Design of potential antiprotozoal daunorubicin derivatives
Daunorubicin is an antitumour antibiotic which is highly active against the sleeping sickness parasite Trypanosoma rhodesiense in vitro, but which lacks in vivo activity. The object of this work was to modify daunorubicin so as to promote in vivo activity, and to study the mechanism by which daunorubicin is trypanocidal. A series of daunorubicin analogues, and derivatives in which daunorubicin was linked to a macromolecular carrier (known as daunorubicin conjugates) were prepared, and tested against trypanosome infected mice. Only daunorubicin conjugates in which drug was linked to the carrier by glutaraldehyde, were active. Treatment with these conjugates increased the survival time of infected mice from three days to as long as eleven days, and temporarily cleared parasites from the bloodstream of infected animals. Conjugates with other types of linkage were inactive. A fluorescence assay method was developed to measure drug release from conjugates. Investigation revealed that glutaraldehyde linked conjugate released about 20% of bound drug over a 2-3 hour period when incubated in murine plasma. In contrast, inactive conjugates either released bound drug very rapidly, or were stable to drug release. Daunorubicin is known to possess several potentially cytotoxic mechanisms of action, the most important being intercalation into the DNA double hel~x and stimulation of superoxide radical formation. In order to discover the contribution of these mechanisms to trypanocidal activity, the trypanocidal potency of a series of daunorubicin analogues was assessed in vitro. The ability of these analogues to intercalate into DNA and to stimulate lipid peroxidation and oxygen consumption was also assessed. The results were used to explore the relationship between these mechanisms and trypanocidal activity. These studies indicate that ability to bind to DNA is important in conferring trypanocidal activity on this group of antibiotics.
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