Intravenous administration of N-(beta-alanyl-L-leucyl-L-alanyl-L-leucyl)doxorubicin (4) induces an acute toxic reaction, killing animals in a few minutes. This results from its positive charge at physiological pH combined with its propensity to form large aggregates in aqueous solutions. Negatively charged N-capped versions of 4 such as the succinyl derivative 5 can be administered by the iv route at more than 10 times the LD(50) of doxorubicin without inducing the acute toxic reaction, and they are active in vivo.

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