Oil-in-Water Nanosized Emulsions for Drug Delivery and Targeting. Tamilvanan Shunmugaperumal

oligos or siRNA are water soluble due to their polyanionic character, the aqueous solution of these compounds need to be added directly to the o/w cationic nanosized emulsion in order to interact electrostatically with the cationic emulsion droplets and thus associate/link superficially at the oil–water interface of the emulsion (Teixeira et al. 1999; Tamilvanan 2004; Hagigit et al. 2010). During in vivo condition when administered via parenteral and ocular routes, the release of the DNA and oligos from the associated emulsion droplet surfaces should therefore initially be dependent solely on the affinity between the physiological anions of the biological fluid and cationic surface of the emulsion droplets. The mono‐ and di‐valent anions containing biological fluid available in parenteral route is plasma and in ocular topical route is tear fluid, aqueous humor, and vitreous. Moreover, these biofluids contain multitude of macromolecules and nucleases. There is a possibility that endogenous negatively charged biofluid's components could dissociate the DNA and oligos from cationic emulsion. It is noteworthy to conduct during the preformulation development stages an in vitro release study for therapeutic DNA and oligos‐containing cationic nanoemulsion in these biological fluids and this type of study could be considered as an indicator for the strength of the interaction occurred between DNA or oligo and the emulsion (Hagigit et al. 2008). Interestingly, the stability of oligos (a 17‐base oligonucleotide, partially phosphorothioated) was validated using a gel‐electrophoresis method. After incorporating the oligos into the cationic nanosized emulsion as well as during in vitro experiments of oligos‐containing emulsion in vitreous fluid at different time periods, the emulsions were phase separated by Triton X‐100 and then the degradation of oligos was also monitored following the same validated gel‐electrophoresis method (Hagigit et al. 2008). No appearance of new band was seen in comparison to the standard aqueous oligos solution. This result indicates that the oligos did not undergo degradation against the conditions applied to prepare a sterile emulsion.

In order to bring the nanosized emulsion closer to otherwise inaccessible pathological target tissues, homing devices/ligands such as antibodies and cell recognition proteins are usually linked somehow onto the particle surfaces. Various methods have been employed to couple ligands to the surface of the nanosized emulsions with reactive groups. These can be divided into covalent and noncovalent couplings. Noncovalent binding by simple physical association of targeting ligands to the nanocarrier surface has the advantage of eliminating the use of rigorous, destructive reaction agents. Common covalent coupling methods involve formation of a disulfide bond, cross‐linking between two primary amines, reaction between a carboxylic acid group and primary amine, reaction between maleimide and thiol, reaction between hydrazide and aldehyde, and reaction between a primary amine and free aldehyde (Nobs et al. 2004). For antibody‐conjugated anionic emulsions, the reaction of the carboxyl derivative of the coemulsifier molecule with free amine groups of the antibody and disulfide bond formation between coemulsifier derivative and reduced antibody were the two reported conjugation techniques so far (Song et al. 1996; Lundberg et al. 1999, 2004). However, by the formation of a thio‐ether bond between the free maleimide reactive group already localized at the o/w interface of the emulsion oil droplets and a reduced monoclonal antibody, the antibody‐tethered cationic emulsion was developed for active targeting to tumor cells (Goldstein et al. 2005, 2007a, b).

2.5. QbD APPROACH TO OPTIMIZE EMULSION

The QbD paradigm, as clearly delineated in the International Council for Harmonisation (ICH) Q8(R2), Q9, and Q10 guidelines, indicates the necessity to follow initial risk assessment studies, factor screening studies, and then the optimization of any pharmaceutical products not only for improving the final product quality but also to provide regulatory flexibility for the industry to improve their manufacturing processes [ICH Q9 guideline 2005; ICH Q10 guideline 2008; ICH Q8(R2) guideline 2009]. The guideline ICH Q8(R2) described a QbD‐based approach of Formulation by Design (FbD). Whereas the ICH Q9 promulgated the need of Quality Risk Management (QRM), the ICH Q10 signified the way of obtaining the quality final products. The typical steps required for the new API products during their formulation development stage are shown in Flowchart 2.1.

Like other dosage forms, the o/w nanosized emulsions do contain many raw materials, manufacturing process, and testing of in‐process materials and final products. At the very outset, the selection of each raw materials and their proper amount is one of the prime steps. Following the raw materials’ assortment, the manufacturing process is to be selected from the myriad of available size‐reduction machineries. In many cases, these two (raw materials and manufacturing process) constraints are often called as independent variables or critical process parameters (CPPs). Studying the influence of CPPs on the so‐called dependent or response variables [also known as critical quality attributes (CQAs)] will eventually determine the final product quality and also provide flexibility for the industry to advance their manufacturing processes for meeting the stringent regulatory procedures. Hence, the studies relating the influence of CPPs on CQAs look in a diverse manner that depends on the changes of regulatory procedures over the time periods. Initially, it was thought that the product quality along with its performance, in vitro and in vivo, could be safeguarded by performing the traditional quality by testing (QbT) approach usually via one‐factor‐at‐a‐time (OFAT) experimental approach. In the QbT approach, the quality of both API and its product is mainly ensured by testing of raw materials, a fixed manufacturing process, and testing of in‐process materials and end product (Yu 2008).

Flowchart 2.1. Typical steps involved for the new drug products during their formulation development stage as per the quality by design (QbD) approach of formulation by design (FbD).

      This traditional framework has certain drawbacks. Any minor changes made in input materials and processes (including equipment) for anticipated variability are empirical and addressed via the OFAT experimental approach. This development practice is not cost‐effective and results in incomplete product and process understandings, which in turn leads to restrictive (or fixed) manufacturing processes that are unable to compensate for the regular variability in input materials, processes, manufacturing equipment, and laboratory instrumentation (Debevec et al. 2018). As mentioned earlier, the QbT approach also requires extensive testing to comply with restrictive FDA‐approved specifications (Yu 2008).

For these reasons, traditional industry practices are often extraordinarily expensive and time consuming and present an overwhelming burden to regulatory agencies for reviewing multiple chemistry, manufacturing, and control (CMC) supplements. Another major drawback of the traditional approach is that it does not differentiate products with regard to complexity (for example, APIs with high potency and/or narrow therapeutic indices) and associated level of quality and health risks to manufacturers and consumers, respectively (Yu 2008). These drawbacks of “minimal” or “traditional approach” and need for higher regulatory flexibility via an “enhanced approach” prompted a paradigm shift in the industry practice [ICH Q8(R2) guideline 2009].

      The need for transition from traditional QbT to an enhanced approach was formally communicated through an ICH Q8 guidance published in May 2006, which emphasized that “quality cannot be tested into products, rather it should be built into products by design” (FDA Guidance for Industry 2006).

      These innovative frameworks are fully reflected in current regulatory guidance on QbD and PAT [FDA Guidance for Industry 2004; ICH Q8(R2) guideline 2009] and are encouraged for industry practice.

      Both QbD and PAT share common goals of providing a rapid and science‐ and risk‐based road map for product development and economically effective strategies for process monitoring and analytical testing. The QbD strategy involves an end‐to‐end integration of six key elements, which are quality target product profile (QTPP), risk assessments related to process and product design, DOEs, design space, control