A rapid, selective and sensitive high pressure liquid chromatographic (HPLC) method for tile determination of theophylline in human serum (or plasma) and saliva was developed.When t, sing 0.5 ml of serum, concentrations down to o.4 mg.I -~ could be accurately measured. Each sample requires only about I5 minutes for the completion of the assay, including sample preparation. The actual chromatography time is about 8 minutes. The theophylline metabolites and other xanthines, as theobromine, caffeine (and its metabolite paraxanthine) are well separated. The sensitivity, precision and accuracy are sufficient for routine monitoring of therapeutic theophylline serum levels in patients (approximately lo to 20 rag.l-t). Rehability of the method was demonstrated during analysis of about 3000 samples on the same column. (Pharm. Weekblad Sci. Ed. 2,(49)(50)(51)(52)(53)

The influence of time of drug administration on pharmacokinetics of theophylline was studied both after ingestion of a sustained-release tablet, containing choline theophyllinate ( Zy 15061-S. R.; Teovent ; Sabidal ; ZYMA S.A.) and after intravenous infusion of aminophylline to eight healthy volunteers. Both drugs were administered in the morning (10 a.m.) and on a separate occasion in the evening (10 p.m.) after a 12 h period of fasting. After oral administration of a dose of 540 mg theophylline, the drug was steadily absorbed, both during day-time and during night-time. In some subjects absorption was slower in the evening. Maximum theophylline plasma concentrations were reached after 3.3 +/- 0.4 h (mean +/- SD) and 3.9 +/- 1.4 h respectively (not significantly different p greater than 0.05). The maximum plasma concentrations were almost identical after administration in the morning and in the evening (12.6 +/- 3.3 mg X l-1 and 13.1 +/- 1.4 mg X l-1 respectively). There was also no significant difference (p greater than 0.05) between the areas under the plasma concentration-time curves after oral and intravenous administration, both at day-time and at night-time. This finding indicates complete bioavailability of the sustained release tablets on both occasions. After administration of the tablets in the morning the plasma concentration 12 h post dosing was significantly lower than after administration in the evening: c1 12 accounted for 6.0 +/- 2.0 mg X l-1 after intake at 10 a.m. and for 7.9 +/- 2.1 mg X l-1 after ingestion at 10 p.m. (p less than 0.01). A similar observation was done after intravenous administration of the drug: c12 was 6.6 +/- 1.6 mg X l-1 after starting the infusion in the morning and 8.0 +/- 1.8 mg X l-1 after infusing the drug in the evening (p less than 0.01). This phenomenon could be explained by the finding of a significantly prolonged half-life of theophylline during night-time, provided that the plasma concentrations were in the range of 5 to 15 mg X l-1 (which coincides approximately with the therapeutic range of the drug).(ABSTRACT TRUNCATED AT 250 WORDS)

1The in vitro characteristics (dissolution rate) of a sustained release theophylline preparation (Theo-Dur®) were first measured in acid medium (pH = 1) for 2 h and after that in a phosphate buffer (pH = 6.8) for 6 h. 2 The tablets released more than 95% of the active ingredient within 6 h at a rate of approximately 11% of the dose per hour at pH = 1 and about 18% at pH = 6.8. 3 Dose dependency of the pharmacokinetics of theophylline was tested in seven healthy volunteers by giving them either 300 mg Theo-Dur(E) or 900 mg Theo-Dur® in two doses a day for 5 days in a cross over design. After the last tablet on day 6 the fall-off curve was followed in order to calculate the pharmacokinetic parameters. 4 Theophylline (300mg) resulted on day 6 in a mean serum concentration of 4.4 0.8 ,ug ml-'The dose of 900 mg resulted in a proportional increase in the serum concentration. The result was 13.3 + 2.2 g ml-'. 5The T½,2 (300 mg) was 9.3 + 1.4 h and the 7T/2 (900 mg) was 8.5 + 2.0 h. These values do not significantly differ (0.10 < P < 0.20). 6 It was concluded that theophylline exhibits the rules of linear pharmacokinetics when serum concentrations are in the therapeutic range.

The correlation between serum and saliva levels of theophylline was investigated in seven healthy volunteers after multiple dose administration of a low dose (300 mg/day) and a high dose (900 mg/day) of a sustained release theophylline preparation (Theo-Dur). Tablets were taken for five days, at 8 a. m. and 8 p. m. and a last dose was taken on Day 6 at 8 a. m. Fourteen serum and saliva samples were collected simultaneously during the dosing period and for up to 32 h after the last dose. On the 300 mg/day regimen the level in saliva was 55.3% of the serum level, with an overall variability of 6.7% and an intrasubject variability of 10.5%. After 900 mg/day, the saliva concentration was 55.5% of the serum concentration, with an overall variability of 7.6% and an intrasubject variability of 12.7%. A good correlation was found between both determinations (r = 0.99), which suggests that saliva levels could be used to monitor theophylline after administration of a sustained release tablet.

The systemic disposition of theophylline after taking a new, sustained release tablet (Theolair Retard 250 mg, Theolair S.R., Riker Laboratories) has been studied in 8 hospitalized patients. Absolute bioavailability was determined from the ratios of the areas under the serum concentration-time curves after intake of the tablet and after intravenous infusion of aminophylline in the same patient. The absolute bioavailability of Theolair Retard 250 mg was 110.9 +/- 20.8% (mean +/-SD). Maximal serum concentrations were reached after 7.3 +/- 3.5 h, the large intersubject variation being due to differences in gastric emptying time. The tablets appear to release theophylline slowly in acid conditions, but more rapidly in an alkaline medium. Invasion was found to be either monophasic with a rate constant of about 0.8 h-1 (intestine), or biphasic with rate constants of 0.2 h-1 (stomach) and 0.8 h-1 (intestine). The peak levels accounted for 7.9 +/- 2.2 mg . 1-1. The profiles of the serum concentration-time curves were such that the concentrations remained above 80% of cmax for 6.5 +/- 3.3 h. The relevant pharmacokinetic parameters (half-life of elimination, total body clearance and volume of distribution) were determined and were used to calculate the individual dosage regimens required to obtain therapeutic serum concentrations. The optimal dosing interval to obtain an average steady state serum concentration of 12.5 mg . l-1 was 9.8 +/- 3.1 h.