Die mittlere Verweildauer — ein Maß zur Bewertung der Bioverfügbarkeit eines retardierten Theophyllin-Präparates

Summary

Dost [1] 1958 defined the mean time („mittlere Verweildauer“) as the statistical mean of all times during which any individual molecule of an amount of drug is held up within a pharmacokinetic system prior to being eliminated from it [2]. Dost‘s approach led to his rule of the areas — a usefull tool in the assessment of the extent of bioavailability — and to the concept of mean times (von Hattingberg and Brockmeier 1978) (2) which provides the basis for the assessment of the rate of bioavailability.

Using the additivity of mean times in pharmacokinetic models the mean time of a drug in the total system (T_sys) is the sum of partial mean times, i. e. the sum of mean time of in vivo dissolution (T_diss-vivo), absorption (T_abs) and mean transit time (T_vss) which characterizes the body model (2, 6, 7, 8).

$$ {T_{sys}} = {T_{diss - vivo}} + {T_{abs}} + {T_{vss}}\quad \left( {time} \right)$$

Analyzing the entrance (3) of drug into the total body model as represented by its steady state volume of distribution (V_ss)the mean time of invasion (T_inv) summarizes mean dissolution time (T_diss-vivo) and mean absorption time (T_abs).

$$ {T_{inv}} = {T_{diss - vivo}} + {T_{abs}}\quad \left( {time} \right)$$

Following a suitably designed experiment mean invasion times (T_inv) can be separated in the form of differences of mean system times (T_sys) and mean transit times (T_vss).

$$ {T_{inv}} = {T_{sys}} + {T_{vss}}\quad \left( {time} \right)$$

The mean time of invasion (T_inv) includes all time consuming processes during a drug’s entrance into the body and therefore is a measure for the rate of bioavailability.

This is demonstrated experimentally using a retarded theophylline preparation. The mean times (T_sys and T_vss) were generated from theophylline plasma data via the development of transit (9) curves by pragmatic numerical integration and exponential extrapolation (6).

The mean system times (T_sys) after linear infusion were corrected to get mean transit times (T_vss) (2, 7).

$$ {T_{vss}} = {T_{sys}} - \tau /2\quad \left( {time} \right)$$

The experimental data of the retarded theophylline formulation (Solosin0 retard film-coated tablets) result in an extent of bioavailability (absolute) of 95 percent and 5.3 hours as the mean time of invasion (Ti_nvasi_on) for the rate of bioavailability.

Conclusions: It has been demonstrated using a theophylline retard preparation that the mean time of invasion — generated without any assumptions as to the nature of the underlying pharmacokinetic model — is an exact measure for the rate of bioavailability.

Zusammenfassung

Die Untersuchung an gesunden jungen Probanden zeigt, daß Theophyllin aus Solosin retard Filmtabletten praktisch vollständig verfügbar ist, und daß das Ausmaß der Retardierung mit Hilfe des Konzeptes der mittleren Verweildauer [2] exakt quantifizierbar ist.