Synthesis, physiochemical and biological evaluation of a polyamine conjugate as a potential drug in breast cancer tumour-targeting
Breast cancer chemotherapy often damages rapidly reproducing cells without discerning
between those which are healthy and those which are neoplastic. In this project the design,
synthesis and evaluation of novel tumour targeting conjugates were investigated.
Polyamines are an attractive tool in targeted cancer therapy since their transport system
(PAT) can be used to facilitate the cellular uptake of cytotoxic drugs conjugated to them.
Furthermore the PAT is upregulated in many cancer cells because of their high demand for
them. Consequently, synthetic analogues which contain a cytotoxic moiety linked with a
polyaminic portion could be transported in higher concentrations into cancer cells than into
those which are normal. Additionally, polyamines exhibit a high affinity for nucleic acids;
therefore drugs containing an antineoplastic moiety which targets DNA and a polyaminic
portion may exhibit a bifunctional mode of binding to the nucleic acid and hence an higher
In order to improve selectivity, a trifunctional conjugate was designed composed of an
oestrogenic ligand and an acridinic moiety, linked by an amido bond to the polyamine
spermine. The acridine is expected give rise to the main cytotoxic action via DNA
intercalation, whereas the oestrogenic portion is predicted to provide an additional
selectivity against those breast cancer cells which are oestrogen receptor positive (ER+).
Synthetic studies towards the target molecule were partly successful with the two halves of
the conjugate, the functionalised oestrogen and polyamine linked acridine (PAC), being
made. However time prevented the final step, linking these moieties, from being achieved.
The cytotoxicity of PAC and the controls mitoxantrone (MTO) and 9-aminoacridine
(AMAC) was found against four ER+ cell lines (MCF7, T 47D, 5637, ZR-75-1) and one
ER- (Hs 578T). PAC had μM IC50 values in each cell line and was more cytotoxic than
AMAC, but less than MTO.
To optimise the activity of the synthetic polyamine conjugate, DFMO, an irreversible
inhibitor of a key enzyme in the synthesis of endogenous polyamines, was co-administered
during cytotoxicity assays. Depletion of the endogenous polyamine pool by inhibition of
biosynthetic enzymes has been shown to upregulate PAT and hence leads to enhanced
uptake of polyamine analogues. The resulting IC50 values were in the nM range suggesting
that the PAC is transported by PAT.
The Chinese Hamster Ovary (CHO) cell line is known to strongly express PAT whilst the
mutant CHO-MG is devoid of it. The IC50 values for PAC were similar (μM) for both cell
types suggesting that PAC is not transported predominantly by PAT and that DFMO acts
synergistically via an alternative mechanism. Interestingly the cytotoxicity of MTO was
increased by a similar order upon co-administration with DFMO.
Spectrophotometric analysis of mixtures of PAC with calf thymus DNA showed clear
isosbestic points indicating that PAC has a single mode of binding. This is consistent with
base pair intercalation, typical of widely used chemotherapics including MTO.