Preclinical and clinical in vitro in vivo correlation of an hGH dextran microsphere formulation.

PURPOSE: To investigate the in vitro in vivo correlation of a sustained release formulation for human growth hormone (hGH) based on hydroxyethyl methacrylated dextran (dex-HEMA) microspheres in Pit-1 deficient Snell dwarf mice and in healthy human volunteers. MATERIALS AND METHODS: A hGH-loaded microsphere formulation was developed and tested in Snell dwarf mice (pharmacodynamic study) and in healthy human volunteers (pharmacokinetic study). RESULTS: Single subcutaneous administration of the microspheres in mice resulted in a good correlation between hGH released in vitro and in vivo effects for the hGH-loaded microsphere formulation similar to daily injected hGH indicating a retained bioactivity. Testing the microspheres in healthy volunteers showed an increase (over 7-8 days) in hGH serum concentrations (peak concentrations: 1-2.5 ng/ml). A good in vitro in vivo correlation was obtained between the measured and calculated (from in vitro release data) hGH serum concentrations. Moreover, an increased serum concentration of biomarkers (insulin-like growth factor-I (IGF-I), IGF binding protein-3 (IGFBP-3) was found again indicating that bioactive hGH was released from the microspheres. CONCLUSIONS: Good in vitro in vivo correlations were obtained for hGH-loaded dex-HEMA microspheres, which is an important advantage in predicting the effect of the controlled drug delivery product in a clinical situations.

Pharmacokinetics of lidocaine and bupivacaine and stable isotope labelled analogues: a study in healthy volunteers.

The pharmacokinetics of lidocaine and bupivacaine and tri-deuteromethyl-labelled lidocaine and bupivacaine were investigated in healthy volunteers. The deuterium-labelled and the unlabelled form of the drug to be investigated were simultaneously infused in 10 min. Plasma concentrations were determined using a combination of capillary gas chromatography and mass fragmentography. Bi-exponential functions were fitted to the plasma concentration-time data. The mean distribution and elimination half-lives were 8.4 +/- 5.9 min and 96 +/- 26 min for lidocaine, 9.2 +/- 7.0 min and 98 +/- 27 min for deuterium-labelled lidocaine, 15.3 +/- 9.9 min and 111 +/- 32 min for bupivacaine, and 15.2 +/- 10.9 min and 109 +/- 31 min for deuterium-labelled bupivacaine, respectively. The mean volumes of the central compartment and mean steady state volumes of distribution were: lidocaine 37 +/- 151 and 97 +/- 201, deuterium-labelled lidocaine 39 +/- 161 and 98 +/- 181, bupivacaine 27 +/- 111 and 66 +/- 231 and deuterium-labelled bupivacaine 28 +/- 121, and 65 +/- 221, respectively. The respective mean plasma clearances were 0.88 +/- 0.181 min-1, 0.87 +/- 0.181 min-1, 0.61 +/- 0.151 min-1, and 0.62 +/- 0.171 min-1. The results of the study indicate that substitution of a deuterated methyl group does not alter the pharmacokinetics of lidocaine and bupivacaine in healthy subjects.

Acceleration of recovery from sciatic nerve damage by the ACTH (4-9) analog Org 2766: different routes of administration.

Functional recovery following a sciatic nerve crush in rats was investigated by measuring the reflex withdrawal of the hindpaw to a hot air stream. The ACTH(4-9) analog Org 2766 accelerated recovery when administered subcutaneously (two-daily injections: 10 micrograms/animal; minipumps: 20-40 micrograms/animal per 24 hr: biodegradable microspheres: 40 micrograms/animal per 24 hr), but oral administration (1.5-20 mg/animal daily, in the drinking water; 1.5-15 mg/animal daily, by gavage) was not effective.

A multi-compartment vaginal ring system for independently adjustable release of contraceptive steroids.

A multi-compartment contraceptive vaginal ring system has been designed for the simultaneous zero-order release of contraceptive steroids, the rates of which can be programmed independently. This vaginal ring system consists of two or more drug-containing Silastic tubes with an outer diameter of 5 mm. The tubes with different lengths, with a total length of 16.5 cm are connected with specially-shaped glass stoppers to obtain a ring with an outer diameter of 60 mm. The stopper prevents migration of the steroids from one compartment to the other and guarantees optimal release characteristics of both steroids even after long-term storage. An additional advantage of glass is the good adherence to Silastic, enabling construction of systems with sufficient tensile strength. The release characteristics have been followed in vitro and can be programmed independently by changing the wall thickness of the tube (membrane thickness) and/or the length of each individual steroid-containing compartment. Multicompartment vaginal rings were made and tested with 3-keto-desogestrel and ethinylestradiol. The rings had an outer diameter of 60 mm, and were fabricated with independent in vitro release rates ranging from 75 to 300 micrograms/day and 10 to 30 micrograms/day for, respectively, 3-keto-desogestrel and ethinylestradiol. Using the multi-compartment vaginal ring system, contraceptive devices can be fabricated relatively simply with pre-programmed release rates for a progestogen and an estrogen to investigate the optimal daily doses for vaginal hormonal contraception.

PIP: A multi-compartment contraceptive vaginal ring system has been designed for the simultaneous zero-order release of contraceptive steroids, the rates of which can be programmed independently. This vaginal ring system consists of 2 or more drug-containing Silastic tubes with an outer diameter of 5 mm. The tubes with different lengths, with a total length of 16.5 cm are connected with specially-shaped glass stoppers to obtain a ring with an outer diameter of 60 mm. The stopper prevents migration of the steroids from 1 compartment to the other and guarantees optimal release characteristics of both steroids even after long-term storage. An additional advantage of glass is the good adherence to Silastic, enabling construction of systems with sufficient tensile strength. The release characteristics have been followed in vitro and can be programmed independently by changing the wall thickness of the tube (membrane thickness) and/or the length of each individual steroid-containing compartment. Multi-compartment vaginal rings were made and tested with 3-ketodesogestrel and ethinylestradiol.

Correlation between antipyrine metabolite formation and theophylline metabolism in humans after simultaneous single-dose administration and at steady state.

To nine healthy male volunteers two model substrates for oxidative drug-metabolizing enzyme activity, viz, antipyrine (A) and theophylline (T) were administered simultaneously by the p.o. route. To six of them, the same two drugs were also administered simultaneously at zero-order rate with an osmotic rectal drug delivery system in order to obtain steady-state plasma concentrations. Plasma A and T concentrations were measured simultaneously by a high-performance liquid chromatographic method and urinary excretions of the major metabolites arising from A (4-hydroxyantipyrine, norantipyrine and 3-hydroxymethylantipyrine) and from T (3-methylxanthine, 1-methyluric acid and 1,3-methyluric acid) were also measured by high-performance liquid chromatography. Correlations between total plasma clearance and metabolic clearances of A and T and clearances for production of the various metabolites were investigated in order to determine whether the metabolic pathways of A and T are mediated by the same or closely related forms of the cytochrome P-450 system. Total plasma clearances of the two drugs were found to correlate reasonably well (r = 0.72) but not well enough to be of useful predictive value. The strongest correlations (r = 0.91) were found between the clearance for production of 4-hydroxyantipyrine and both total and metabolic clearances of T. The clearances for production of all metabolites of T also correlated better with the clearance for production of 4-hydroxyantipyrine (r ranging from 0.79-0.86) than with the clearance of norantipyrine and 3-hydroxymethylantipyrine (r ranging from 0.42-0.58).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of cimetidine on steady state concentration and metabolite formation from antipyrine infused with a rectal osmotic mini pump.

The utility of an osmotic rectal drug delivery system as a tool in steady-state pharmacokinetic interaction studies has been investigated using the cimetidine-antipyrine interaction. Antipyrine was administered to six healthy male volunteers at the rate of 15 mg/h until steady-state was reached. Cimetidine 400 mg was then given followed by 200 mg cimetidine after 2, 4 and 6 h. Antipyrine kinetics in plasma and saliva were assessed, and metabolite excretion was determined in urine. Antipyrine levels in plasma and saliva increased shortly after cimetidine administration, indicating inhibition of antipyrine metabolizing enzymes. From the metabolite data it was concluded that all major metabolic pathways of antipyrine were affected to the same extent. The effect lasted somewhat longer than anticipated on the basis of the plasma cimetidine concentrations, but it had disappeared within 48 hours after cessation of treatment. It is concluded that the osmotic rectal drug delivery system is a useful tool in pharmacokinetic interaction studies, because it provides very constant steady-state concentrations, thus permitting investigation of the time course of drug interactions.

Nifedipine kinetics and dynamics during rectal infusion to steady state with an osmotic system.

Nifedipine steady-state kinetics and dynamics were investigated in a placebo-controlled study of six healthy subjects. Nifedipine was given rectally through an osmotic system at a zero-order rate for 24 hr. Steady-state plasma concentrations of approximately 20 ng/ml were achieved within 6 to 8 hr. Nifedipine lowered diastolic blood pressure (DBP) and increased forearm blood flow (FBF) and plasma norepinephrine concentration. On the other hand, heart rate (HR) and systolic blood pressure were not affected. Changes in DBP and FBF were closely related to nifedipine plasma concentrations during and immediately after the infusion period. Our data indicate that nifedipine lowers blood pressure in subjects with normotension and that it is possible by infusing the drug at a relatively low rate to dissociate its effect on blood pressure from that on HR.

Rectal and intravenous propranolol infusion to steady state: kinetics and beta-receptor blockade.

The effects of intravenous propranolol infusion for 24 hr was compared with those of zero-order rectal administration by an osmotic delivery system in six healthy subjects. In plasma and urine, levels of propranolol, 4-OH-propranolol (4-OH-P), and conjugates were determined just before and at 6 hr and at 20 hr during drug administration. With the rectally applied osmotic delivery system providing zero-order release, fairly constant steady-state levels of propranolol in plasma were produced within 12 to 15 hr (four to five times elimination t1/2). The mean steady-state levels were 25 ng/ml after 1.1 micrograms/min/kg rectally and 60 ng/ml after 0.8 micrograms/min/kg IV. The mean rectal systemic availability was 33%; the elimination t1/2s for the two routes did not differ. The results of analysis of plasma for metabolites indicate that after rectal propranolol different metabolic pathways are followed and that there is partial avoidance of first-pass elimination. The isoproterenol challenge resulted in a reproducible assessment of beta-receptor blockade that was closely related to the propranolol concentration in plasma in all subjects and for both routes of administration. With rate-controlled release of propranolol from an osmotic delivery device, the rectal route provides an alternative to intravenous infusion to achieve constant steady-state propranolol concentrations. This may be useful for research purposes or during the perioperative period in surgical patients.

Site specific rectal drug administration in man with an osmotic system: influence on "first-pass" elimination of lidocaine.

Lidocaine was administered to healthy volunteers at different sites in the rectum. Unchanged drug and monoethylglycinexylidide (MEGX) concentrations were measured in plasma with a newly developed gas chromatographic method. Lidocaine was given rectally by means of an osmotic system (Osmet(®)) which delivered 25 mg/h at zero-order rate. In a pilot experiment in two subjects it was shown that lidocaine administration close to the anus for 5 h resulted in higher lidocaine plasma levels as compared to administration at 15 cm from the anus. Six other subjects participated in three separate experiments, in which lidocaine was administered rectally close to the anus and at 7.5 and 15 cm from the anus. A zero-order infusion plasma level profile was found for both the parent compound and its metabolite. The MEGX/lidocaine plasma concentration ratio was calculated for all experiments. After administration most proximal to the anus the mean metabolite/parent drug concentration ratio was significantly less than that obtained after administration at 15 cm from the anus, whereas at approximately 7.5 cm from the anus the values were in-between. Comparison of the AUC lidocaine/AUC MEGX ratios gave similar results; the highest value, 3.2 ± 1.3 (mean ± S. D.), was found after administration close to the anus, while at 15 cm from the anus the ratio was 1.6 ± 0.3 (p < 0.01). The terminal elimination half-lives of lidocaine and MEGX did not differ for the three sites of administration, and the mean values were 110 and 180 min respectively. The results of this study demonstrate that the site of drug administration in the human rectum determines the degree of hepatic "first-pass" elimination of high-clearance drugs. Maximal avoidance of presystemic elimination is achieved when administration takes place close to the anus.

Avoidance of "first-pass" elimination of rectally administered propranolol in relation to the site of absorption in rats.

The extent of "first-pass" elimination of racemic propranolol and dextropropranolol in doses of 0.25 or 0.50 mg was investigated in relation to the site of drug administration in the rectum of rats. The compounds were given orally, i.v., and rectally at distances of 2 and 1 cm from and directly at the anus by low volume zero-order 30 min infusion. Unchanged propranolol was determined in blood, and propranolol and three metabolites were measured in urine. The systemic availability of propranolol after oral administration was approximately 6 %. Rectal administration at 2 cm, at 1 cm and directly at the anus (0.2 cm) gave two, three and six times higher values, respectively. The more distal application site produced urinary metabolite profiles that were comparable to those observed after oral administration, while application directly at the anus was similar to i.v. dosing. In all experiments log-linear elimination phases with comparable elimination half-lives (range 12-18 min) were found, except with the 0.50 mg dose after i.v. and rectal administration close to the anus which showed a non-linear profile. The mean systemic availability after rectal administration of 0.25 mg dextro-propranolol close to the anus was 50 and 64 % as compared to a 0.25 and 0.125 mg i.v. dose, respectively. The rectal route may be used for propranolol to partially prevent hepatic first-pass metabolism. However, avoidance of presys-temic elimination is maximal only in the immediate vicinity of the anus as the venous blood supply of the upper part of the rectum of rats appears to be connected to the portal system and the lower part to the general circulation.

Avoidance of "first-pass" elimination of rectally administered lidocaine in relation to the site of absorption in rats.

The extent of "first-pass" elimination of lidocaine in relation to the site of absorption in the rectum of rats was investigated. Male Wistar rats received lidocaine orally, i.v. or rectally at 4, 2, 1 and at about 0.2 cm from the anus. In all cases 5.0 mg of a tritium-labeled lidocaine solution was administered by zero-order infusion into the rectum. In blood, unchanged lidocaine and urine and feces total radioactivity were measured. Lidocaine was absorbed almost completely in all cases as assessed relative to i.v. administration. The average elimination half-life was about the same for all routes of administration (approximately 27 min). Systemic availability of lidocaine when given by the oral route was 16%; a similar value was found after rectal administration at 4 cm distance from the anus. At 2 cm distance from the anus the mean systemic availability was 21%, at 1 cm, 45% and as closely as possible to the anus, 72%. It is concluded that the degree of avoidance of first-pass elimination of rectally administered lidocaine is very much dependent on the site of drug administration. When the drug is absorbed very closely to the anus, little first-pass elimination occurs.

Zero-order rectal delivery of theophylline in man with an osmotic system.

In vivo and in vitro performances of a rectally applied osmotic delivery system containing a water-soluble derivative of theophylline were compared. The system was applied in six healthy volunteers during 72 h, and a comparison was made with two conventional dosage forms, a suppository and a solution administered orally, given once. In vitro, a perfect zero-order release rate with the osmotic system was obtained. In all subjects, application of the osmotic system resulted in a constant steady-state theophylline level just as in a long time zero-order i.v. infusion, which strongly suggests that the release rate in vivo is zero-order too. The release rate was not influenced by renewing the dosage form after 36 hr or by defecation. The in vivo and in vitro release rates were almost identical and subject-independent.

Rectal drug administration: clinical pharmacokinetic considerations.

The human rectum represents a body cavity in which drugs can be easily introduced and retained and from which absorption is well possible. There are important therapeutic reasons why it is sometimes preferable to give a drug rectally rather than orally, e.g. in cases of nausea and vomiting. Drawbacks of rectal drug administration include the interruption of absorption by defaecation and lack of patient acceptability. The mechanism of drug absorption from the rectum is probably no different to that in the upper part of the gastrointestinal tract, despite the fact that the physiological circumstances (e.g. pH, fluid content) differ substantially, Absorption from aqueous and alcoholic solutions may occur very rapidly, which has proved to be of considerable therapeutic value in the rapid suppression of acute convulsive attacks by diazepam (e.g. in children), but absorption from suppositories is generally slower and very much dependent on the nature of the suppository base, the use of surfactants or other additives, particle size of the active ingredient, etc. There is some evidence that hepatic first-pass elimination of high clearance drugs is partially avoided after rectal administration, e.g. lignocaine. This can be explained by the rectal venous blood supply: the upper part is connected with the portal system, whereas the lower part is directly connected with the systemic circulation. Plasma concentration data following rectal administration of representatives of several classes of drugs are reviewed: anticonvulsants, non-narcotic analgesics and non-steroidal anti-inflammatory agents, hypnosedatives and anaesthetics, strong analgesics, theophylline and derivatives, corticosteroids, antibacterial agents, thiazinamium, promethazine, hyoscine-N-butyl-bromide, streptokinase, progesterone, ergotamine tartrate and levodopa. Only limited number of cases has it been adequately shown that the rectal route of administration gives plasma concentrations which are comparable to the oral route. Potentially the rectal route offers the same possibilities as the oral route, but the influence of the formulation seems to be very critical. It is also likely that the future novel drug delivery systems with zero order release characteristics will be applied rectally. Interesting preliminary results have already been obtained with theophylline administered by 2ml osmotic pumps.

Rectal infusion of the model drug antipyrine with an osmotic delivery system.

An osmotically-powered rectal drug delivery system, was used for the rectal infusion of the model drug antipyrine. The system, which is slightly larger than a normal suppository, has a nominal pumping rate of 43 microliter h-1 over at least 30 h. Four healthy volunteers kept two such systems in their rectum for a sum total of 98 h. Saliva and plasma concentrations were determined at regular intervals and in all cases a very constant steady-state saliva and plasma concentration was reached and maintained. Defecation and reinsertion of the drug delivery system did not cause any irregularities in the concentration profile. The system was very well tolerated by the volunteers.