Pharmacokinetic/pharmacodynamic modelling of effects of dexamethasone and prednisolone in combination with endogenous cortisol on lymphocyte counts and systemic markers of bone turn over and inflammation in healthy and asthmatic men

Objective. To compare potency and efficacy of dexamethasone (DEXA) and prednisolone (PRED) in assumed equipotent doses in combination with endogenous cortisol, using lymphocyte counts, plasma osteocalcin (OC), and eosinophilic cationic protein (ECP) as effect variables and to evaluate potential differences between healthy subjects and asthmatic patients. Methods. Eight healthy subjects and six asthmatic patients who had stopped taking their regular inhaled glucocorticosteroid treatment (ICS) for 1 week, were given an IV bolus of DEXA and PRED in assumed equipotent doses of 2.0 mg and 12.5 mg, respectively, on separate occasions, in combination with subcutaneously injected granulocyte-colony-stimulating factor (G-CSF) as a stimulant for ECP production. Plasma levels of DEXA, PRED, cortisol and effect variables were determined over 25 h and pharmacokinetic-pharmacodynamic (PK-PD) modelling was performed. Results. Baseline cortisol concentration was lower in patients than in healthy subjects. Both of the exogenous glucocorticoids (GCs) diminished cortisol production. In the healthy subjects, the cortisol production remained suppressed for the full duration of the study day after DEXA but not after PRED. In the asthmatic patients though, the reappearance of the endogenous production of cortisol was seen after both DEXA and PRED. The E-max values for lymphocyte counts and OC showed that cortisol acted as partial, and DEXA and PRED as full agonists. The observed responses of DEXA and PRED suppressing cortisol, OC and lymphocyte counts were all of the same relative order of magnitude, in accordance with the estimated PD parameters. However, cortisol was estimated to have very little effect on ECP and modelling further predicted that DEXA and PRED were only partial agonists for this effect, without a difference between healthy and asthmatic subjects. Yet, in healthy subjects, the area under the concentration-time curves (AUCs) indicated unexpectedly that ECP was only suppressed after PRED and not after DEXA, while in patients it was suppressed after both GCs. The rank order of potency on lymphocyte counts, OC and ECP was DEXA>PRED>cortisol, although the different relative potencies of the three GCs involved were not the same for all of the three effect variables and differences were also found between healthy and asthmatic subjects. Conclusion. PK-PD modelling studies of GCs demonstrated not only differences in potency of DEXA and PRED on the measured systemic markers, but also different potencies per target tissue and differences between healthy and asthmatic men. The effects caused by the achieved blood concentrations of DEXA and PRED, expressed as AUCs of the effect variables, were in accordance with their respective E-max values in case of the lymphocytes and OC but not for ECP