Hydroxyapatite (HAP) has been the gold standard in the biomedical field due to its composition and similarity to human bone. Properties such as shape, size, morphology, and ionic substitution can be tailored through the use of different synthesis techniques and compounds. Regardless of the ability to determine its physicochemical properties, a conclusion for the correlation with the biological response it is yet to be found. Hence, a special focus on the most desirable properties for an appropriate biological response needs to be addressed. This review provides an overview of the fundamental properties of hydroxyapatite nanoparticles and the characterization of physicochemical properties involved in their biological response and role as a drug delivery system. A summary of the main chemical properties and applications of hydroxyapatite, the advantages of using nanoparticles, and the influence of shape, size, functional group, morphology, and crystalline phase in the biological response is presented. A special emphasis was placed on the analysis of chemical and physical interactions of the nanoparticles and the cargo, which was explained through the use of spectroscopic and physical techniques such as FTIR, Raman, XRD, SEM, DLS, and BET. We discuss the properties tailored for hydroxyapatite nanoparticles for a specific biomolecule based on the compilation of studies performed on proteins, peptides, drugs, and genetic material.

Glaucoma is associated with ocular hypertension and lowering intraocular pressure is a key objective of glaucoma therapies. Recent studies have established a role for the Schlemm's canal endothelium in this pressure increase and have shown it to have a unique, lymphatic-like, hybrid phenotype. However, the role of these lymphatic phenotypes in the adult canal remains uncertain. Long-term functional studies have been limited by systemic importance of lymphatic genes and lack of Schlemm's canal-specific animal models. Here, we designed and validated a strategy using 4OH-tamoxifen-loaded nanocarriers to generate targeted, Schlemm's canal specific knockout mice lacking lymphatic phenotypes. Using this system, we selectively deleted Prox1, the master transcription factor governing lymphatic fate. Within four weeks, intraocular pressure significantly increased, and ocular hypertension was maintained for at least 24 weeks. Unlike lymphatic vessels, which degenerate following Prox1 deletion, Schlemm's canal reverted to a less functional vein-like phenotype with no change in size or morphology. These results highlight the utility of nanocarriers for tissue-specific genetic recombination and demonstrate that changes in lymphatic phenotypes alter intraocular pressure, providing new targets for glaucoma therapy. Moreover, as we found that PROX1 was downregulated with age in human Schlemm's canal, these canal-specific conditional Prox1 knockout mice are a valuable new adult-onset model of ocular hypertension that captures key features of age-related human disease.