Introduction

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After oral administration in the rat, d-propoxyphene (DP) is subjected to a major first-pass effect and is metabolized through various pathways (1, 2). The major pathway involves demethylation to norpropoxyphene (NP) (2, 3). NP is detected in the plasma after portal administration of DP but was not found after systemic injection by Oguma and Levy (3). This difference, which was attributed to the saturation of enzymatic pathways in the liver, might indicate that the hepatic and extrahepatic metabolisms of the drug are not identical. We compared the absolute bioavailability of DP obtained after oral and portal administration. We also examined differences in DP metabolism when tracer doses of (dimethylamino-¹⁴C) DP were administered in the duodenum, the portal vein, or a peripheral vein. The demethylation of DP to NP was evaluated by the measurement of exhaled ¹⁴CO₂. With both pharmacological and tracer doses, it was shown that DP is metabolized differently in the intestine and in the liver, suggesting that the route of administration may play a critical role in its transformation.

	Materials and Methods

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Animals. Wistar male rats weighing 230-270 g were purchased from University Catholic of Louvain animal facilities. All animals were deprived of food 16 hr prior to drug administration. They were handled in accordance with Catholic University of Louvain regulations.

Methods. In experiments using pharmacological doses of DP, DP and NP determinations in plasma were performed by capillary gas chromatography as previously described (4) or by GC-MS following the method described by Kintz and Mangin (courtesy of Prof. Gielen, University of Liège, Belgium) (5). Low cut-off value for plasma NP was 50 and 10 ng/ml, respectively.

In Vivo Study Design. To elucidate the importance of the route of administration and hepatic metabolism, either pharmacological doses of DP or ¹⁴C-DP tracer doses were administered by bolus in 200-µl saline solution, except if otherwise specified.

Route of administration. To take into account the intravenous toxicity of 20 mg/kg DP dose and data from Oguma and Levy (3), various pharmacological doses of DP were administered by gavage (4, 8, 12, and 20 mg/kg), in the portal vein (12 mg/kg), or in the femoral vein (4 mg/kg). Blood samples were collected from the jugular vein, and plasma samples were stored at 20°C until assay.

Role of the liver. ¹⁴C-DP was administered in rats that had undergone, either a total hepatectomy, a surgical portacaval shunt, or a bile duct diversion.

In Vitro Study Design. Liver and intestinal microsomes were prepared as previously described (9) and kept frozen at 80°C until used. DP (2.5 nmol in 600-µl final incubation volume) was incubated for 3 min at 37°C with 70 µg microsomal proteins in 40-µM sodium phosphate buffer (pH 7.4), 65 µmol of NADP, 25 µmol of MgCl₂, 0.7 U glucose 6-phosphate dehydrogenase, and 15 µmol of glucose-6-phosphate. The reaction was stopped by the addition of ZnSO₄ (15%). Extraction procedure was similar to that used for plasma, and NP determinations were further performed using capillary gas chromatography. With these optimal conditions, linear reaction rates were ensured and intra- and interday coefficients of variation were lower than 5%.

	Results

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Pharmacological Doses. DP AUC values were 255, 220, and 538 ng · hr/ml after portal, systemic, and duodenal administration, respectively. Absolute DP bioavailability obtained after duodenal administration was two times higher than that observed after portal administration (48.9% vs. 23.2%, respectively).

Tracer Doses (Table 1). After duodenal administration, breath ¹⁴CO₂ 2 hr cumulative excretion (CE) reached 13.6 ± 5.3%. In preliminary experiments in which ¹⁴C-DP was also administered through a duodenal canula in animals with external biliary diversion, only 4.1% of total radioactivity was found in the gut 24 hr after drug administration. In the presence of portacaval shunt, ¹⁴CO₂ 2 hr CE fell to 5.4 ± 4.1% (p = 0.066) in comparison with that in normal rats.

	Discussion

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In the rat doses of DP lethal when given intravenously are well tolerated after oral administration, a fact in agreement with an important first-pass effect reducing the systemic availability of the drug and its central toxic effect. The liver is still considered the major site of drug transformation. Surprisingly, however, DP absolute bioavailability was much greater after duodenal administration than after direct injection in the portal vein. As shown by others (1), the intestinal absorption of DP is complete, a feature confirmed by our tracer studies performed in the presence of biliary diversion. Moreover, the existence of an incomplete absorption could not explain the greater bioavailability after duodenal administration. Saturation of an hepatic metabolic pathway, an hypothesis which is not supported by our tracer experiments, is also unlikely. Such a phenomenon would indeed lead to the opposite results since sinusoidal DP concentration would be much higher after portal than after oral administration. This observation suggests that DP is transformed into the gut in a metabolite which largely escapes transformation by the liver and remains recognized as DP by the analytical method used. This could explain the greater bioavailability obtained after duodenal administration.

Data concerning the detection of NP after administration of pharmacological doses of DP are in agreement with those of Sullivan et al. (11) and Oguma and Levy (3), although a more sensitive method to measure NP concentration of 10 ng/ml was used. To explain the absence of NP after systemic injection, two hypotheses based on saturation mechanisms have been proposed by Oguma and Levy (3). First, NP is very rapidly transformed and is only detected when the enzyme responsible for its metabolism is saturated. Second, NP is only produced in significant amounts when another major metabolic pathway of DP is saturated. The fact that ¹⁴CO₂ production was still greater after IP than after ID or systemic administration tends to rule out the second hypothesis. Not being able to selectively measure NP at these concentrations, we instead relied on an indirect method to evaluate its production, the measurement of ¹⁴CO₂ resulting from the demethylation of ¹⁴C-DP. The ¹⁴CO₂ breath elimination and thus likely the demethylation of DP into NP was greater after portal than after systemic or duodenal administration. This difference persisted at the end of the 24-hr period of collection, indicating that it was not because of an initial dilution in a peripheral reservoir after duodenal or systemic administration. The saturation of an enzymatic pathway is unlikely to explain our results obtained using tracer doses of the drug. Furthermore, the level of ¹⁴CO₂ elimination was not influenced by the rate of administration in the portal vein either as a bolus or by a continuous infusion within 20 min. The fact that the production of ¹⁴CO₂ mainly varied according to the route of DP administration indicates that it might be related to the amount of the drug reaching the liver prior to any transformation. This hypothesis is further supported by the 50% reduction in ¹⁴CO₂ elimination observed after portacaval shunting, a procedure that is likely to increase DP extrahepatic elimination.

Presystemic extrahepatic transformation of DP should have two consequences. First, it may reduce the amount of DP demethylated to NP which only occurs when sufficient amounts of untransformed DP reach the liver (such as that obtained after direct IP administration or after administration of very high doses in the duodenum). Second and when compared with IP administration, it may increase the relative amount of the label excreted into bile. The total amounts of excreted labeled material were indeed similar after duodenal or after portal administration, but there was an inverse relationship between the amount of excreted in bile and that excreted in the exhaled air, respectively (Table 1).

As further shown by experiments performed in hepatectomized animals, the liver is the main site of ¹⁴CO₂ production. This observation is in agreement with the current knowledge about the demethylation of the drug, even if it is difficult to exclude that only the final transformation of the drug, i.e. the transformation of formaldehyde into ¹⁴CO₂, occurs in the organ. As supported by our in vitro studies with isolated microsomes, it is likely that the gut contributes little to the production of ¹⁴CO₂ and to in vivo demethylation of DP. On the other hand, various arguments suggest that the extrahepatic transformation of DP mainly occurs in the gut. ¹⁴CO₂ excretion was indeed very similar after duodenal and systemic administrations, but in the two instances significantly lower than after portal administration. A major role for the lung has been excluded on the basis of results obtained following intracardiac or systemic injection.

Our experiments thus suggest that DP is transformed in the gut in such a way that its further handling by the liver is modified, an observation well in agreement with the data described by Giacomini et al. (12) who found that portacaval shunting is not able to suppress completely the first-pass effect. The nature of this prehepatic metabolism was however not investigated. Prehepatic metabolism of various drugs has already been described for drugs such as cyclosporine, propranolol, and verapamil (9, 13, 14). Our work indicates that the prehepatic transformation of DP is likely to influence not only quantitatively but also qualitatively its metabolism. This new concept may be of potential pharmacological importance since it may be anticipated that organs responsible for various biotransformations exhibit different metabolic behaviors in terms of pharmacological induction (14) or response to various pathological conditions.

In conclusion, it is proposed that DP is transformed in the gut wall in such a way that its hepatic metabolism is modified.