Chiral Switch Drugs for Asthma and Allergies: True Benefit or Marketing Hype

Chiral Switch: Between Therapeutical Benefit and Marketing Strategy

Chirality of pharmaceutical substances is an important aspect in drug research because it determines how enantiomers will interact with chiral biological targets. Enantiomers of a chiral drug can have different pharmacokinetic and pharmacological profiles; consequently, using a single pure enantiomer instead of a racemate can enhance the effectiveness and/or safety of the treatment. The tendencies of modern pharmaceutical industry regarding the current market of chiral drugs are divided between the chiral switch of previously used racemates and the development of new enantiopure drugs. The term chiral switch refers to the replacement on the market of a previously approved racemate with its single enantiomer version. The potential advantages of chiral switch can be related to a higher therapeutic index due to better potency, selectivity and fewer adverse effects, faster onset of action and exposure of the patient to lower drug dosages. However, chiral switch is also a strategy that permits manufacturers to keep market exclusivity for chiral pharmaceuticals that have lost their patent protection, even if the pure enantiomers have not demonstrated higher effectiveness or safety profile compared with the racemates.

Long-Acting β2-Agonists in Asthma: Enantioselective Safety Studies are Needed

Long-acting β2-agonists (LABAs) such as formoterol and salmeterol are used for prolonged bronchodilatation in asthma, usually in combination with inhaled corticosteroids (ICSs). Unexplained paradoxical asthma exacerbations and deaths have been associated with LABAs, particularly when used without ICS. LABAs clearly demonstrate effective bronchodilatation and steroid-sparing activity, but long-term treatment can lead to tolerance of their bronchodilator effects. There are also concerns with regard to the effects of LABAs on bronchial hyperresponsiveness (BHR), where long-term use is associated with increased BHR and loss of bronchoprotection. A complicating factor is that formoterol and salmeterol are both chiral compounds, usually administered as 50:50 racemic (rac-) mixtures of two enantiomers. The chiral nature of these compounds has been largely forgotten in the debate regarding LABA safety and effects on BHR, particularly that (S)-enantiomers of β2-agonists may be deleterious to asthma control. LABAs display enantioselective pharmacokinetics and pharmacodynamics. Biological plausibility of the deleterious effects of β2-agonists (S)-enantiomers is provided by in vitro and in vivo studies from the short-acting β2-agonist (SABA) salbutamol. Supportive clinical findings include the fact that patients in emergency departments who demonstrate a blunted response to salbutamol are more likely to benefit from (R)-salbutamol than rac-salbutamol, and resistance to salbutamol appears to be a contributory mechanism in rapid asthma deaths. More effort should therefore be applied to investigating potential enantiospecific effects of LABAs on safety, specifically bronchoprotection. Safety studies directly assessing the effects of LABA (S)-enantiomers on BHR are long overdue.

Modeling and simulation to probe the pharmacokinetic disposition of pomalidomide R- and S-enantiomers

Pomalidomide, a potent novel immunomodulatory agent, has been developed as a racemic mixture of its R- and S-isomers. Pharmacokinetic (PK) analyses were conducted to determine the PK disposition of the isomers from their PK profiles in humans and monkeys. Modeling and simulation were performed to describe the observed PK profiles and explore potential differences in isomer disposition and exposure. PK profiles of S- and R-isomers were measured in a human absorption, distribution, metabolism, and excretion study after oral administration of racemate. PK profiles of S- and R-isomers were measured in monkeys after intravenous and oral administration of S- or R-isomers and pomalidomide racemate. Modeling and simulation were performed using NONMEM 7.2 (Globomax, Ellicott City, MD) to describe the observed PK profiles of S- and R-isomers in humans and monkeys. The results showed that in humans, the in vivo elimination rate of pomalidomide isomers was lower than the R-/S-interconversion rate, resulting in no clinically relevant difference in overall exposure to the two isomers. However, in monkeys, the in vivo elimination rate was higher than the R-/S-interconversion rate, resulting in 1.72- and 1.55-fold differences in R- versus S-isomer exposures. Monte Carlo simulation indicated that exposure to R- and S-enantiomers in humans should be comparable even if single isomers are administered. Thus, in humans, rapid isomeric interconversion of pomalidomide isomers results in comparable exposure to R- and S-enantiomers regardless of whether pomalidomide is administered as a single enantiomer or as a racemate, therefore justifying the clinical development of pomalidomide as a racemate.