Distribution of Small Molecular Weight Drugs into the Porcine Lens: Studies on Imaging Mass Spectrometry, Partition Coefficients, and Implications in Ocular Pharmacokinetics

Abstract

Lens is the avascular tissue in the eye between the aqueous humor and vitreous. Drug binding to the lens might affect ocular pharmacokinetics, and the binding may also have a pharmacological role in drug-induced cataract and cataract treatment. Drug distribution in the lens has been studied in vitro with many compounds; however, the experimental methods vary, no detailed information on distribution between the lens sublayers exist, and the partition coefficients are reported rarely. Therefore, our objectives were to clarify drug localization in the lens layers and establish partition coefficients for a wide range of molecules. Furthermore, we aimed to illustrate the effect of lenticular drug binding on overall ocular drug pharmacokinetics. We studied the distribution of 16 drugs and three fluorescent dyes in whole porcine lenses in vitro with imaging mass spectrometry and fluorescence microscopy techniques. Furthermore, we determined lens/buffer partition coefficients with the same experimental setup for 28 drugs with mass spectrometry. Finally, the effect of lenticular binding of drugs on aqueous humor drug exposure was explored with pharmacokinetic simulations. After 4 h, the drugs and the dyes distributed only to the outermost lens layers (capsule and cortex). The lens/buffer partition coefficients for the drugs were low, ranging from 0.05 to 0.8. On the basis of the pharmacokinetic simulations, a high lens-aqueous humor partition coefficient increases drug exposure in the lens but does not significantly alter the pharmacokinetics in the aqueous humor. To conclude, the lens seems to act mainly as a physical barrier for drug distribution in the eye, and drug binding to the lens affects mainly the drug pharmacokinetics in the lens.

Carboxylesterase Activities and Protein Expression in Rabbit and Pig Ocular Tissues

“…Numerous ocular drugs or prodrugs containing an ester or amide bond are liable to hydrolysis in ocular tissues of various species. 33 , 38 , 62 , 68 Although esterase activity is well-presented, the detailed activity and expression data on CES isoforms in the eye is still ambiguous. 22 , 40 For the first time in ocular tissues, we adopted substrates and inhibitors that have been used to characterize human CES 13 , 16 , 25 and AADAC 18 , 21 activities, because the respective rabbit and pig enzymes have not been expressed in vitro and characterized for their substrate specificity.…”

Mapping glucose metabolites in the normal bovine lens: Evaluation and optimisation of a matrix‐assisted laser desorption/ionisation imaging mass spectrometry method

“…While human lens GSH does not appear to suffer significantly from ionisation differences in different lens regions, 44 the effect of lens tissue microenvironment on the ionisation of glucose and related molecules is unknown and was therefore tested in our bovine lens samples (Figure 4). Additionally, the method of IS application was tested, because the best method of application, that is, application underneath tissue section, on top of tissue section prior to matrix application or simultaneous IS/matrix application seems to differ depending on the molecule of interest 54–56 . The glucose analogue 3‐OMG was chosen as the sprayed IS due to its chemical similarity yet different molecular weight to both endogenous glucose and its metabolites and the targeted SIL glucose and its metabolites in the ex vivo cultured lenses.…”

“…Additionally, the method of IS application was tested, because the best method of application, that is, application underneath tissue section, on top of tissue section prior to matrix application or simultaneous IS/matrix application seems to differ depending on the molecule of interest. [54][55][56] The glucose analogue 3-OMG was chosen as the sprayed IS due to its chemical similarity yet different molecular weight to both endogenous glucose and its metabolites and the targeted SIL glucose and its metabolites in the ex vivo cultured lenses. MALDI images were plotted and signal intensity plots along the optical axis, that is, from anterior to posterior pole, were generated following sequential IS/matrix application (Figure 4A…”

Integrating ion mobility and imaging mass spectrometry for comprehensive analysis of biological tissues: A brief review and perspective

“…11,12 IMS offers an untargeted mapping capability for hundreds to thousands of molecules in a single experiment. Moreover, IMS provides the analytical flexibility to investigate different classes of biomolecules including small metabolites, 13,14 lipids, 15,16 drugs, 17,18 glycans, 19,20 peptides, 21,22 and proteins. 23,24 However, IMS faces challenges of dynamic range, peak capacity, and the ability to structurally identify as defined by the Mason-Schamp equation.…”