The relation between the vapour pressure of a drug and its concentration emerging in the air stream from a nasal inhaler

Abstract

A method of determining the absolute vapour pressures of volatile drugs and drug-adjuvant mixtures is described. The absolute vapour pressures of methylamphetamine, propylhexedrine and eucalyptol have been determined over a range of temperatures. The concentrations of these drugs emerging from a nasal inhaler system have also been measured under similar experimental conditions. From the inhaler results, it has been possible to derive values for the vapour pressures of the drugs in the system and compare them with absolute vapour pressure values. The derived pressures were lower than the absolute pressures at any given temperature. Nevertheless, there seems to be reasonable agreement among the latent heats of vaporization of the three drugs using either parameter. The factors that may give rise to the low values of derived vapour pressure are discussed. Mixtures of eucalyptol and methylamphetamine examined using the same techniques, show that both with the inhaler system and with mixtures of the pure drugs a similar liquid-vapour equilibrium exists. The vapour pressure-composition diagram constructed from either set of results shows a positive deviation from Raoult's law.