Tilorone-induced lysosomal lesions: the bisbasic character of the drug is essential for its high potency to cause storage of sulphated glycosaminoglycans

Abstract

The immunomodulatory agent tilorone {2,7-bis-[2-(diethylamino)ethoxy]fluoren-9-one} and congeners are potent inducers of lysosomal storage of sulphated glycosaminoglycans (GAGs) in animals and cultured fibroblasts of animals and man. All potent inducers of GAG storage hitherto described are bisbasic polycyclic aromatic compounds. They are accumulated in lysosomes and disturb the degradation of GAGs, mainly dermatan sulphate. It has been proposed that the drugs cross-link the polyanionic GAG chains giving rise to undegradable drug–GAG complexes. This hypothesis implies that the bisbasic character of the drug molecules is essential for the side effect in question. In the present study, this was tested by comparing tilorone and its monobasic derivative (MT) with respect to (i) induction of GAG storage in cultured bovine corneal fibroblasts and (ii) physicochemical interactions with GAGs in vitro. The intralysosomal concentration of MT achieved after 1–3 days was of the same order of magnitude as previously shown for tilorone. Nevertheless, under conditions that did not enhance the secretion of a lysosomal enzyme (β-hexosaminidase, EC 3.2.1.52), the ability of MT to cause storage of [35S]GAGs was significantly lower than that of tilorone. Morphological observations showed that MT was much more potent in causing lysosomal storage of polar lipids than of GAGs. CD spectroscopy with tilorone revealed that the presence of GAGs caused the primarily achiral drug molecules to display CD. This suggested a helical orientation of the tilorone molecules within GAG–drug complexes, and short intermolecular distances which allowed electronic coupling of the aromatic ring systems of adjacent drug molecules. In contrast, MT failed to display any induced optical activity, indicating the absence of highly ordered GAG–drug complexes. In conclusion, the present results show that the substitution of the planar aromatic ring system with two basic side chains is essential for the high potency of tilorone in inducing lysosomal GAG storage. This is paralleled by, and presumably causally related to, strong physicochemical interactions with GAGs.