It is known that the effective antibiotic clofazimine bioaccumulates in macrophages. Clofazimine form highly organized, insoluble molecular complexes within these cells. Although bioaccumulation is an unwanted side effect of drugs, clofazimine is a well-tolerated antibiotic. In the case of clofazimine it is thought that the CLDIs (crystal-like drug inclusions) function as an intracellular drug depot. It is known that other compounds related to clofazimine bioaccumulate in different compartments but less intense than clofazimine.

Now researchers from America and England have demonstrated that phenazine compounds can bioaccumulate and self-assemble in macrophages and epithelial cells.

Quantitaive polarization microsocpy of macrophages and eithelial cells incubated with different phenazine compounds.

The researchers examined if lipophilicity and chemical structure of phenazine have an impact on the intracellular uptake and therefore of the bioaccumulation within the cell. To do this, they synthesized a small library of phenazine derivatives of clofazimine. They replaced the chlorophenyl group with different aromatic substituents and modified the R-imino group. Afterwards they analyzed the grade of bioaccumulation of each compound through chemical methods. Due to phenazines being chromophores, they confirmed the intracellular localization of the inclusions with fluorescence confocal microscopy.

By using quantitative polarization microscopy, they determined how the inclusions influence the transmittance of polarized light. Through this method they can detect, observe and analyze the formation of insoluble drug complexes inside live cells.

The researchers showed that replacing the chlorophenyl group with different aromatic substituents inhibits the uptake of clofazimine. The modification of the R-imino group did not suppress the bioaccumulation. Indeed a few modifications result in greater bioaccumulation than clofazimine does. Overall phenazine compounds tended to accumulate more in macrophages than in epithelial cells.

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