May a different kinetic mode explain the high efficacy/safety profile of inhaled budesonide?

The claimed functional basis for ICSs in asthma and COPD is airway selectivity, attained by inhaling a potent, lipophilic compound with long local dissolution/absorption time. The development has been empirically based, resulting in five widely used ICSs. Among them, budesonide (BUD) deviates by being less lipophilic, leading to a more rapid systemic uptake with plasma peaks with some systemic anti-inflammatory activity. By this, BUD fits less well into the current pharmacological dogma of optimal ICS profile. In this review we compared the physicochemical, pharmacological and clinical properties of BUD, fluticasone propionate (FP) and fluticasone furoate (FF), representing different levels of lipophilicity, airway and systemic kinetics, focusing on their long-acting β2-agonist (LABA) combinations, in line with current GINA and GOLD recommendations. We are aware of the differences between formoterol (FORM) and the not rapid acting LABAs such as e.g. salmeterol and vilanterol but our comparisons are based on currently available combination products. A beclomethasone dipropionate (BDP)/FORM combination is also commented upon. Based on clinical comparisons in asthma and COPD, we conclude that the BUD/formoterol (BUD/FORM) combination is as effective and safe as the FP and FF combinations, and is in some cases even better as it can be used as "maintenance plus reliever therapy" (MART) in asthma and as maintenance in COPD. This is difficult to explain by current views of required ICS's/LABAs pharmacokinetic profiles. We propose that BUD achieves its efficacy by a combination of airway and systemic activity. The airway activity is dominating. The systemic activity contributes by plasma peaks, which are high enough for supportive anti-inflammatory actions at the blood and bone marrow levels but not sufficiently long to trigger a similar level of systemic adverse effects. This may be due to BUD's capacity to exploit a systemic differentiation mechanism as programmed for cortisol's various actions. This differentiation prospect can be reached only for an ICS with short plasma half-life. Here we present an alternative mode for an ICS to reach combined efficacy and safety, based on a poorly investigated and exploited physiological mechanism. A preference of this mode is broader versatility, due to that its straighter dose-response should allow a better adaptation to disease fluctuations, and that its rapid activity enables use as "anti-inflammatory reliever".