Inherent Fluorescence Demonstrates Immunotropic Properties for Novel Janus Kinase 3 Inhibitors

There is a general question in small molecule pharmacology
about
how apparent compound concentrations in blood, plasma, and organs
actually relate to actual amounts at the target site of a compound.
In this study, we used inherently fluorescent JAK3 ligands and their
macrolide conjugates to investigate the relationship between physical
properties, apparent bulk concentration, and organ and subcellular
distribution. In vitro uptake into immune cells suggested that much
of the substance was associated with granules or organelles. Samples
from murine pharmacokinetic studies were analyzed by both conventional
mass spectrometry and cryofluorescence microscopy methods to show
the distribution of a compound within organs and cells without artifacts
of fixation. These observations confirm the uptake of granules observed
in vitro. Data from macrolides carrying either a coumarin fluorophore
or a JAK3 inhibitor were similar, suggesting that the distribution
is directed by the properties of the larger macrolide. These data
show a propensity for azalide macrolides to concentrate in the lung
and gut epithelia and suggest that the plasma- or whole-blood-derived
estimates of drug levels almost certainly underestimate concentrations
of macrolides in the mucous membranes. Thus, their apparent efficacy
at sub-bacteriostatic doses may reflect their higher levels in barrier
layers.