[0039] FIG. 7B provides a graph showing release rates (fraction released as a function of time) of polymyxin B NCs when using sodium dodecyl sulfate (SDS) IP for several API:IP ratios. The charge ratio is given as 1 :x, where x is the number of ion pair (IP) charge equivalents of the positively› charged API. 

[0040] FIG. 7C provides a graph showing release rates (fraction released as a function of time) of polymyxin B NCs when using sodium oleate (OA) IP for several API:IP ratios. The charge ratio is given as 1 :x, where x is the number of ion pair (IP) charge equivalents of the positively-charged APL 

[0041] FIG. 8 provides a graph showing decrease in the size over time of an s. aureus infected lesion on a mouse’s back upon injection with control NPs without MP7 and upon injection with NPs containing NP7. 

[0042] FIG. 9 shows a comparison of the chemical struc› tures of polymyxin B sulfate and colistin ("polymyxin E"). 

[0043] FIG. 10 provides a graph showing release rates (fraction released as a function of time) of polymyxin B (poly) from NCs formed using sodium oleate (OA) IP in phosphate buffered saline (PBS) and of colistin from NCs formed using sodium oleate (OA) IP in simulated lung fluid (SLF3). 

[0044] Embodiments of the invention are discussed in detail below. In describing embodiments, specific terminol› ogy is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent parts can be employed and other methods developed without parting from the spirit and scope of the invention. All references cited herein are hereby incorporated by reference in their entirety as if each had been individually incorporated. 

[0045] Nanoprecipitation of active pharmaceutical ingre› dients (APis) into drug carriers, e.g., to form nanocarriers (NCs), for example, nanoparticles (NPs), can be used to produce formulations with stable nanoparticles and biologi› cal efficacy. (In this text, unless otherwise indicated, the terms "nanocarrier" (NC) and "nanoparticle" (NP) are used interchangeably.) However, many nanoprecipitation tech› niques are applicable for only hydrophobic and water› insoluble APis, and have not been demonstrated for highly soluble therapeutics. Presented here is a model and tech› nique to encapsulate water-soluble APis using nanoprecipi› tation techniques by manipulating API salt forms. APis are ion paired with hydrophobic counterions to produce new API salts that exhibit altered solubilities suitable for nano› precipitation processing. Nanocarrier properties, such as stability and release rates, can be tuned by varying ion-pair (IP) molecular structure and ion-pair to API molar ratios. The tools set forth herein can be used to process hydrophilic APis into nanocarrier systems. The governing rules of ion pair identity, and processing conditions required for suc› cessful encapsulation are experimentally determined and assessed with theoretical models. Successful NC, e.g, nano› particle, formation for the antibiotic polymyxin B can be facilitated with hydrophobicity of the ion pair acid being greater than log P=2 for strong acids and greater than log P=S for weak acids. Oleic acid with a log P=S, and pKa=5, can be suitable as an ion pair agent, because it is biocom› patible and forms ion pair complexes. NC formation from 

## DETAILED DESCRIPTION OF THE INVENTION 

pre-formed, organic soluble ion pairs, can be compared to in situ ion pairs where NCs are made in a single precipitation step. NC properties, such as stability and release rates, can be tuned by varying ion pair molecular structure and ion pair-to-API molar ratios. For polymyxin B, NCs -10°-200 nm in size and with characteristic API release rates over three days were produced. The embodiments of the inven› tion set forth herein represent new tools and demonstrate a new approach that enables the formation of nanoparticles from previously intractable compounds, for example, enabling the processing of hydrophilic APis into nanocarrier systems. 

[0046] Thus, set forth herein is encapsulation of hydro› philic actives into nanocarriers via hydrophobic ion-pairing. This approach may be employed, inter alia, in treatment of infectious diseases, such as bacterial, fungal, viral, and parasitic infections. More particularly, the controlled deliv› ery of active therapeutic ingredients (APis) through nano› carriers can result in improved bioavailability, reduced tox› icity, sustained activity, simplified dosing regimens, improved patient adherence, and enhanced overall efficacy. 

[0047] In a process that is an embodiment of the invention, charged functional groups on the API are ion-paired with a counter ion to produce an API with transiently altered solubilities. Flash NanoPrecipitation (FNP) was used to self-assemble nanoparticles (NPs) containing an encapsu› lated API-IP complex. Two routes of assembly were explored: (1) complexation oftheAPI with the hydrophobic counterion during FNP (termed "in situ ion pairing"); and (2) complexation prior to FNP (termed "pre-ion pairing"). For the pre-ion paired method, particles were formed by rapid micro-mixing of a hydrophobic-hydrophilic block› copolymer and API-IP complex dissolved in an organic solvent against deionized (DI) water through a confined impingement jet (CIJ) mixer. For the in situ ion-pairing method, the organic stream containing block copolymer and anionic IP dissolved in organic solvent were rapidly micro› mixed with the water stream containing cationic hydrophilic API dissolved in water through a CIJ mixer. For both methods, the organic and water streams were mixed in a 1: 1 volume ratio and the resulting mixed streams were subse› quently diluted tenfold into water. 

[0048] In nanoprecipitation processes, APis may be dis› solved in organic solvents and mixed with water as an antisolvent to induce precipitation and nanocarrier self› assembly. However, such direct precipitation methods may be only feasible for water insoluble compounds ( c Log P>4), and may not be feasible for encapsulating hydrophilic bio› logics into nanocarriers. Water soluble biologics may be encapsulated through alternative water-in-oil-in-water emul› sion (W/O/W) or liposomal processes that require multiple steps, suffer from poor encapsulation efficiencies, or exhibit low drug mass loadings. 

[0049] Experimental verification showed the approach of the invention to be capable of encapsulating hydrophilic APis using Flash NanoPrecipitation via hydrophobic ion pairing. For example, the followingAPis were encapsulated: the antimicrobial small molecules polymyxin B and colistin; the peptides mastoparan 7 and sub5; the protein ovalbumin; and the aminoglycoside gentamycin. 

[0050] The successful formulation of tobramycin, gen› tamycin, and polymyxin B into NPs with the FNP process can expand the applicability of these therapeutics. For example, these APis, which are used to manage P. aerugi- 

