Although available evidence supports the use of combined bronchodilator treatment in COPD, there are still some unknowns. In this respect, a close review of the literature may reveal two phenomena that require explanation.

The first stems from combination trials evaluating the functional effect of adding a twice daily LABA to a once daily LAMA (tiotropium).3,5 These studies similarly describe a very illustrative graph that shows the resulting functional improvement after the addition of the first and second dose of twice daily LABA in patients on tiotropium treatment. However, attention must be drawn to the fact that the second dose of LABA, the dose administered in the evening, achieves smaller functional improvements than those achieved with the morning dose.

This phenomenon may have several explanations. On one hand, the smaller effect of tiotropium 12h after the onset of action means that the onset of bronchodilation after the second dose of LABA occurs in a different part of the tiotropium action curve. Nevertheless, although this is true, the bronchodilator impact is more minor even if we take into account this decline in the action of tiotropium.8

Another possible explanation could be that the LABA acts differently according to the time of day at which it is administered. In this respect, the existence of a circadian rhythm that influences airway calibre has been described. This circadian rhythm causes the calibre of the airways to become smaller during the night, regardless of the state of sleep or wakefulness.9 To establish the importance of this phenomenon, various studies have been conducted on the efficacy of bronchodilators according to the time of day at which they are administered. Although detailed analysis of their results is beyond the scope of this editorial, most studies did not find major differences between administering a long-acting bronchodilator in the morning or at night, when given separately.10,11 However, if we study the joint action of two bronchodilators, a once daily LAMA (tiotropium) and twice daily LABA, we come across an interesting finding. A recent study explored the clinical efficacy of the combination of tiotropium and formoterol, in order to study different regimens according to the time of administration of both drugs (morning or evening) and the number of doses in the case of formoterol (1 or 2 doses daily), so five treatment groups were established.12 In this study, both treatment groups that received formoterol every 12h plus tiotropium (one group in the morning and the other in the evening) obtained better efficacy in lung function, symptoms and use of rescue medication than the other groups. However, patients with nighttime symptoms obtained a greater benefit in dyspnoea and use of rescue medication if the tiotropium was administered at night together with the formoterol instead of in the morning.12 This finding had not been described previously, and suggests that the joint action of both bronchodilators is more effective when the administration of the two drugs is closer, which constitutes the second phenomenon that requires explanation.

In order to understand why this functional and clinical improvement occurs with the joint administration of a LABA and a LAMA, we must briefly recall the mechanism of action of bronchodilators.

The mechanism of action of the β2 agonists is known and has been recently reviewed.13 Briefly, the stimulation of these receptors causes activation of G proteins coupled to the β2 receptor, which in turn regulate the action of an adenylate cyclase. This enzyme mediates the conversion of adenosine triphosphate to cyclic adenosine monophosphate (cAMP). cAMP regulates the expression of several cellular proteins, among which are the calcium channel, which in turn permits relaxation of the bronchial muscles. Therefore, adenylate cyclase and intracellular cAMP levels play a crucial role in the mechanism of action of the β2 receptors.

There are currently five known types of muscarinic receptors, termed M1–M5.14 Of these, M1–M3 are expressed in the human respiratory tract. Specifically, airway smooth muscle contains a mixed population of M2 and M3 receptors. The main bronchodilator effect derived from their inhibition is related with the M3 receptors, which through G proteins stimulate a phospholipase that in turn promotes the influx of calcium into the cell.15 However, despite the clear role of the M3 receptors in muscle contraction, the number of M3 receptors in the airways is much lower than the M2 receptors, with a ratio of 4:1.16

The role of the M2 receptor in the action of the LAMAs has not been sufficiently considered until now. However, the function of the M2 receptors is key for understanding the interaction with the β2 adrenergic receptors in the airways. It is known that the M2 receptors inhibit (via the G proteins) the generation and accumulation of cAMP in the cell, which is the mechanism of action of the β2 receptor. Thus, stimulation of an M2 receptor would inhibit the action of the β2 receptor. Although the exact site of interaction where this inhibition occurs is unknown, the relationship between both receptors contributes jointly to the contraction of the bronchial smooth muscle.17

From the above, it appears that it would be plausible to suggest that the effect of stimulating the β2 receptor would be greater during M2 receptor inhibition. To understand the clinical implications of this suggestion, we must remember the tiotropium pharmacodynamic studies. It is well known that tiotropium exerts its bronchodilatory action by inhibiting the M3 receptors. However, it is equally certain that it also inhibits the M2 receptors, only that this M2 inhibition is transient, with a half life of 3.6h.8 Thus, M2 receptor inhibition and the subsequent release of β2 last barely a few hours, so if we want to take advantage of the potential benefits of this M2–β2 interaction, it follows that both drugs must be administered at the same time, in order for the M2 inhibition to unblock the β2 receptor and allow it to have greater action on being stimulated. This would explain the smaller effect of the second dose in the LABA–LAMA combination studies,3,5 as it lacks this interaction in the second administration, or the greater clinical effect when the formoterol and tiotropium doses are given simultaneously in the study by Terzano et al.12

If this hypothesis is true, it would be advisable to use a LABA together with a LAMA to be able to take advantage of the M2–β2 interaction. Thus, if once daily bronchodilators are administered, it would be reasonable to recommend their simultaneous administration at the same time of the day, instead of one in the morning and the other at night. On the contrary, if a twice daily LABA is used, it would be reasonable to recommend the administration of a twice daily LAMA. At present there are two twice daily (salmeterol, formoterol) and one once daily LABAs (indacaterol) available. Only one once daily LAMA has been available to date (tiotropium), although new LAMA molecules have recently been approved with an administration profile every 12h (aclidinium18) and every 24h (glycopyrronium19). Clinical trials exploring the clinical relevance of this interaction on combining once or twice daily bronchodilators are therefore required.

In recent years, we have been witnessing an expansion in the development of bronchodilator drugs towards more potent, longer-lasting molecules with a better safety profile. Knowledge of the mechanisms of action of both bronchodilator families, LABA and LAMA, provides us with information that helps us to understand the interaction between both, with a significant role between the M2 and β2 receptors. Understanding this receptor interaction is crucial for suggesting the optimal form of administration of these drugs, so that the best clinical benefits can be obtained for patients. Future clinical studies must ascertain the clinical importance of this M2–β2 interaction in specific clinical contexts.

The author has received fees for giving lectures, scientific advice, participation in clinical studies and writing articles for (alphabetical order): Almirall, AstraZeneca, Bayer, Boehringer Ingelheim, Cantabria Pharma, Chiesi, Esteve, Faes, Ferrer, GlaxoSmithKline, MSD, Novartis and Pfizer.