ang II binding to angiotensin type 2 receptors in membrane preparations of adrenal medulla. These data indicate that D-Pro7-Ang-(1-7) is a selective antagonist for Ang-(1-7), which can be useful to clarify the functional role of this heptapeptide.

Peritubular AVP regulates bicarbonate reabsorption in cortical distal tubule via V(1) and V(2) receptors

Peritubular arginine vasopressin (AVP) regulates bicarbonate reabsorption in the cortical distal tubule via V(1) and V(2) receptors. The dose-dependent effects of peritubular AVP on net bicarbonate reabsorption (J(HCO)) were evaluated by stationary microperfusion of in vivo early (ED; distal convoluted tubule) and late distal (LD; connecting tubule and initial collecting duct) segments of rat kidney, using double-barreled H(+)-sensitive, ion-exchange resin/reference (1 M KCl) microelectrodes. AVP (10(-11) M) perfused into peritubular capillaries increased J(HCO), compared with basal levels during intact capillary perfusion with blood, in ED and LD segments. AVP (10(-9) M) also increased J(HCO) in both segments, but the effect of AVP (10(-11) M) was significantly higher. A specificV(1)-receptor antagonist alone or with AVP (10(-11) or 10(-9) M) reduced J(HCO) below basal levels. A specific V(2)-receptor antagonist alone or plus AVP (10(-11) M) did not affect J(HCO) but increased AVP (10(-9) M)-mediated stimulation. 8-Bromoadenosine 3',5'-cyclic monophosphate alone reduced J(HCO) below basal levels and also reduced AVP (10(-11) M)-mediated stimulation. (Deamino-Cys(1), D-Arg(8)) vasopressin (a V(2)-selective agonist) also reduced J(HCO) below basal levels. These results show that peritubular AVP stimulates J(HCO) in ED and LD segments via basolateral V(1) receptors and that basolateral V(2) receptors have a dose-dependent inhibitory effect mediated by cAMP. The data also indicate that endogenous AVP stimulates distal J(HCO) via basolateral V(1) receptors.

Inhibition by transforming growth factor-beta1 of the cellular action of arginine vasopressin in cultured rat glomerular mesangial cells

The present study was undertaken to determine whether transforming growth factor (TGF)-beta1 modulates the cellular actions of arginine vasopressin (AVP) in cultured rat glomerular mesangial cells. AVP increased cytosolic free calcium ([Ca2+]i), and TGF-beta1 dose-dependently reduced the AVP-mobilized [Ca2+]i. Such an inhibition by exogenous TGF-beta1 was abolished by liposomal transfection of antisense oligodeoxynucleotide for the TGF-beta type II receptor. AVP activated mitogen-activated protein (MAP) kinase, which was significantly reduced by 1 ng/ml TGF-beta1. AVP increased [3H]thymidine incorporation into mesangial cells in a dose-dependent manner, and 1 ng/ml TGF-beta1 significantly reduced the AVP-stimulated [3H]thymidine incorporation. However, 10 microM antisense oligodeoxynucleotide for the TGF-beta type II receptor seemed to attenuate the inhibition by TGF-beta1. 1 X 10(-7) M AVP significantly increased inositol 1,4,5-trisphosphate (IP3) production by 1.8-fold, but this production was totally blunted by 1 ng/ml TGF-beta1. TGF-beta1 did not affect [3H]AVP receptor binding. 1 X 10(-6) M AVP concentration stimulated TGF-beta1 production in mesangial cells by 4-fold. These results indicate that TGF-beta1 inhibits the cellular signaling of AVP at steps beyond the AVP receptors and prior to the phospholipase C activation, and that TGF-beta1 may participate in a negative feedback regulation on the cellular action of AVP in glomerular mesangial cells.

Stability and perfusion studies of Desmopressin (dDAVP) and prodrugs in the rat jejunum

Three aliphatic carboxylic acid esters of the tyrosine phenolic group in Desmopressin (dDAVP) were investigated in vitro for their stability and metabolism in rat gastrointestinal media. The degradation followed strictly first-order kinetics and the prodrugs were quantitatively converted to dDAVP. The n-hexanoyl (II) and n-octanoyl (III) esters were rapidly hydrolysed in 10% rat jejunal fluid showing half-lives of 1.1+/-0.2 min and 1.4+/-0.1 min, respectively. In 5 % rat jejunal homogenate the half-lives were 3.2+/-0.2 min and <30 sec, respectively. The sterically hindered pivalate ester (I) proved to be more stable. The half-lives were 10.3+/-0.3 min in 10% rat jejunal fluid and 1.5+/-0.1 min in 10% rat jejunal homogenate, respectively. The presence of paraoxon, an inhibitor of type B esterases significantly decreased the degradation rate of the pivalate ester (I) in rat jejunal fluid (t1/2 > 5 hrs) indicating that the prodrug is converted to dDAVP by rapid luminal breakdown of the ester bond. It was shown that approximately 13 % of prodrug I disappeared from the gut lumen during a single-pass perfusion experiment in rat jejunum. Our results indicate that the disappearance from the jejunal lumen was primarily caused by degradation of the prodrug to dDAVP by esterases rather than absorption. The better stability of the sterically hindered prodrug (I) indicate that even more sterically hindered prodrugs will be a better choice for a further optimization of stability and lipophilicity, and consequently a potentially improved intestinal absorption of dDAVP.

Pathophysiology of hyponatremia after transsphenoidal pituitary surgery

Hyponatremia after pituitary surgery is presumed to be due to antidiuresis; however, detailed prospective investigations of water balance that would define its pathophysiology and true incidence have not been established. In this prospective study, the authors documented water balance in patients for 10 days after surgery, monitored any sodium dysregulation, further characterized the pathophysiology of hyponatremia, and correlated the degree of intraoperative stalk and posterior pituitary damage with water balance dysfunction. Ninety-two patients who underwent transsphenoidal pituitary surgery were studied. To evaluate posterior pituitary damage, a questionnaire was completed immediately after surgery in 61 patients. To examine the osmotic regulation of vasopressin secretion in normonatremic patients, water loads were administered 7 days after surgery. Patients were categorized on the basis of postoperative plasma sodium patterns. After pituitary surgery, 25% of the patients developed spontaneous isolated hyponatremia (Day 7 +/- 0.4). Twenty percent of the patients developed diabetes insipidus and 46% remained normonatremic. Plasma arginine vasopressin (AVP) was not suppressed in hyponatremic patients during hypoosmolality or in two-thirds of the normonatremic patients after water-load testing. Only one-third of the normonatremic patients excreted the water load and suppressed AVP normally. Hyponatremic patients were more natriuretic, had lower dietary sodium intake, and had similar fluid intake and cortisol and atrial natriuretic peptide (ANP) levels compared with normonatremic patients. Normnonatremia, hyponatremia, and diabetes insipidus were associated with increasing degrees of surgical manipulation of the posterior lobe and pituitary stalk during surgery. The pathophysiology of hyponatremia after transsphenoidal surgery is complex. It is initiated by pituitary damage that produces AVP secretion and dysfunctional osmoregulation in most surgically treated patients. Additional events that act together to promote the clinical expression of hyponatremia include nonatrial natriuretic peptide-related excess natriuresis, inappropriately normal fluid intake and thirst, as well as low dietary sodium intake. Patients should be monitored closely for plasma sodium, plentiful dietary sodium replacement, mild fluid restriction, and attention to symptoms of hyponatremia during the first 2 weeks after transsphenoidal surgery.

[Evaluation of 20 years' experience of enuresis in children]

The clinical and urodynamic approach to enuretic children over a period of more than 20 years has allowed the authors to develop a multifactorial pathophysiological concept of this disorder. The main factors involved are psychological, familial, genetic, vesical, due to bladder immaturity, hormonal, due to a defect of nocturnal ADH secretion, hygiene and dietary habits, etc. The sleep factor is predominant in the majority of cases. Although nocturnal enuresis is apparently isolated in many cases, it is often associated with a state of bladder immaturity, sometimes latent during the day, but occurring at night with episodes of detrusor hyperactivity, occurring during various phases of sleep. In practice, the recognition, in children, of these factors, some of which require specific treatments, implies a management combining several of these therapeutic modalities.

Hyponatremia in the rat in the absence of positive water balance

The purpose of this report is to determine the mechanisms that lead to hyponatremia when isotonic saline was the only fluid infused into rats given antidiuretic hormone (ADH), and what might minimize the degree of this hyponatremia. Normal rats were deprived of food and water for the 24-hr study period. They received an infusion of isotonic saline to expand their extracellular fluid (ECF) volume with and without exogenous ADH administration (N = 8 in each of the four groups). Similar studies were also carried out in 32 rats fed a low electrolyte diet for 72 hr before the experiment. An additional control group was fed the low electrolyte diet supplemented with sodium (Na), potassium (K), and chloride (Cl). Hyponatremia developed over 24 hr in rats fed their usual diet if treated with ADH and isotonic saline (fall, 13 +/- 2 mM, P < 0.01). The hyponatremia was caused by negative balance for Na + K salts. Hyponatremia did not develop after the saline + ADH treatment if rats were pretreated for 3 days with a low electrolyte diet. Two factors were required to develop this hyponatremia--generation of electrolyte-free water as a result of the excretion of a large quantity of Na + K salts at a high concentration in the urine, and prevention of the excretion of this electrolyte-free water by ADH. Increasing the avidity for Na reabsorption by the kidney prevented this type of hyponatremia from developing.

Receptor subtype for vasopressin-induced release of nitric oxide from rat kidney

The vasopressin receptor subtype that causes nitric oxide (NO) release remains controversial. To elucidate this receptor-ligand interaction, we examined the effects of vasopressin receptor antagonists on vasopressin-induced release of NO from isolated perfused rat kidneys by using a sensitive chemiluminescence assay. Vasopressin increased renal perfusion pressure and NO signals in the perfusate in a dose-dependent manner. N omega-Monomethyl-L-arginine abolished this increase in NO release; however, a similar increase in renal perfusion pressure induced by prostaglandin F2 alpha was not associated with the increase in NO release. OPC-21268, a V1 receptor antagonist, significantly reduced the vasopressin-evoked renal vasoconstriction and NO release, whereas OPC-31260, a V2 receptor antagonist, had no effects. Moreover, desmopressin, a selective V2 receptor agonist, did not increase the NO signal. NO release by vasopressin was markedly attenuated in deoxycorticosterone acetate (DOCA)-salt hypertensive rat kidneys compared with control kidneys (10(-10) mol/L vasopressin: +0.8 +/- 0.3 versus +6.9 +/- 1.4 fmol/min per gram kidney, DOCA versus control; P < .001). Histochemical analysis for renal NO synthase revealed a substantial attenuation of the staining of endothelial NO synthase in DOCA-salt rats. These results directly demonstrate that vasopressin stimulates NO release via the endothelial V1 receptor in the rat kidney.

Cellular signaling and proliferative action of AVP in mesangium of SHR: effect of low density lipoprotein

The present study was undertaken to determine whether low density lipoprotein (LDL) modulates the cellular action of arginine vasopressin (AVP) in cultured glomerular mesangial cells of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The AVP-induced cellular signal transduction, including inositol 1,4,5-trisphosphate (IP3) production, fura-2 intracellular calcium measurements and cellular alkalinization, was significantly greater in cells of SHR than those of WKY. This is based on an increase in AVP V1 receptor number in cells of the SHR. Also, the AVP activation of mitogen-activated protein (MAP) kinase and [3H]thymidine incorporation was significantly exaggerated in cells of SHR compared with those of WKY. LDL at a concentration of 10 micrograms/ml augmented the cellular signaling and proliferative action of AVP in cells of WKY, but not in those of SHR. Since [3H]AVP receptor binding was not affected by the LDL pretreatment, LDL modulates the signal transduction between a location distal to the AVP receptors and proximal from the production of IP3 and diacylglycerol. These results indicate that an increase in AVP receptor capacity has a profound effect on the AVP-induced cellular signaling and proliferation, and that LDL has a slight alteration on the action of AVP in glomerular mesangial cells of SHR.

A low-affinity vasopressin V2-receptor gene in a kindred with X-linked nephrogenic diabetes insipidus

In this study, a mutation in vasopressin Type 2 receptor (V2R) in a patient with hereditary nephrogenic diabetes insipidus (NDI) has been identified and characterized. The sequencing of the V2R gene from the patient revealed that there was a missense mutation (TAT to TGT) resulting in the substitution of 205Tyr for Cys in the putative third extracellular domain. The expression analysis in COS cells showed that the binding affinity of the mutant receptor (KD = 19.8 nM) for arginine vasopressin was much lower than that of the wild-type receptor (KD = 1.8 nM) so that intracellular cAMP production stimulated by arginine vasopressin was impaired in cells with the mutant V2R. From these results, it was concluded that the single amino-acid substitution of V2R is responsible for this familial disease.

The effect of vasopressin on phosphate transport in the proximal tubule of the dog

Recollection micropuncture study was performed in 11 thyroparathyroidectomized dogs during antidiuresis to determine the effect of continuous vasopressin infusion at 50 mU/kg/h on proximal tubule phosphate and sodium transport. The animals were divided into two groups according to changes in mean arterial blood pressure. In the first group (five dogs) with increased blood pressure and glomerular filtration rate (GFR), mean proximal tubule fluid-to-plasma inulin ratio fell significantly from 1.69 to 1.53, whereas it remained unchanged at 1.60 in the second group (six dogs) with no change in blood pressure. In contrast, mean proximal tubule fluid-to-plasma ultrafilterable phosphate ratio increased consistently in both groups, regardless of blood pressure changes. Since natriuresis as well as phosphaturia were observed in all animals, the sodium effect of vasopressin in the distal nephron must be mainly responsible for the natriuresis. It was concluded that vasopressin, when given in the doses employed, inhibits phosphate transport in the proximal tubule and sodium reabsorption in the distal nephron. An additional effect on proximal tubule sodium reabsorption appears to be related to the rise in blood pressure and GFR secondary to vasopressin administration.