Peroxidative Metabolism of β₂-Agonists Salbutamol and Fenoterol and Their Analogues

Abstract

Phenolic β 2 -adrenoreceptor agonists salbutamol, fenoterol and terbutaline relax smooth muscle cells that relieve acute airway bronchospasm associated with asthma. Why their use sometimes fails to relieve bronchospasm, and why the drugs appear to be less effective in patients with severe asthma exacerbations, remains unclear. We show that in the presence of hydrogen peroxide, both myeloperoxidase, secreted by activated neutrophils present in inflamed airways, and lactoperoxidase, which is naturally present in the respiratory system, catalyze oxidation of these β 2 -agonists. Azide, cyanide, thiocyanate, ascorbate, glutathione, and methimazole inhibited this process, while methionine was without effect. Inhibition by ascorbate and glutathione was associated with their oxidation to corresponding radical species by the agonists'-derived phenoxyl radicals. Using electron paramagnetic resonance (EPR), we detected free radical metabolites from β 2 -agonists by spin trapping with 2-methyl-2-nitrosopropane (MNP). Formation of these radicals was inhibited by pharmacologically-relevant concentrations of methimazole and dapsone. In alkaline buffers radicals from fenoterol and its structural analog, metaproteronol, were detected by direct EPR. Analysis of these spectra suggests that oxidation of fenoterol and metaproterenol, but not terbutaline, causes their transformation through intramolecular cyclization by addition of their amino nitrogen to the aromatic ring. Together, these results indicate that phenolic β 2 -agonists function as substrates for airway peroxidases and that the resulting products differ in their structural and functional properties from their parent compounds. They also suggest that these transformations can be modulated by pharmacological approaches using appropriate peroxidase inhibitors or alternative substrates. These processes may affect therapeutic efficacy and also play a role in adverse reactions of the β 2 -agonists.

Airway Peroxidases Catalyze Nitration of the β2-Agonist Salbutamol and Decrease Its Pharmacological Activity

“…1) are potentially subject to peroxidative metabolism. Consistent with this notion, we have shown that the airway peroxidases LPO and MPO can catalyze oxidation of ␤ 2 -agonists in vitro (Reszka et al, 2009). Given that inflamed airways are also the source of NO 2 Ϫ , we were interested to examine the effect of NO 2 Ϫ on the peroxidative metabolism of the agonist.…”

“…Both species can be formed enzymatically by LPO, MPO, and EPO systems (Burner et al, 2000;Brü ck et al, 2001;Brennan et al, 2002;Monzani et al, 2004;Van Dalen et al, 2006;Reszka et al, 2009). At concentrations of NO 2 Ϫ markedly higher than the concentration of salbutamol, oxidation of NO 2 Ϫ to NO 2 • may prevail, and oxidation of salbutamol may be accomplished by its reaction with NO 2…”

“…We have previously reported that salbutamol undergoes oxidation by LPO/H 2 O 2 alone, but the process is not very efficient, because only a small portion of the drug was oxidized (Reszka et al, 2009). It was concluded that this was caused by the slow interaction of the drug with LPO compound II, which accumulates in the system.…”

“…The cAMP content for each sample was determined by extrapolating values to a standard curve prepared with known concentrations of cAMP. (Reszka et al, 2009). The reaction was, however, slow, presumably because of the inability of the drug to react efficiently with compound II of the peroxidases.…”

“…Figure 6A shows that AA in a concentration-dependent manner delayed the appearance of the peak at 410 nm, suggesting that nitration of salbutamol by LPO/H 2 O 2 /NO 2 Ϫ is also delayed. However, the resumption of the reaction after a lag period suggests that the inhibitory action of AA is not caused by inhibition/inactivation of the enzyme, but rather by its interaction with the drug-derived phenoxyl radicals (Reszka et al, 2009) and/or with NO 2…”