Br. J. clin. Pharmac. (1975), 2, 277-280

BIOAVAILABILITY AND DISSOLUTION OF DIFFERENT FORMULATIONS OF OXYTETRACYCLINE PREPARATIONS A. HART Department of Pharmacy, Liverpool Polytechnic, Liverpool L3 3AF

H.E. BARBER & T.N. CALVEY Department of Pharmacology and Therapeutics, University of Liverpool, Liverpool L69 3BX

1 The concentration of oxytetracycline in plasma was studied by microbiological assay after oral administration of five different preparations of the antibiotic. None of these preparations had been studied previously. 2 There was a statistically significant correlation between the time required for 50% dissolution at pH 2 and biological availability, as assessed by the peak plasma level or the area under the plasma concentration-time curve. 3 The mean bioavailability of oxytetracycline was greatest with preparations of the hydrochloride, and with film-coated tablets of the dihydrate. In contrast, sugar-coated tablets of oxytetracycline dihydrate were associated with poorer dissolution characteristics and reduced biological availabiiity.

Introduction

Several chemically equivalent preparations of oxytetracycline can be currently prescribed in Britain. Thus, the official preparation is available as film-coated or sugar-coated tablets of the dihydrate, or as capsules of the hydrochloride (British Pharmacopoeia, 1973), and there are many analogous commercial products. The biological availability of these different preparations is partly dependent on their rapid solution in gastric secretions. In in vivo conditions, the solubility of oxytetracycline hydrochloride may be considerably greater than the dihydrate (Barber, Calvey, Muir & Hart, 1974), although there are no published studies on the relative bioavailability of these two compounds. We have therefore compared the biological availability of three preparations of oxytetracycline dihydrate (including both film-coated and sugar-coated tablets), as well as two formulations of the hydrochloride. None of these preparations was studied in our previous paper (Barber et al., 1974); all of them were made by one manufacturer. Methods

Formulation details of the five preparations of oxytetracycline which were used in these experiments are shown in Table 1. The four official preparations (i.e., A, B, C and E) all complied with

the requirements of the British Pharmacopoeia

(1973). In vitro studies

Dissolution tests on at least five samples of each preparation were carried out in water at pH 2.0. The dissolution method used was an autoriated modification of the rotating basket method described in the United States PharmacopoeiaNational Formulary XIII, 1970 (Randall & Goldsmith, 1975).

Table 1 Formulation details of the five oxytetracycline preparations Preparation A B C

D E

Formulation

Oxytetracycline dihydrate tablets (non-shellaced core; sugar-coated) Oxytetracycline dihydrate tablets (shellaced core; sugar-coated) Oxytetracycline dihydrate tablets (film-coated) Oxytetracycline hydrochloride tablets (film-coated) Oxytetracycline hydrochloride capsules

278

A. HART, H.E. BARBER & T.N. CALVEY

In vivo studies

loor

Five healthy male student volunteers (mean body weight = 75 kg; range = 68-81 kg) were used in the in vivo studies. A total of 25 experiments were carried out, using a factorial design; thus, each subject received all five preparations in a randomized sequence on different occasions. In all subjects, an interval of at least 2 weeks elapsed between successive experiments. No other drugs were taken during the period of study, and the informed consent of each subject was obtained. After overnight fasting, a control blood sample (about 10 ml) was removed by venepuncture and added to a tube containing lithium heparin. Two tablets or two capsules (500 mg) of one of the oxytetracycline preparations (Table 1) were then swallowed with approximately 250 ml water. Fasting conditions were maintained for at least 3-4 hours. Samples of venous blood were obtained at 30, 60, 90, 120, 180, 270, 360 and 405 min, and added to lithium heparin as described above. Plasma was immediately obtained from all blood samples by centrifugation, and stored at -20°C. Oxytetracycline in specimens of plasma was assayed in triplicate by a microbiological method (Grove & Randall, 1955), using the test organism Bacillus cereus (var. mycoides). Further details of the assay are described in a previous paper (Barber et al., 1974).

Results In vitro studies The mean dissolution profiles of the five preparations are shown in Figure 1. In general, the formulations of oxytetracycline hydrochloride (D and E) and film-coated tablets of the dihydrate (C)

.5SC

-w

sa

8 60

0)

.ig>%4C 0

10

20

30

40

50

Time(min) Figure I Mean dissolution curves (pH 2.0) of the five oxytetracycline preparations.

had very similar dissolution characteristics. More than half the oxytetracycine in these preparations was released within 15 min, and solution was complete by 20 minutes. Both sugar-coated oxytetracycline dihydrate tablets had slower dissolution rates; the tablets without shellac (preparation A) dissolved more rapidly than the shellaced preparation B. Differences between the mean time required for 25%, 50%, and 75% dissolution of the five preparations were usually statistically significant (Table 2). In vivo studies

There were considerable differences in the mean bioavailability of the five preparations, as assessed

Table 2 In vitro dissolution data of the five oxytetracycline preparations. The dissolution parameters t25, tso, and t7, represent the times required for 25%, 50%, and 75% of the drug to dissolve at pH 2.0.

Preparation A B C D

E

t2S 16.6 ± 0.5*** 16.4 ± 0.3*** 12.7 ± 1.0** 10.6 ± 0.3** 8.9 ± 0.2*

tso

20.4 ± 22.1 ± 14.5 ± 12.7 ± 10.3 ±

0.9*** 0.8*** 0.9** 0.4** 0.3*

t75 28.1 ± 33.4 ± 17.0± 14.8 ± 12.2 ±

1.1* 1.5* 1.0** 0.3** 0.3*

* Significantly different (P < 0.05) from all other preparations. ** Significantly different (P < 0.05) from preparations A, B & E. *** Significantly different (P < 0.05) from preparations C, D & E. Values correspond to the mean ± s.e. mean of at least five experiments; differences between the means were analysed by the Student's t test.

BIOAVAILABILITY OF OXYTETRACYCLINE PREPARATIONS

by the area under the plasma concentration-time curve between 0 and 405 min (Table 3). Thus, there was nearly a two-fold difference between the availability of preparation B and preparation D. There was a statistically significant correlation (r = -0.947; 0.005 > P > 0.001) between the tso values (Table 2) and mean bioavailability, as assessed by the area under the plasma concentration-time curve (Table 3) for the five preparations. Although the maximum concentration of oxytetracycline in plasma invariably occurred in the samplc obtained 1 20 or 180 min after oral administration, differences between the levels achieved by the five preparations were observed. The mean maximum plasma level produced by preparations C, D, and E was almost identical (1.43 gg/ml, 1.51 ,ug/ml, and 1.45 jig/ml respectively). In contrast, lower peak concentrations were achieved by tablet A (1.20 jig/ml) and tablet B (0.88 ,ug/ml). The correlation between the mean maximum plasma levels (Table 3) and the mean t5o values (Table 2) was statistically significant (r = -0.900; 0.02 > P > 0.01). As shown in other studies (Scales & Assinder, 1973; Barber et al., 1974), there was a marked variation in individual plasma levels with all five preparations, as indicated by the large standard errors (Table 3). In consequence, statistically significant differences (P < 0.05) were not obtained between the mean plasma levels of different preparations, except with tablets B and D. As shown in Table 3, differences between these two preparations were statistically significant at 90, 120, 180, 270, 360 and 405 minutes.

Discussion

Differences in the biological availability of commercial preparations of tetracyclines have been reported by authors in the United States (Brice & Hammer, 1969; Macdonald, Pisano, Burger, Dornbush & Pelcak, 1969), Holland (Altmann, Beeuwkes, Brombacher, Buytendijk, Gigen & Maesen, 1968), Scandinavia (Bergen, Oydvin & Lunde, 1972; Tuomisto & Mannisto, 1973) and Britain (Barnett, Smith, Greenwood & Hetherington, 1974; Barber et al., 1974). In most of these studies bioavailability was assessed by the area under the plasma concentration-time curve or the urinary elimination of the antibiotic. In one instance, there was a rank correlation between tetracycline excretion in urine and dissolution in vitro in simulated gastric juice (Macdonald et al., 1969), although the relationship between bioavailability and acid dissolution was generally less well defined (Brice & Hammer, 1969; Barnett et al., 1974; Barber et al., 1974). These studies have generally been concerned with standard products of several manufacturers, and it is usually accepted that variations in both bioavailability and dissolution are related to differences in the formulation of the antibiotic. In our previous paper (Barber et al., 1974), it was concluded that there was no apparent relationship between the bioavailability of four oxytetracycline dihydrate preparations and acid dissolution in in vitro conditions. Other authors have shown that two formulations of the drug with similar dissolution profiles at pH 2 may

Table 3

Plasma levels and bioavailability of the five oxytetracycline preparations. Values represent the mean ± s.e. mean of at least five experiments

Time

Plasma concentration

(min)

(9ig/mi) A

30 60 90 120 180 270 360 405 Bioavailability* *

0.21 0.13 0.44 ± 0.24 1.01 ± 0.24 1.20 ± 0.27 1.17 ± 0.21 0.95 ± 0.16 0.77 ± 0.13 0.66 ± 0.10 343.4 ± 58.6 ±

B

0.16 0.10 0.53 ± 0.19 0.71 ± 0.23 0.82 ± 0.27 0.88 ± 0.29 0.74 ± 0.26 0.64 ± 0.21 0.56 ± 0.20 267.2 ± 89.5 ±

C

0.31 ± 0.22 0.72 ± 0.37 0.76 ± 0.36 1.13 ± 0.37 1.43 ± 0.28 1.36 ± 0.27 1.10 ± 0.23 1.04 ± 0.19 431.6 ± 93.8

D 0.21 ± 0.13 0.92 ± 0.21 1.33 ± 0.28** 1.51 ± 0.29** 1.46 ± 0.26** 1.43 ± 0.28** 1.34 ± 0.31 ** 1.17 ± 0.31**

496.6 ± 98.4

Area under plasma concentration-time curve between 0 and 405 min (jIg/ml x min). Significantly different (P < 0.05) from the mean values for preparation B (paired t test).

**

279

E

0.38 ± 1.08 ± 1.23 ± 1.33 ± 1.45 ± 1.35 ± 1.09 ± 1.05 ± 468.2 ±

0.17 0.34 0.35 0.27 0.26 0.30 0.27 0.25 101.5

280

A. HART, H.E. BARBER & T.N. CALVEY

produce different peak plasma concentrations (Jones, Risdall & Frier, 1974). In the present experiments, in vitro dissolution tests and in vivo studies were carried out independently, and the results were not compared until the study was complete. Nevertheless, there was a statistically significant correlation between the time required for 50% dissolution at pH 2 and the mean bioavailability of the five preparations, as assessed by the area under the plasma concentration-time curve or the maximum level in plasma. The explanation for these differences between our present and our previous results is a matter of conjecture. Although an improved dissolution method was used in the current experiments, it is unlikely that this is entirely responsible for the differences observed. Our experience with oxytetracycline therefore suggests that in vitro and in vivo data can be correlated with some preparations, but not with others. As anticipated, the dissolution characteristics of the non-shellaced tablet of oxytetracycline dihydrate (tablet A) were marginally better than the normal, shellac-coated tablet (tablet B). These differehces were reflected in the plasma levels and the relative bioavailability of the two preparations. On the other hand, the dissolution profile and mean biological availability of the film-coated dihydrate tablets (preparation C) was only slightly less than film-coated tablets and capsules of the hydrochloride (preparations D and E). Indeed,

there was little difference (either in vitro or in vivo) between film-coated tablets of the watersoluble hydrochloride and the relatively insoluble dihydrate, although theoretical considerations suggest that the dissolution of the latter preparation is totally dependent on gastric acidity (Barber et al., 1974). It is therefore possible that the biological availability of oxytetracycline preparations may be improved by the exclusive use of film-coated tablets. The variations in bioavailability observed in the present experiments may well be of practical importance. Thus, it is possible that the inferior bioavailability of some oxytetracycine preparations can occasionally be the cause of therapeutic failure; in addition, the incidence of side-effects and secondary effects in the gastro-intestinal tract may be increased by enhancing the amount and concentration of non-absorbed antibiotic in the gut (Garrod, Lambert & O'Grady, 1973). Nevertheless, since steady state plasma concentrations in in vivo conditions are usually much greater than minimum inhibitory concentrations in vitro, the therapeutic significance of these results is a matter of conjecture. We wish to thank Berk Pharmaceuticals Ltd for their help with the dissolution tests. The technical assistance of Miss S. Newby and Miss A. Smith is also gratefully acknowledged. This investigation was supported by a generous grant from the McAlpine Foundation. Reprint requests should be addressed to T.N.C.

References ALTMANN, A.E., BEEUWKES, H., BROMBACHER, P.J., BUYTENDIJK, H.J., GIJEN, A.H. & MAESEN, F.P.V. (1968). Serum levels of tetracycline after

different administration forms. Cin chim Acta, 20, 185-1 88. BARBER, H.E., CALVEY, T.N., MUIR, K. & HART, A. (1974). Biological availability and in vitro dissolution of oxytetracycline dihydrate tablets. Br. J. clirL

Pharmac., 1, 405408.

BARNETT, D.B., SMITH, R.N., GREENWOOD, N.D. & HETHERINGTON, C. (1974). Bioavailability of commercial tetracycline products. Br. J. clii

Pharmac., 1, 319-323.

BERGEN, T., OYDVIN, B. & LUNDE, I. (1972). Biological availability and in vitro release from oral

oxytetracycline and tetracycline preparations. Acta pharmac. tox., 33, 138-156. BRICE, G.W. & HAMMER, H.F. (1969). Therapeutic non-equivalence of oxytetracycline capsules. J. Am. med. Ass, 208, 1189-1190. BRITISH PHARMACOPOEIA (1973). pp. 337-338. London: H.M.S.O. GARROD, L.P., LAMBERT, H.P. & O'GRADY, F.

(1973). In Antibiotic and Chemotherapy, p. 153. Edinburgh and London: Churchill Livingstone. GROVE, D.C. & RANDALL, W.A. (1955). In Assay Methods of Antibiotics: a Laboratory Manual, p. 50. New York: Medical Encyclopedia, Inc. JONES, T.M., RISDALL, P.C. & FRIER, M. (1974). The equivalence of oxytetracycline tablets BP. J. Pharm Pharmac., 26, Suppl., 1 16P. MACDONALD,

H.,

PISANO,

F.,

BURGER,

J.,

DORNBUSH, A. & PELCAK, E. (1969). Physiological availability of various tetracyclines. Clin Med., 76, 30-33. RANDALL, N. & GOLDSMITH, J.A. (1975). An automated dissolution test for tablets and capsules. Lab. Pract., 35, 24, 77. SCALES, B. & ASSINDER, D.A. (1973). Fluorometric estimation of oxytetracycline in blood and plasma. J. Pharnm Sci, 62, 913-917. TUOMISTO, J. & MANNISTO, P. (1973). Cross over study of ten tetracycline preparations. Eur. J. clirL Pharmac., 6, 64-68.

(Received October 24, 1974)