Regional pharmacokinetics. II. Experimental methods

Human subcutaneous tissue distribution of fluconazole: comparison of microdialysis and suction blister techniques

AIMS

To investigate uptake of fluconazole into the interstitial fluid of human subcutaneous tissue using the microdialysis and suction blister techniques.

METHODS

A sterile microdialysis probe (CMA/60) was inserted subcutaneously into the upper arm of five healthy volunteers following an overnight fast. Blisters were induced on the lower arm using gentle suction prior to ingestion of a single oral dose of fluconazole (200 mg). Microdialysate, blister fluid and blood were sampled over 8 h. Fluconazole concentrations were determined in each sample using a validated HPLC assay. In vivo recovery of fluconazole from the microdialysis probe was determined in each subject by perfusing the probe with fluconazole solution at the end of the 8 h sampling period. Individual in vivo recovery was used to calculate fluconazole concentrations in subcutaneous interstitial fluid. A physiologically based pharmacokinetic (PBPK) model was used to predict fluconazole concentrations in human subcutaneous interstitial fluid.

RESULTS

There was a lag-time (approximately 0.5 h) between detection of fluconazole in microdialysate compared with plasma in each subject. The in vivo recovery of fluconazole from the microdialysis probe ranged from 57.0 to 67.2%. The subcutaneous interstitial fluid concentrations obtained by microdialysis were very similar to the unbound concentrations of fluconazole in plasma with maximum concentration of 4.29 +/- 1.19 microg ml(-1) in subcutaneous interstitial fluid and 3.58 +/- 0.14 microg ml(-1) in plasma. Subcutaneous interstitial fluid-to-plasma partition coefficient (Kp) of fluconazole was 1.16 +/- 0.22 (95% CI 0.96, 1.35). By contrast, fluconazole concentrations in blister fluid were significantly lower (P < 0.05, paired t-test) than unbound plasma concentrations over the first 3 h and maximum concentrations in blister fluid had not been achieved at the end of the sampling period. There was good agreement between fluconazole concentrations derived from microdialysis sampling and those estimated using a blood flow-limited PBPK model.

CONCLUSIONS

Microdialysis and suction blister techniques did not yield comparable results. It appears that microdialysis is a more appropriate technique for studying the rate of uptake of fluconazole into subcutaneous tissue. PBPK model simulation suggested that the distribution of fluconazole into subcutaneous interstitial fluid is dependent on tissue blood flow.

An in vitro-in vivo validation of the isolated perfused tumor and skin flap preparation as a model of cisplatin delivery to tumors

The isolated perfused tumor and skin flap (IPTSF) is a unique model system in which drug disposition is evaluated in tumor tissue and surrounding normal tissue, both of which are supplied by the same vascular system. We compared tissue Pt concentrations obtained following systemic administration of cisplatin (CDDP) to whole pigs bearing tumored skin flaps with data obtained from IPTSF treated similarly. During the in vivo study, CDDP was administered intravenously to six pigs that had tumor and skin flaps. IPTSF were created in four pigs and isolated in a perfusion chamber and perfused with medium containing CDDP for 180 min. Venous plasma or perfusate samples were serially collected throughout perfusion. Tissue samples were collected after perfusion was complete. All samples were assayed for Pt by atomic absorption spectroscopy. Area under the curve of Pt profiles from IPTSF and in vivo perfused flaps were not significantly different. Pt concentrations were significantly higher in tumor samples from in vivo perfused flaps than in samples from IPTSF. Pt concentrations in skin and subcutaneous tissue were not significantly different. When consideration is given to all of the potential variables that were operative in these experiments, the results of this study demonstrate that Pt distribution within the IPTSF was comparable to that obtained in vivo.

Effect of hyperthermia on cisplatin and carboplatin disposition in the isolated, perfused tumour and skin flap

The effect of hyperthermia on the disposition of platinum (Pt) from cisplatin (CDDP) and carboplatin (CBDCA) in the isolated, perfused tumour and skin flap (IPTSF) was evaluated. Flaps (n = 4/treatment) were perfused with 3.0 micrograms CDDP or 15 micrograms CBDCA/ml perfusion medium at a rate of 1 ml/min for 3 h. Two-hour (CDDP experiments) or 3 h (CBDCA experiments) washout phases were then performed. The disposition kinetics of free Pt were characterized using a four-compartment, physiologically relevant, pharmacokinetic model. Hyperthermia (HT) may have enhanced the mobility of Pt but it did not increase total Pt mass in the tissue compartments in CDDP experiments. Conversely, HT significantly increased Pt mass in the fixed, non-tumour tissue compartment (p < 0.05) in CBDCA experiments. While a similar trend was noted in the fixed, tumour tissue compartment of CBDCA-treated flaps, the difference was not significant (p = 0.17). Total tissue Pt mass was significantly greater in CDDP compared with CBDCA experiments (p < 0.05). In conclusion, HT alters the disposition of Pt from CDDP and CBDCA under conditions of constant rate infusion. Further characterization of factors influencing drug disposition to non-tumour and tumour tissues can be systematically accomplished using the IPTSF.

Regional pharmacokinetics. III. Modelling methods

Regional pharmacokinetics is the study of drug concentrations in specific regions of the body due to drug uptake and elution. Mathematical methods of interpreting regional pharmacokinetic data can vary greatly in their complexity depending on their intended use (i.e. to describe or predict), but must reinforce rather than replace experimental pharmacokinetics. 'Black box' analysis provides and empirical method for the study of complex pharmacokinetic systems using either statistical moment or linear systems analysis. However, these methods are only applicable to linear and time-invariant systems, and ignore the large body of information concerning the physiological and physiochemical basis of regional pharmacokinetics. Clearance concepts are suitable for describing linear drug uptake processes, but mass balance principles have wider applications in describing the rate and extent of both drug uptake and elution. Compartmental models of a region can vary from single compartment descriptions based on the concept of venous equilibrium to complex multi-compartmental models of the intravascular, interstitial, and intracellular spaces, in which drug transport between compartments is a function of drug binding and ionization. Ultimately, as more regional pharmacokinetic information is obtained, more complex three dimensional models may be necessary such as those used to describe the uptake of oxygen from capillaries.