The purpose of this study was to explore retina-targeted delivery of 17β-estradiol (E2), a powerful neuroprotectant, by its bioprecursor prodrug 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED) administered as eye drops in animal models. Compared to the parent hormone, DHED displayed increased transcorneal flux ex vivo both with and without the presence of 2-hydroxypropyl-β-cyclodextrin used as a penetration-enhancing excipient in rat, rabbit, and pig. In vitro, the prodrug also showed facile bioactivation to E2 in the retina but not in the cornea. After topical administration to rats and rabbits, peak DHED-derived E2 concentrations reached 13 ± 5 ng/g and 18 ± 7 ng/g in the retina of female rats and rabbits, respectively. However, the prodrug remained inert in the rest of the body and, therefore, did not cause increase in circulating hormone concentration, as well as wet uterine and anterior pituitary weights as typical markers of E2′s endocrine impact. Altogether, our studies presented here have demonstrated the premise of topical retina-selective estrogen therapy by the DHED prodrug approach for the first time and provide compelling support for further investigation into the full potential of DHED for an efficacious and safe ocular neurotherapy.

We report a three-pronged phenotypic evaluation of the bioprecursor prodrug 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED) that selectively produces 17β-estradiol (E2) in the retina after topical administration and halts glaucomatous neurodegeneration in a male rat model of the disease. Ocular hypertension (OHT) was induced by hyperosmotic saline injection into an episcleral vein of the eye. Animals received daily DHED eye drops for 12 weeks. Deterioration of visual acuity and contrast sensitivity by OHT in these animals were markedly prevented by the DHED-derived E2 with concomitant preservation of retinal ganglion cells and their axons. In addition, we utilized targeted retina proteomics and a previously established panel of proteins as preclinical biomarkers in the context of OHT-induced neurodegeneration as a characteristic process of the disease. The prodrug treatment provided retina-targeted remediation against the glaucomatous dysregulations of these surrogate endpoints without increasing circulating E2 levels. Collectively, the demonstrated significant neuroprotective effect by the DHED-derived E2 in the selected animal model of glaucoma supports the translational potential of our presented ocular neuroprotective approach owing to its inherent therapeutic safety and efficacy.

To facilitate the development of broad-spectrum retina neuroprotectants that can be delivered through topical dosage forms, this proteomics study focused on analyzing target engagements through the identification of functional protein networks impacted after delivery of 17β-estradiol in eye drops. Specifically, the retinae of ovariectomized Brown Norway rats treated with daily eye drops of 17β-estradiol for three weeks were compared to those of vehicle-treated ovariectomized control animals. We searched the acquired raw data against a composite protein sequence database by using Mascot, as well as employed label-free quantification to detect changes in protein abundances. Our investigation using rigorous validation criteria revealed 331 estrogen-regulated proteins in the rat retina (158 were up-regulated, while 173 were down-regulated by 17β-estradiol delivered in eye drops). Comprehensive pathway analyses indicate that these proteins are relevant overall to nervous system development and function, tissue development, organ development, as well as visual system development and function. We also present 18 protein networks with associated canonical pathways showing the effects of treatments for the detailed analyses of target engagements regarding potential application of estrogens as topically delivered broad-spectrum retina neuroprotectants. Profound impact on crystallins is discussed as one of the plausible neuroprotective mechanisms.

Using thyrotropin-releasing hormone (TRH) as a model, we explored whether synergistic combination of lipoamino acid(s) and a linker cleaved by prolyl oligopeptidase (POP) can be used as a promoiety for prodrug design for the preferential brain delivery of the peptide. A representative prodrug based on this design principle was synthesized, and its membrane affinity and in vitro metabolic stability, with or without the presence of a POP inhibitor, were studied. The in vivo formation of TRH from the prodrug construct was probed by utilizing the antidepressant effect of the peptide, as well as its ability to increase acetylcholine (ACh) synthesis and release. We found that the prototype prodrug showed excellent membrane affinity and greatly increased metabolic stability in mouse blood and brain homogenate compared to the parent peptide, yet a POP inhibitor completely prevented prodrug metabolism in brain homogenate. In vivo, administration of the prodrug triggered antidepressant-like effect, and microdialysis sampling showed greatly increased ACh release that was also antagonized upon a POP inhibitor treatment. Altogether, the obtained promising exploratory data warrant further investigations on the utility of the prodrug approach introduced here for brain-enhanced delivery of small peptides with neurotherapeutic potential.

The widely used rat uterotrophic assay to assess known and potential estrogenic compounds only considers uterine weight gain as endpoint measurement. To complement this method with an advanced technology that reveals molecular targets, we analyzed changes in protein expression using label-free quantitative proteomics by nanoflow liquid chromatography coupled with high-resolution mass spectrometry and tandem mass spectrometry from uterine protein extracts of ovariectomized rats after daily 17β-estradiol exposure for five days in comparison with those of vehicle-treated control animals. Our discovery-driven study revealed 165 uterine proteins significantly regulated by estrogen treatment and mapped by pathway analyses. Estrogen-regulated proteins represented cell death, survival and development, cellular growth and proliferation, and protein synthesis as top molecular and cellular functions, and a network found with the presence of nuclear estrogen receptor(s) as a prominent molecular node confirmed the relevance of our findings to hormone-associated events. An exploratory application of targeted proteomics to bisphenol A as a well-known example of an estrogenic endocrine disruptor is also presented. Overall, the results of this study have demonstrated the power of combining untargeted and targeted quantitative proteomic strategies to identify and verify candidate molecular markers for the evaluation of endocrine-disrupting chemicals to complement a conventional bioassay.

Hot flushes are best-known for affecting menopausal women, but men who undergo life-saving castration due to androgen-sensitive prostate cancer also suffer from these vasomotor symptoms. Estrogen deficiency in these patients is a direct consequence of androgen deprivation, because estrogens (notably 17β-estradiol, E₂) are produced from testosterone. Although estrogens alleviate hot flushes in these patients, they also cause adverse systemic side effects. Because only estrogens can provide mitigation of hot flushes on the basis of current clinical practices, there is an unmet need for an effective and safe pharmacotherapeutic intervention that would also greatly enhance patient adherence. To this end, we evaluated treatment of orchidectomized (ORDX) rats with 10β, 17β-dihydroxyestra-1,4-dien-3-one (DHED), a brain-selective bioprecursor prodrug of E₂. A pilot pharmacokinetic study using oral administration of DHED to these animals revealed the formation of E₂ in the brain without the appearance of the hormone in the circulation. Therefore, DHED treatment alleviated androgen deprivation-associated hot flushes without peripheral impact in the ORDX rat model. Concomitantly, we showed that DHED-derived E₂ induced progesterone receptor gene expression in the hypothalamus without stimulating galanin expression in the anterior pituitary, further indicating the lack of systemic estrogen exposure upon oral treatment with DHED.