Elimination of drugs by passive diffusion from blood to intestinal lumen: factors influencing nonbiliary excretion by the intestinal tract

Intestinal Excretion, Intestinal Recirculation, and Renal Tubule Reabsorption Are Underappreciated Mechanisms That Drive the Distribution and Pharmacokinetic Behavior of Small Molecule Drugs

Drug reabsorption following biliary excretion is well-known as enterohepatic recirculation (EHR). Renal tubular reabsorption (RTR) following renal excretion is also common but not easily assessed. Intestinal excretion (IE) and enteroenteric recirculation (EER) have not been recognized as common disposition mechanisms for metabolically stable and permeable drugs. IE and intestinal reabsorption (IR:EHR/EER), as well as RTR, are governed by dug concentration gradients, passive diffusion, active transport, and metabolism, and together they markedly impact disposition and pharmacokinetics (PK) of small molecule drugs. Disruption of IE, IR, or RTR through applications of active charcoal (AC), transporter knockout (KO), and transporter inhibitors can lead to changes in PK parameters. The impacts of intestinal and renal reabsorption on PK are under-appreciated. Although IE and EER/RTR can be an intrinsic drug property, there is no apparent strategy to optimize compounds based on this property. This review seeks to improve understanding and applications of IE, IR, and RTR mechanisms.

Concentration-dependent Exsorption of Quinidine in the Rat Intestine

During intravenous infusion, the luminal concentration of quinidine was higher than the plasma concentration. The intestinal clearance (CLi) of the drug was measured by dividing the rate of appearance of the drug in the intestinal luminal perfusate by the plasma concentration. The CLi of quinidine was therefore much higher than the rate of luminal perfusion. Over the infusion dose range of 0·1–2 mg h−1, the CLi of quinidine decreased with increasing plasma concentration of quinidine. Adding quinidine into the luminal perfusate had little effect on the CLi of quinidine. Co-administration of quinidine with other agents intravenously did not alter the CLi of salicylic acid and urea, while the same treatment decreased the CLi of theophylline and 5-disopyramide. In-vitro experiments on brush-border membrane vesicles showed that quinidine decreased the rate of Na+ uptake and H+ efflux. The inhibition was significant at quinidine concentrations above 20 μm. Quinidine was a more potent inhibitor than amiloride. At quinidine infusion rates less than 2 mg h−1, quinidine concentration in plasma or in the luminal perfusate was at the lower limit of the inhibitory concentration. Microclimate pH at the intestinal surface was also measured. At mid-jejunum, the microclimate pH increased 0·3 pH units by infusing 2 mg h−1 of quinidine, while the microclimate pH at most other measuring sites was not significantly altered by quinidine infusion. It was concluded that quinidine is exsorbed from blood into the intestinal lumen by a carrier-mediated pathway in addition to the passive diffusion. At high plasma concentration, quinidine exsorption becomes saturated. Quinidine inhibited the intestinal exsorption of theophylline and S-disopyramide possibly by competition on the carrier.

Intestinal excretion of unconjugated bilirubin in man and rats with inherited unconjugated hyperbilirubinemia

Patients with Crigler-Najjar syndrome and Gunn rats cannot form bilirubin glucuronides owing to a lack of bilirubin UDP-glucuronosyltransferase activity. Because increased serum and tissue bilirubin levels remain constant, an alternative excretory route has to substitute for this deficiency. Gunn rats excrete in bile only 2-13% of the bilirubins eliminated in Wistar rats. In contrast, the biliary excretion rate of urobilinogen in Gunn and Wistar rats is comparable. The sum of bilirubins and urobilinogen excreted in the bile of Gunn rats amounts to 10-30% of pigments excreted in Wistar rats. Despite this low biliary excretion, the intestinal content and fecal excretion of bile pigments in Gunn and Wistar rats were similar. These data support an extrabiliary entrance of unconjugated bilirubin into the intestine. Additional proof for this was found in that the intestinal lumen of Gunn rats still contains a high amount of bilirubins and urobilinogen after 3 d of external biliary drainage. A similar procedure in Wistar rats resulted in the complete disappearance of bile pigments from the intestine. The direct transmural transport of bilirubin from blood to all parts of the intestinal lumen was demonstrated by injecting 14C-bilirubin i.v. into Gunn rats with isolated parts of small and large intestine. In Crigler-Najjar and Gilbert's syndrome patients, the biliary excretion of bile pigments has previously been shown to be strongly reduced. Their stools, however, contained approximately the same amount of bile pigments as in normal subjects. Although only traces of unconjugated bilirubin were detected in the stool of normal persons (4 +/- 3% of total bile pigments), higher amounts were found in patients with Crigler-Najjar disease (20 +/- 12&). These results suggest a direct intestinal permeation of unconjugated bilirubin in severe unconjugated hyperbilirubinemia both in man and rats.

Modipafant, a new PAF antagonist: pharmacokinetics and disposition in rat, dog and man

1. The pharmacokinetics and disposition of modipafant, a dihydropyridine PAF antagonist, were studied in rat and dog following intravenous and oral administration of the drug or its radiolabelled analogue. In addition, the pharmacokinetics were studied in man following single administration of escalating oral doses of the drug. Modipafant is a lipophilic weak base with log D(octanol) 7.4 and pKa of 4.3 and 5.3 respectively. 2. Following intravenous administration of [14C]-modipafant to rat, radioactivity is rapidly distributed throughout the body, except for the brain. A significant amount of radioactivity (probably modipafant) is rapidly distributed to the alimentary tract, particularly in the stomach. This is believed to be due to 'ion trapping' of modipafant in the acidic environment of the upper GI tract. The re-circulated modipafant may be subject to reabsorption and/or faecal excretion. 3. Following intravenous administration to rats, systemic clearance is five times greater in the male than female. The magnitude of this difference is in keeping with the clearance of other dihydropyridines such as nilvadipine. In dog, the clearance values are similar for both sexes, as expected. In this latter species, the systemic clearance decreases 6-fold with increasing dose size, indicative of saturation of a pathway of metabolism. 4. Following oral administration over a dose range of 1-12 mg/kg, modipafant is incompletely (27-67%) bioavailable in rat and dog. In the male dog, systemic exposure to drug (AUC/infinity) increased non-linearly with dose. Following oral administration to man, absorption was rapid with a mean value for Tmax of 1 h, and Cmax's ranging non-linearly from 90 to 2100 ng/ml following dosing at 12.5 to 150 mg respectively. 5. The elimination of modipafant is characterized by short half-life (mean values for t1/2 range from 1 to 3 h). However, the nature of the receptor kinetics of modipafant (slow offset) means that the drug shows a long duration of action in spite of short pharmacokinetics at pharmacologically relevant doses. 6. Following oral and intravenous administration of 14C-modipafant to rat and dog, the majority of radioactivity (mean 92%) is recovered in the faeces. The excretion of modipafant in rat and dog is characterized by metabolism, mostly to pyridine metabolites, accounting for between 38 and 75% of total clearance, the rest being cleared as unchanged drug.

Are we doing too many animal biodisposition investigations before phase I studies in man? A re-evaluation of the timing and extent of ADME studies

This commentary attempts to re-evaluate, from a scientific standpoint, the timing and usefulness of undertaking detailed kinetic and metabolic studies in many animals from several species, and extrapolating the findings to man. This reappraisal is now possible due to the conditional acceptance of harmonised guidelines on toxicokinetics at ICH2 in Orlando in 1993, the results from which can provide much of the information that is required to design and validate safety studies and to extrapolate exposure or safety margins to man.

Propachlor-S-cysteine: a major circulating metabolite in the calf, pig and rat after administration of propachlor

1. Propachlor-S-cysteine was the major metabolite found in systemic blood from rat, pig and calf given propachlor via the stomach. It was also the major metabolite found in the portal blood of pig; the portal blood of rat and calf was not examined. 2. Erythrocytes were the major transporter of propachlor metabolites in rat blood whereas plasma was the major transporter of these metabolites in pig and calf. 3. There was no evidence for metabolism of propachlor-S-cysteine by rat blood or by cytosol from rat, pig and calf erythrocytes.

Role of intestinal excretion in the effect of subcutaneously administered sedecamycin on cecal infection caused by Treponema hyodysenteriae in mice

The therapeutic effects of subcutaneously administered sedecamycin on experimental Treponema hyodysenteriae infection in mice were evaluated. Sedecamycin was more active than tiamulin and lincomycin. The efficacy of sedecamycin upon subcutaneous administration was similar to that upon oral administration. Sedecamycin given subcutaneously provided similar degrees of protection in bile duct-ligated and intact mice. Pharmacokinetic studies utilizing a liquid chromatographic technique were carried out to determine the concentration of sedecamycin in the cecum, the site of T. hyodysenteriae infection in mice. Little sedecamycin was found; however, lankacidinol, a major metabolite of sedecamycin, was found in the cecal contents of intact mice after subcutaneous or oral administration of sedecamycin. Lankacidinol was also found in the cecal contents of bile duct-ligated mice, although the concentration found after subcutaneous administration of sedecamycin was much lower than that found after subcutaneous or oral administration to intact mice. These results indicate that sedecamycin is excreted directly into the intestinal tract as an active metabolite by a route other than the bile duct. It is suggested that this intestinal excretion plays an important role in the efficacy of subcutaneously administered sedecamycin against cecal infection of mice by T. hyodysenteriae.

In situ intestinal absorption of 2-chloro-N-isopropylacetanilide (propachlor) and non-biliary excretion of metabolites into the intestinal tract of rats, pigs and chickens

1. Propachlor was absorbed from in situ intestinal loops of rats and pigs, with absorption half-times of 7.5 and 16.5 min, respectively. 2. Water-soluble 14C-labelled metabolites that accumulated in the intestinal loops accounted for 31%, 53%, and 25% of the starting 14C for rats, pigs and chickens, respectively. 3. Propachlor(S)cysteine was identified as the major metabolite in the pig intestinal lumen (43% of the water-soluble 14C). 4. It is concluded that intestinal metabolism and intestinal excretion of water-soluble metabolites of propachlor are important physiological processes that occur in a variety of animal species. These processes provide a route by which metabolites of xenobiotics may reach the intestinal lumen in animals which are poor biliary excretors.

Comparative drug exsorption in the perfused rat intestine

The factors affecting drug exsorption into the gastrointestinal tract are uncertain. In this study, the intestinal clearance (CLi) of compounds which vary in their lipophilicity, serum protein binding, molecular weight and ionic charge at physiological pH, has been measured. Male Sprague-Dawley rats with ligated bile ducts were infused with the test compounds through the jugular vein. The small intestine was intubated and perfused with Tyrode solution at 20 mL h-1. The CLi of the compounds investigated (urea, polyethylene glycol, inulin, albumin, dextran, barbituric acid, salicylic acid, thiobarbital, thiopental, thioseconal, theophylline, S-disopyramide and quinidine) was determined under anaesthesia by dividing the rate of a component's appearance rate in the perfusate by its carotid arterial concentration. Serum protein binding of the compounds was determined by equilibrium dialysis. The n-octanol-water partition coefficients of the compounds were measured as indices of lipophilicity. The CLi values of dextran, albumin, inulin, polyethylene glycol and urea were 0.56, 1.03, 4.5, 4.8 and 12.0 mL h-1, respectively. The larger the molecular weight of a compound, the smaller its CLi. The molecular weight is apparently one of the major determinants of CLi. Thiobarbital, thiopental and thioseconal are compounds of similar structure with increasing lipophilicity and serum protein binding. The CLi of thiobarbital, thiopental and thioseconal was proportional to the unbound fraction in serum. The unbound clearance (CLui) of three thiobarbiturates were similar (approximately 11 mL h-1). The unbound fraction of drug in serum appears to be a factor determining their CLi. Barbituric acid and salicylic acid, two acidic compounds, showed a low CLi (less than 1 mL h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic modelling of the effect of activated charcoal on the intestinal secretion of theophylline, using the isolated vascularly perfused rat small intestine

The effect of activated charcoal administration on the secretion of theophylline from the blood into the intestinal lumen has been examined by use of the rat isolated vascularly perfused small intestine. A closed two compartment model was used to analyse the vascular and luminal concentration-time curves obtained. An equation was derived to calculate the time-dependent intestinal clearance. From control experiments it was concluded that theophylline is secreted by a diffusional transport system through the intestinal wall. The intestinal clearance declined rapidly with time as a result of the concomitant increase in luminal theophylline concentration. After 120 min a steady state between the vascular and luminal perfusate was established. Administration of activated charcoal in the lumen had a profound effect on the kinetics of the drug. The vascular steady state concentration was depressed dramatically. The theophylline clearance remained nearly constant with time, because the blood to lumen concentration gradient was maximized. The maximal value for the intestinal theophylline clearance was estimated to be 0.88 mL min-1 and it equalled the value for the intestinal blood flow at the absorptive site. By use of the concept of absorptive site blood flow, the maximal effect of charcoal on systemic theophylline clearance could be adequately predicted for rats, dogs and man. Activated charcoal administration is only useful to enhance the systemic clearance of drugs or toxicants if that clearance is of the same order of magnitude as the absorptive site blood flow or lower.

The percutaneous fate of the rodenticide flocoumafen in the rat: role of non-biliary intestinal excretion

1. Appreciable penetration of radioacticity occurred through rat skin following percutaneous administration of 14C-flocoumafen. At 7 days after dosing 12% of the administered radioactivity remained at the site of application, while 25% was located in the liver as unchanged flocoumafen. 2. Excretion of flocoumafen metabolites via the urine accounted for 10% dose over the 7 day experiment, this is some 30-fold greater than that seen after a single oral dose. 3. Unchanged flocoumafen comprised the major product detected in faeces. Biliary elimination was a very minor route of excretion and did not account for all of the unmodified flocoumafen present in faeces. 4. Considerable amounts of unchanged flocoumafen found associated with the contents of the large intestine after intraperitoneal administration to rats fitted with biliary fistulae indicates that, in the intact rat, flocoumafen enters the intestine by a non-biliary intestinal excretion mechanism.

Evidence for involvement of non-biliary excretion into the intestines in the formation of methylsulphonyl-containing metabolites of 2-chloro-N-isopropylacetanilide (propachlor) by swine and rats

1. Radiolabelled metabolites excreted in the urine of swine after an oral dose of 14C-propachlor were similar to those previously reported with rats and included methylsulphonyl (CH3SO2-)-containing metabolites. 2. A bile-duct-cannulated pig dosed orally with 14C-propachlor excreted 7.6% dose in the bile compared with approx. 75% dose with rats. Although enterohepatic circulation had been prevented with the bile-duct-cannulated pig. CH3SO3 metabolites of propachlor were excreted in the urine. Enterohepatic circulation had been reported to be necessary for formation of CH3SO2 metabolites of propachlor with rats. 3. Germ-free pigs dosed orally with 14-C-propachlor did not excrete urinary CH3SO2 metabolites, indicating involvement of the enteric flora in the production of these metabolites as shown previously with rats. 4. A g.l.c.-mass spectrometric specific ion monitoring technique indicated that trace amounts of CH3SO2 metabolites were present in the urine and bile of bile-duct-cannulated rats dosed orally with 14C-propachlor. This was confirmed by isolation and mass-spectral analyses. 5. It is concluded that after oral administration of propachlor to rats and swine, glutathione (GSH) conjugation and cysteine conjugate beta-lyase activity in the enteric flora are involved in formation of CH3SO2 metabolites. Presumably, the GSH conjugate is formed in mammalian tissue and must be returned to the lumen of the gut to be acted upon by the gut microflora. In rats this is accomplished primarily by biliary excretion, whereas in swine a non-biliary mechanism is much more important.

Kinetics of theophylline clearance in gastrointestinal dialysis with charcoal

The pharmacokinetic response of theophylline following the oral administration of activated charcoal was investigated in rabbits. Rabbits were continuously infused with a theophylline solution at a rate of 2.12 mg/h. At the fourth hour of theophylline infusion, 20 g of activated charcoal was administered by intubation to the rabbit (n = 12). The concentration of theophylline in serum gradually decreased after the charcoal treatment. The magnitude of the change in theophylline concentration induced by charcoal varied among animals. By comparing the steady-state theophylline concentration in the control and treated animals, the total body clearance was found to increase from 94.4 +/- 7.5 to 210 +/- 27 mL/h/kg (mean +/- SE). The rate of change of theophylline concentration in serum following charcoal treatment was fitted with a simulation curve by assuming a stepwise increase in clearance immediately following the charcoal treatment. The results indicated that activated charcoal exerted its maximum effect in increasing theophylline clearance immediately after its administration.