Vaping Liquids: A Formulator’s Dream?
Philippe Rogueda1, Philip Kwok2
1Aedestra Ltd, 16 D & E, Neich Tower, 128 Gloucester Road, Wanchai, Hong Kong
2Faculty of Pharmacy, The University of Sydney, Pharmacy and Bank Building, Science Road, Camper down, NSW 2006, Australia
e-liquids used in e-cigarettes are treasure troves of excipients: propylene glycol, ethylene glycol, vegetable glycerine, flavourings of all sorts, nicotine, and even water! This is an e armoury that scientists would love to access to develop more stable, more flexible inhalation formulations.
Propylene glycol and vegetable glycerine are the main constituents of e-liquids, with concentrations in excess of 90% w/w. This is magnitudes higher than the nearest inhalation products: Clenil Modulite, Qvar and Symbicort pMDI are formulated with ethanol (typically 5 to 10% w/w) or glycerol (< 5% w/w) or polyethylene glycol (< 0.3 % w/w).
The pharmaceutical industry has been very conservative about using new excipients in inhaled delivery. The high levels of excipients used in e-liquids are an opportunity for formulations scientists to explore new formulation spaces. Molecules that are insoluble in water of HFA propellants could be formulated as solutions.
The mass use of e-liquids is akin to a giant worldwide open source clinical and epidemiology trials of new excipients. The evidence of their side effects is currently ambiguous, often clouded by the presence of flavourings, but also because of the combustion in vaping hardware. So far, the dangers of the inhalation of high levels of propylene glycol of glycerol remain moot.
This is good news for formulators. What should the industry do? Test new excipients? Or sit on the fence and let new formulation opportunities go to the vaping industry? Inhaled drug delivery has missed the vaping wave, will it miss the new excipients wave?