When utilized as a macromolecular drug targeting ligand, folic acid (Pte-Glu) has traditionally been coupled to peptides, proteins and lipids via one of its two carboxylate groups fortuitously located within a distal glutamyl moiety. It has been assumed in the literature that the gamma-glutamyl carboxylate of Pte-Glu is the preferred conjugation site for macromolecules enduring endocytosis via the folate-binding protein receptor. However, it is also possible that the steric placement of the attached macromolecule around the vitamin's pteridine moiety may be the more influential parameter controlling this delivery mechanism. Using solid-phase chemistries, we have synthesized dipeptide derivatives of pteroic acid for the purpose of identifying the preferred site onto which a macromolecule can be chemically attached without compromising its endocytosis potential. Thus, using fluorescent and radiolabeled conjugates, we have determined that macromolecules attached to Pte-Glu by either an alpha- or gamma-glutamyl linkage could associate with receptor-bearing cells at virtually identical levels. We further discovered that removal of the remaining un-conjugated glutamyl carboxylate had no inhibitory effect on cell uptake; and, the cytotoxicity of related momordin toxin conjugates were comparable among the various pteroate derivatives tested. From these observations we suggest that the preparation of endocytosis-competent pteroate-macromolecule conjugates is strongly influenced by the steric environment around the ligand's para-aminobenzoic acid moiety, and that no selective isomeric (i.e. alphaGlu versus gammaGlu) conjugation requirement necessarily exists.

Poly(ethylene glycol) of various sizes was used as a molecular spacer to separate the cell-targeting ligand, folate, from the surface of poly-L-lysine. The resulting ternary macromolecule (pLys-PEG-folate) was investigated in various formulations for its ability to transfect reporter plasmids into receptor-bearing HeLa and IGROV cell lines. Formulations were optimized with respect to DNA content, +/- charge ratio, and the size and amount of PEG substitution off the pLys backbone. Transfection activity was highest 48 h after sample introduction, and PEG 3400 was determined to be the most favorable spacer size tested. pLys-PEG-folate:DNA transfection was also found to be both concentration dependent and saturable; plus, it was blocked by the addition of excess-free folate, indicative of a specific mechanism of uptake. Transfection activity was virtually identical for complexes formed in 10% serum-supplemented media, deionized water, or Hepes buffer. And, cell viability remained greater than 85% at the highest concentrations of pLys-PEG-folate:DNA complexes tested (4.8 microg/mL pLys 331 000; 12 microg/mL DNA). Taken together, these observations provide evidence that pLys-PEG-folate:DNA complexes are taken up specifically by the folate endocytosis pathway, and that the intramolecular spatial distance of the ligand from the pLys backbone dramatically influences transfection.

Triple negative breast cancer (TNBC) is a subtype characterized by lack of gene expression for the estrogen receptor, progesterone receptor and the Her2/neu protein. Hence, the lack of hormonal or Her2 targeted therapy options makes TNBC difficult to treat, resulting in a death rate which is disproportionately higher than for other breast cancer subtypes. Recently, the folate receptor (FR) was shown to be highly expressed in TNBC, and expression significantly correlated with the higher grades of malignancy as well as poor outcomes. These observations suggest that TNBC patients may benefit from treatment with an FR-targeted therapy. To address this question, an established FR-positive subcutaneous TNBC pre-clinical model (MDA-MB-231) was chosen to evaluate a panel of FR-targeted small molecule drug conjugates (SMDC) with high potencies and varying mechanisms of action. Thus, EC1456 (folate-tubulysin B; IC50 ∼ 1 nM) and EC1744 (a folate-DNA cross-linking agent; IC50 ∼0.1 nM) were dosed intravenously at 2 μmol/kg following a three times per week, two week schedule in tumor-bearing mice. Both FR-targeted agents were found to be highly active against the MDA-MB-231 tumors, with 80-100% cure rates. In all cases, the observed anti-tumor activity was not accompanied by any significant weight loss in the test animals. These findings suggest that folate-targeted SMDCs may be active against TNBC and that further preclinical studies are warranted. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-16-24.