Introduction

The fill of a soft capsule is a lipophilic or hydrophilic liquid and can be a solution or a suspension.

Historically, fill formulations for soft capsules were developed mainly with the solubility of the drug substance in mind. Nowadays soft capsule formulations are employed for advanced pharmaceutical drugs delivery systems with the aim to increase both solubility and finally, bioavailability. Most fill formulations for soft capsules can, with some exceptions, be classified as lipid drug delivery systems (LDDS). A first comprehensive classification of LDDS, the Lipid Formulation Classificaiton System, LFCS, was published in 2010 by CW Pouton[i], and has been discussed and extended ever since [ii] [iii].

Table 1 gives an overview of LFCS with respective examples of marketed products.

Moreover, various drug substances have been formulated in a solution of polyethylene glycol alone, especially in PEG 400. PEG 400 is typically classified as a cosolvent, and as such these formulations do not fit into the LFCS. Examples are presented separately. As the formulation principles for LDDS alone have been discussed extensively in various publications, this article focuses on the practical basis for selection of liquid formulations as fills for soft capsules.