In addition to being the major route of uptake of dietary nitrogen from the small intestine, expression of the proton-coupled di- and tri-peptide transporter PepT1 is also up-regulated in many cell lines derived from cancerous tissue, despite not being expressed in the wild-type tissue (Mitsuoka et al. 2008). Included amongst these tissues is the pancreas, and PepT1 expression in the human pancreatic cell line AsPc1 has been documented (Gonzalez et al 1998). Here we report that some small modified dipeptides that are PepT1 substrates have an anti-proliferative effect on the growth of AsPc1 cells. To test the effect on proliferation rates, compounds were included at a concentration of approximately twice their affinity for PepT1 in the medium of AsPc1 cells plated at an initial density of 50% in 96-well plates. After 2-3 days growth under standard conditions (RMPI 1640 medium, plus or minus test compound, supplemented with 10% FCS and penicillin/streptomycin, 37°C, 5% CO2), the proliferation was measured using a colourmetric assay, either MTT (ATCC, LGC Promochem, UK) or XTT (Cambridge Bioscience, UK). Data in the presence or absence of the compounds (control) was compared by Student’s paired t-test between the conditions, and are presented as normalised to the relevant control, mean +/- SEM of n independent experiments. As can be seen from the data in Figure 1, the compounds DF5:15A and DF5:15B, which are modified simple dipeptides, were able to significantly inhibit the proliferation of AsPc1 cells at 0.1mM (both p<0.05, n=4), whilst control dipeptides (Gly-L-Gln at 0.2mM and the hydrolysis-resistant L-Phe-(ψS)-L-Ala at 1.2mM) and the free modified amino acid component of the dipeptide (X at 1mM) did not (p>0.05, n=11, 8 and 7, respectively). As a positive control, the anti-cancer agent 5-fluorouracil (5FU, 100 μM) produced a similar level of inhibition of cell proliferation in our assays (p<0.05, n=17). The findings above are suggestive of an involvement of PepT1 in the observed anti-proliferative effects on AsPc1 cells in that only the addition to the medium of intact modified dipeptides had an effect, supported by the fact that the concentration of compound needed was similar to the PepT1 affinity measured in rabbit PepT1 (expressed in Xenopus laevis oocytes, data not shown). If this is indeed proved to be the case, then compounds like these could be therapeutically useful as anti-proliferative agents against cancerous cells that ectopically express PepT1.