Many
tumor cells, including murine ADJ/
PC6 plasmacytoma cells, possess an active energy dependent
polyamine uptake system which selectively accumulates endogenous
polyamines and structurally related compounds. We have attempted to target the cytotoxic
drug chlorambucil to a
tumor possessing this uptake system by conjugating it to the
polyamine spermidine. Furthermore, since
polyamines have a high affinity for
DNA, the attachment of
spermidine to
chlorambucil should also facilitate its targeting to
DNA. This was supported by the observation that the
chlorambucil-spermidine conjugate was approximately 10,000-fold more active than
chlorambucil at forming interstrand crosslinks with naked
DNA. In vitro cytotoxicity and in vivo antitumor studies were carried out using the ADJ/
PC6 plasmacytoma. In vitro, using [3H]
thymidine incorporation to assess cell viability following a 1-h exposure to control and
polyamine depleted ADJ/
PC6 cells,
chlorambucil-
spermidine was 35- and 225-fold, respectively, more toxic than
chlorambucil. The increased toxicity of the conjugate compared to
chlorambucil was possibly due to enhanced
DNA binding and/or facilitated uptake via the
polyamine uptake system. The enhanced toxicity of the conjugate but not
chlorambucil by prior
polyamine depletion with
difluoromethylornithine, together with the observation that the conjugate but not
chlorambucil competitively inhibited
spermidine uptake into
tumor cells, supported the suggestion that the conjugate utilized the
polyamine uptake system. In vivo following a single i.p. dose, the conjugate was 4-fold more potent than
chlorambucil in its ability to inhibit ADJ/
PC6 tumor growth in BALB/c mice. However, the therapeutic index was not increased. Our results support the hypothesis that
polyamines linked to cytotoxics facilitate their entry into
tumor cells possessing a
polyamine uptake system and increase their selectivity to
DNA. This may have therapeutic application in the delivery of
cytotoxic agents linked to
polyamines to certain
tumors.