**7. Enhancement of PTS Bioavailability**

The phenolic group at the 4 position of PTS is an attractive target for phase 2 conjugative enzymes, which negatively impacts its bioavailability. PTS is also insoluble in water, limiting its concentration in the aqueous environment [77]. Safe, bioavailable formulations of PTS can help increase its bioavailability in the body and reduce known and unknown side effects [30,131]. Some of the ongoing trials for PTS include using dietary blueberry supplements, 250 mg of Nicotinamide Riboside, and 50 mg of PTS, commercially known as Basis™, developed by Elysium, or ElevATP, a combination of polyphenols developed by FutureCeuticals (Table 3). Further clinical trials are warranted to improve the therapeutic bioavailability and optimal administration methods of PTS.

Crystal engineering has been a promising field for bioavailability enhancement due to the advantageous, unique solubility and dissolution rates of drugs [132]. Formulations of cocrystals can improve the dissolution profile and bioavailability of PTS with improved performance. Notably, picolinic acid, an endogenous L-tryptophan metabolite, exhibits neuroprotective, immunological, and anti-proliferative properties. The oral administration of PTS–picolinic acid cocrystals to rats revealed a 10-fold amplification of the bioavailability of PTS when compared to solid oral forms [131]. PTS–caffeine cocrystal solubility was also observed to have over 27 times higher solubility [132].

In the form of a prodrug, a bis (hydroxymethyl) propionate-based analog of PTS exhibited superior tumor inhibitory activity in cisplatin-resistant oral squamous (CAR) cancer cell lines. An increase in absorption, a reduction in metabolism, and the maintenance of high concentrations of PTS were some of the observed benefits of prodrug administration. The concentration of PTS in the blood was significantly higher when the amino acids isoleucine or β-alanine were utilized. Oral gavage administration of PTS with megestrol acetate at 10 mg/kg/day reduced the tumor weight and volume in endometrial cancer rats [61].

Notably, the chronic administration of PTS facilitated burn-wound healing in diabetes, facilitated by a significant reduction in diabetes-induced oxidative stress and the suppression of hypoxia-induced factor1α (HIF1α) activity [130]. Liposome-engulfed PTS is also efficient for the topical administration of the drug [41]. Poly (3-acrylamidophenyl boric acid-b-PTS) [p(AAPBA-b-PTE)] is a round nanoparticle of PTS, with a size ranging from 150 to 250 nm. Notably, they possess an appropriate pH and sensitivity to glucose. In vitro and in vivo studies showed that PTS nanoparticles were nontoxic and safe. Upon administration, these nanoparticles were observed to reduce glucose levels, reverse micro-inflammation, and improve the antioxidant profile in mice [133]. Further, antibody-4arm-polyethylene glycol-PT conjugate, as an antibody–drug conjugate, was used for the targeting and co-delivery of drugs to tumors [134]. A composition with Zein/fucoidan composite nanoparticles as carriers of PTS was prepared using an anti-solvent precipitation method. The findings indicated that this method provided a better-controlled release and could be utilized as a potential carrier for the protective encapsulation of PTS [135]. Poly(2 oxazoline)–PTS block-copolymer nanoparticles are also being used for dual anti-cancer drug deliveries in chemotherapy [136].

Solubilizing PTS in 2-hydroxypropyl-b-cyclodextrin (HP-b-CD) improved its bioavailability by 3.7 times [25]. Furthermore, a lipid-based encapsulation system has been used to enhance the stability of PTS in the aqueous phase. In particular, encapsulation in nanoemulsions made of flaxseed oil and olive oil was studied by Sun et al. The bio-accessibility was high in the flaxseed-oil nano-emulsion. However, greater amounts of intact PTS were transported across intestinal enterocytes in olive-oil nano-emulsions [137]. The oral administration of zinc pectinate beads, prepared through the ionic gelation method, successfully distributed PTS to the colonic tissue [138]. Interestingly, physical exercise promoted the biosynthesis of the anti-inflammatory mediator MaR1, leading to a preventive effect in rheumatoid arthritis. The oral administration of PTS, when accompanied by moderate physical activity, attenuated the pathological process of rheumatoid arthritis in a bovine type II collagen (BIIC)-stimulated rat model [86].

#### **8. Conclusions and Future Prospects**

Numerous studies that have evaluated PTS for its therapeutic potential have demonstrated its role as a promising candidate drug for health benefits in a broad spectrum of disease conditions. Various experimental studies have confirmed that PTS has anti-cancer, anti-diabetic, anti-hypertensive, antimicrobial, anti-aging, anti-atherosclerotic, and neuroprotective properties. PTS has been documented to exert its beneficial effects mainly by modulating antioxidant, anti-apoptotic, and anti-inflammatory pathways. Of particular interest, the activation of the Nrf2 signaling pathway by PTS has been an essential focus of our lab. Our findings indicate its potential as a promising drug candidate for diabetes and associated complications. Even at higher doses, PTS did not exhibit toxicity in animal studies, providing further encouragement to explore the use of the compound in more human clinical trials. However, considering that some of the studies have employed the

co-administration of PTS in combination with other compounds to improve its therapeutic efficiency, the potential effect of drug interactions should be considered.

Although PTS has been identified to exert marked therapeutic benefits, most findings have been proven only in experimental models. Human clinical trials have been largely limited due to the less-than-desired bioavailability of the compound. To overcome this limitation, various strategies have been implemented, which involve modifying the administration routes and formulations of PTS, including cocrystals, prodrugs, nanoparticles, lipid-based encapsulation, and beads. The potential effects of many drug interactions with PTS remain unclear. Of note, the mode of administration of PTS seems to play an important role in its bioavailability, as the administration of intravenous doses shows a higher distribution when compared to oral intake. Further research that carefully considers the dose, drug interactions, administration route, disease-specific formulations, and shortand long-term biomedical implications is warranted before the clinical adoption of this promising natural compound.

**Author Contributions:** Conceptualization, K.M.R., B.X., K.G., S.N. and S.M.; methodology and analysis, S.N. and S.M.; writing—original draft preparation, S.N. and S.M.; writing—review and editing, K.M.R., S.M., K.G. and B.X.; supervision, K.M.R. and B.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Science and Engineering Research Board (SERB) (Grant No. EMR/2016/006196), Government of India.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data availability Statement:** Not applicable.

**Acknowledgments:** The authors gratefully acknowledge the Science and Engineering Research Board (SERB) (Grant No. EMR/2016/006196), Government of India, for financial assistance.

**Conflicts of Interest:** The authors declare no conflict of interest.
