1. Introduction

Poorly water-soluble molecules are a challenging problem in the development of suitable pharmaceutical drug formulation. This situation gets further complicated by the fact that most of the newly developed drugs exhibit poor solubility in organic media as well. Consequently, erratic absorption characteristics and low systemic bioavailability are typical issues with poorly water-soluble drugs [1,2]. The parenteral route of administration (viz. intravenous, intradermal, intramuscular, intraarterial, subcutaneous, etc.), offers a significantly high absorption profile and hence enhanced bioavailability [3]. Owing to the low solubility of drugs, it is impossible and actually dangerous to administer solutions intravenously since it can potentially precipitate and clog the vessel [2]. Drug delivery scientists have used various formulation approaches to deal with problems associated with the delivery of hydrophobic drugs via the parenteral route [3]. The term parenteral is coined out of two words that are Greek in origin, viz. “para” meaning besides and “enteron” meaning gut. Thus, routes of drug administration that bypass the gastrointestinal tract are referred to as parenteral routes of drug delivery. The most critical method of drug delivery is the I.V. (intravenous) route and widely used heterogeneous systems for this route are simple oil-in-water (o/w) emulsions and multiple water-in-oil-in-water (w/o/w) formulations [4].

The traditional, and most common, approaches for parenteral delivery of poorly soluble drugs involve complexation, solubilization of hydrophobic agents in micelles and liposomes as drug carrier systems, among a few others. Although the aforementioned approaches are used for hydrophobic drug delivery, they have several limitations hindering the employment of their full potential. Cyclodextrins are expensive and may exhibit poor complexation with the drug under consideration, limited micellar solubilization capacity and complexity coupled with the high cost of the manufacturing process of liposomes [3]. Therefore, there is a growing need to improve formulation strategies to improve the parenteral delivery of hydrophobic drugs.

In the pharmaceutical arena, emulsions, as well as micro-emulsions, are widely accepted carriers for the delivery of both lipophilic (hydrophobic) and lipophobic drugs, including the ones with low permeability. Lately, micro-nano-emulsions have acquired increased focus in pharma applications as drug carriers [4], since they have great potential to deal with problems that are related to drug delivery of poorly water and also lipid-soluble drugs [5]. This brief and succinct review focuses on the diverse aspects of submicron emulsions and nano-suspensions including the structure of colloidal systems, scientific and regulatory considerations in development, FDA approved colloidal systems for parenteral delivery, and key characterization techniques needed for the successful approval of these colloidal systems. Finally, the application of these carrier systems as promising formulation approaches in parenteral drug delivery, including Total Parenteral Nutrition, Vaccine Delivery, Long-Acting Injectable Therapy, and Anti-Cancer Drugs and Diagnostic Agents, will also be highlighted.