The new ketolide HMR3647 accumulates in the azurophil granules of human polymorphonuclear cells

HMR3647 is a semisynthetic representative of a new group of drugs, the keto lides, derived from erythromycin A. Since macrolides have been shown to acc umulate in human polymorphonuclear cells (PMNs), we have investigated the a bility of the molecule HMR3647 to enter human PMNs as well as other cell ty pes, such as peripheral blood mononuclear cells and cell lines of hematopoi etic and nonhematopoietic origin. In these experiments, HMR3647 was compare d to erythromycin A, azithromycin, clarithromycin, and roxithromycin, Our r esults show that HMR3647 is specifically trapped in PMNs, where it is conce ntrated up to 300 times. In addition, it is poorly released by these cells, 80% of the compound remaining cell associated after 2 h in fresh medium. B y contrast, it is poorly internalized and quickly released by the other cel l types studied. This differs from the results obtained with the macrolide molecules, which behaved similarly in the different cells studied. In addit ion, subcellular fractionation of PMNs allowed us to identify the intracell ular compartment where HMR3647 was trapped. In PMNs, more than 75% of the m olecule was recovered in the azurophil granule fraction. Similarly, in NB4 cells differentiated into PMN-like cells, almost 60% of the molecules accum ulated in the azurophil granule fraction. In addition, when HMR3647 was add ed to disrupted PMNs, 63% accumulated in the azurophil granules, Therefore, this study shows that the ketolide HMR3647 specifically accumulates in PMN azurophil granules, thus favoring its delivery to bacteria phagocytosed in these cells.