Toxicology Letters, 39 (1987) 85-91 Elsevier

85

TXL 01872

ORAL KINETICS AND BIOAVAILABILITY OF THE CHOLINESTERASE REACTIVATOR HI-6 AFTER ADMINISTRATION OF 2 DIFFERENT FORMULATIONS OF TABLETS TO DOGS

(Organophosphorus poisoning; antidotes; oximes; pharmacokinetics)

MATEJ MAKSIMOVIC, DUSAN JOVANOVIC, VELJKO KOVACEVIC and

DUBRAVKO BOKONJIC

Military Technical Institute, Belgrade (Yugoslavia)

(Received 12 May 1987)

(Revision received 6 July 1987)

(Accepted 7 July 1987)

SUMMARY

A one-compartment open model with first-order absorption was used for comparing new oral formula-

tions of the potent acetylcholinesterase reactivating oxime HI-6. Although mean peak plasma levels did

not differ between retard and conventional tablets (21.38 and 20.74 pmol/I), the time for reaching peak

levels was significantly longer (5.5 h) with retard than with conventional tablets (2.86 h). Among other

pharmacokinetic estimates only absorption half-lives and areas under the concentration-time curve

(AUC) were significantly different (P < 0.05). The AUC with retard tablets was 8.07% and that of con-

ventional tablets 5.42% of intravenous AUC, indicating low bioavailability of oral HI-6 formulations.

Potential therapeutic use of HI-6 requires, therefore, further investigations in order to improve its

gastrointestinal absorption.

INTRODUCTION

Various oximes (pralidoxime, toxogonin and HI-6) are effective antidotes in organophosphorus poisoning. Therefore, the oximes have been accepted as part of the therapeutic regimen against poisoning by anticholinesterase compounds. In most studies the oximes were administered by the intravenous and intramuscular route. The oral route is less recommended because these drugs, being charged com-

Address for correspondence: M. Maksimovic, Military Technical Institute, Kataniceva 15, 11002

Belgrade, Yugoslavia.

Abbreviations: AUC, area under the concentration-time curve; HI-6, pyridinium-1-( [(4carbamoyl-

pyridiniohnethoxylmethyl ) -2-(hydroxyiminomethyI)dichloride monohydrate.

03784274/87/$ 03.50 0 1987 Elsevier Science Publishers B.V. (Biomedical Division)

86

pounds, are poorly absorbed from the gastrointestinal tract [l-4]. However, as sug-

gested earlier [2,5], in cases of mild poisoning or when oximes are given

prophylactically, the oral administration of oximes may be feasible. In such cases

oximes have to be taken 2-4 times daily because of their relatively short elimination

half-lives of 2-3 h. If a simpler treatment regimen is needed, it is more convenient

to use long-acting formulations of oximes. Consequently, regarding the persistence

of the oxime in the blood of dogs, Cvetkovic et al. [6] reported that pralidoxime

retard tablets were superior to the conventional ones.

To assess how well HI-6 (pyridinium-1-([(4-carbamoyl-pyridinio)methoxy]-

methyl ) -2-(hydroxyiminomethyl)dichloride monohydrate), one of the most effec-

tive oximes, is absorbed after oral administration, the absorption and elimination

profiles of conventional and retard tablets of HI-6 were studied in dogs and com-

pared to those following i.v. injection of HI-6.

MATERIALS AND METHODS

Animals and test drugs Two studies of oral and one study of intravenous HI-6 kinetics were carried out

in 11 German shepherd dogs of both sexes, weighing 20-28 kg (mean 25.1 kg). Prior

to the experiment, the animals were fasted for 24 h; water was available ad libitum.

Conventional and retard tablets of HI-6, containing 472.3 and 257.1 mg of HI-6,

respectively, were formulated by the Bosnalijek drug company (Sarajevo).

Procedures

Hi-6 retard tablets were administered to all 11 dogs (mean HI-6 dose was 0.27

mmol/kg). To 7 of them conventional tablets were administered (mean dose 0.24

mmol/kg), and 3 other dogs received HI-6 i.v. (0.27 mmol/kg), with one week be-

tween each treatment period.

In the oral studies the dogs swallowed intact tablets together with minced raw

meat. For oxime determination blood samples were drawn from the femoral vein

at 0.5, 1 and every following hour up to 11 h after administration. Samples were

centrifuged and plasma was analysed for HI-6 by the spectrophotometric method

of Maksimovic and Vojvodic [7], with a detection limit of 2.5 pmol/l.

In the i.v. study blood samples were taken at 2, 5, 7, 10, 15, 20, 40, 60, 80, 120,

180, 240, 300 and 360 min, and plasma was analysed for HI-6.

Data analysis Plasma HI-6 concentrations versus time curves were analysed using a two-

compartment model for the i.v. data and a one-compartment model with first-order

absorption for the oral data. Experimental points were fitted by linear regression

(HP-97 calculator). All pharmacokinetic data were calculated according to Ritschel

[8]. Statistical analysis and comparison of the various pharmacokinetic parameters

obtained with retard and conventional tablets of HI-6 were performed using Stu- dent’s paired t-test. Statistics were calculated with a HP-97 calculator. Differences were considered to be significant at P<O.OS.

RESULTS

The plot of log HI-6 plasma concentrations versus time following 0.27 mmol/kg i.v. in 3 dogs is shown in Fig. 1. Mean initial plasma HI-6 concentration was 1458.8 -t 204.1 pmol/l, falling to 22.0 + 1.9 pmol/l at 6 h. The plasma concentration ver-

0 40 80 120 160 200 240 280 320 360

TIME (m(n)

Fig. 1. Log plasma HI-6 concentrations (mean + SD) versus time following an i.v. dose of 0.27 mmol/kg

to 3 dogs (best computer-fitted line to mean data).

TABLE I

PHARMACOKINETIC PARAMETERS OF HI-6 IN DOGS GIVEN 0.27 mmol/kg i.v.

Pharmacokinetic Dog Mean + SD parameter?

1 2 3

cf, min-’ 0.1962 0.328 0.2658 0.2633 f 0.0659

110~. min 3.53 2.11 2.61 2.75 + 0.72

0, min- ’ 0.0105 0.0109 0.0101 0.0105 f 0.004

tt/2, min 65.9 63.3 68.9 66.03 k 2.8

AUC, pmol/ml/min 93.5 102.87 82.48 92.95 * 10.21

Cl ml/min/kg I0I. 2.98 2.71 3.38 3.02 k 0.34

Vdr I/kg 0.28 0.25 0.33 0.29 k 0.04

a OL, distribution rate constant ti/za, distribution half-life; f3, elimination rate constant; fi/2, elimination

half-life; AUC, area under the curve from zero to infinite; Clt,t, total body clearance, Vd, volume of

distribution.

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sus time curve was biexponential so that the two-compartment model used for data

analysis. The various pharmacokinetic parameters calculated for the i.v. dose are

shown in Table I.

The mean distribution half-life was 2.75 min, and distribution was complete in

5-15 min. The elimination half-life was 66.03 min and the AUC was 92.95

pmol/ml/min. Total body clearance was 3.02 ml/min/kg, and volume of distribu-

tion was 0.29 l/kg.

During the first 15 min after HI-6 injection all dogs showed tremor, moderate

hypersalivation and irregular respiration that were transient and ceased without se-

quelae.

Fig. 2 shows the plasma concentration vs. time curves for conventional and retard

tablets. The absorption of HI-6 from conventional tablets was more rapid than from

retard tabiets. The mean values of maximal HI-6 plasma levels were similar while

the times at which the maxima were reached were statistically different (Table II).

The elimination rate constants and elimination half-lives for conventional tablets

were very close to those observed for retard tablets. Among other pharmacokinetic

estimates only the AUCs were found to be significantly different. The AUC of con-

ventional tablets was 67% of that for retard tablets.

Only 5.42% of HI-6 was absorbed from conventional tablets and 8.07% HI-6

from slow-release tablets, indicating poor absolute bioavailability. No behavioral

disturbances followed the administration of both forms of tablets to dogs.

l CONVENTIONAL TBL, N=7

0 RETARD TEL, N=ll

u 9-

0 1 2 3 4 5 6 7 a 9 10 11

Fig. 2. Conventional and retard HI-6 tablets: comparison of plasma HI-6 concentrations in dogs. Open

and filled circles are the means. The solid line is the best computer-fitted curve to mean data.

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TABLE II

PHARMACOKINETIC PARAMETERS OF HI-6 IN DOGS AFTER ADMINISTRATION OF CON-

VENTIONAL AND RETARD TABLETS

Pharmacokinetic parametersa HI-6 tablets (mean + SD)

Conventional (n = 7) Retard (n = 11)

k,, h-’

t1/2a, h

C maX, pmol/l

tln,x, h

k,, h-’

ft/z, h

AUC, gmol/ml/h

Cl,,,, l/h/kg

I’d, l/kg

Fb, %

0.715 + 0.141

1.0 + 0.21

20.74 f 7.34

2.86 * 1.21

0.321 * 0.12

2,51 + 1.15

0.084 + 0.033

0.19 ?Z 0.09

0.70 f 0.47

5.42

0.557 + 0.173

1.34 + 0.37*

21.38 k 7.02

5.5 + 1.0*

0.324 + 0.185

2.75 + 1.35

0.125 + 0.024*

0.19 * 0.03

0.75 * 0.4

8.07

a k,, absorption rate constant; tt/za. absorption half-life; C,,,, peak plasma level; tmax, time to reach

peak plasma level; k,, elimination rate constant; tt/z, elimination half-life; AUC, area under the curve

from zero to infinite; Cl,,,, total body clearance; Vd, volume of distribution.

b Absolute bioavailability = mean value of AUC, p.o.

x 100. mean value of AUC, i.v.

* P < 0.05.

DISCUSSION

Plasma HI-6 levels after single i.v. injection to dogs have been previously deter-

mined by Simons and Briggs [9] and by Jovanovic et al. [ 101. Simons and Briggs

reported a slower distribution and faster elimination of HI-6 in beagle dogs (tr,zol

= 6.3 min; tl/z = 48.2 min). Jovanovic et al. have not determined distribution half-

life of HI-6, but the elimination of HI-6 in German shepherd dogs (ti/2 = 44.4 min)

is comparable to that found by Simons and Briggs. Additionally, Simons and Briggs

and Jovanovic et al. also ascertained a higher total body clearance (5.16 and 7.98

ml/min/kg) and a higher volume of distribution (0.37 and 0.49 l/kg), respectively.

A difference between the present study and those of Simons and Briggs [9] and

Jovanovic et al. [lo] consists mainly in the doses used. Jovanovic et al. [lo] ad-

ministered 0.027 mmol/kg of HI-6 and Simons and Briggs [9] 0.054 mmol/kg. Some

other factors may also contribute to explain the differences in the kinetic

parameters, including the speed and force of injection, species variations, the age

and weight of the dogs. Therefore, these factors might be responsible for distinc-

tions reported in tl/za, tl/z, CL,, and vd. However, the results obtained by Simons

and Briggs [9] and by Jovanovic et al. [ 101 agree with the present experimental con-

clusions that an open two-compartment pharmacokinetic model has to be used to

accurately describe the pharmacokinetics of HI-6 in the dog following intravenous

HI-6 injection.

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In a study of new oral preparations of HI-6 we have compared the kinetics of con- ventional to retard HI-6 tablets following a single dose. The peak plasma HI-6 levels of both conventional and retard tablets were similar, but the time to reach the peak was significantly longer after administration of retard tablets. Similar results were obtained in dogs using two formulations of pralidoxime [6].

Cvetkovic et al. [6] studied the pharmacokinetics of pralidoxime at about 2-fold higher dose (0.58 mmol/kg), and, in contrast to the present report, found 5-6 times higher levels of pralidoxime following retard and conventional tablets. Evaluating pharmacokinetic parameters from that study [6] we found significant differences (P<O.O5) between conventional and retard pralidoxime tablets in the half-lives of absorption (1.21 and 1.99 h) and in the times at which peak plasma pralidoxime levels were reached (4.2 and 8.3 h). Calculated AUC values for both formulations were similar: 1.21 and 1.18 pmol/ml/h respectively. As the AUC reflect the amount of oxime absorbed, there is a disparity between the plasma levels of HI-6 and pralidoxime. Within the ranges studied, 2-3 times more HI-6 than pralidoxime is needed for a similar blood level.

The low bioavailability of HI-6 after oral administration may reflect a poor ab- sorption of the oxime itself, or could be due to the biotransformation of HI-6 to compounds not detectable by our method. From kinetic studies with HI-6 in rats [4,11,12], dogs [9,13] and man [14] emerges that 40-90’70 of HI-6 is excreted un- changed in urine after i.m. or i.v. administration. Hence extensive biotransforma- tion is unlikely, although the metabolism of HI-6 may be different when it is administered by various routes.

In conclusion, the data presented show significantly different rates of absorption and amounts of HI-6 absorbed (in terms of AUC values) from both oral formula- tions. Very high oral doses of HI-6 will be necessary to achieve therapeutic plasma levels in dogs, since the bioavailability of oral formulations is poor. From the point of further therapeutic promise of HI-6 additional investigations aimed on improve- ment of its absorption are indicated.

ACKNOWLEDGEMENTS

We are indebted to them. Azra Ciranov and them. Svetlana Lazarev from Bosnalijek drug company, Sarajevo, for the skillful synthesis of HI-6, to Ph.c. Lea Levi and Ph.c. Sanja Stjepanovic from Bosnalijek for preparing the tablets and to Mr. Ljubomir TaSiC and Mr. Vladimir Vlaisavljevic for competent technical assistance.

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