Folate antagonists: toward improving the therapeutic index and development of new analogs

Gadolinium texaphyrin-methotrexate conjugates. Towards improved cancer chemotherapeutic agents

Conjugates between methotrexate (MTX, Matrex, N-[4-[[(2,4-diamino-6-pteridinyl)methyl]methylamino]benzoyl]-l-glutamic acid), an antifolate cancer chemotherapeutic to which resistance is often observed, and motexafin gadolinium (MGd), an experimental agent demonstrating selective tumor localization, are described. These systems were prepared in order to test whether linking these two species would produce agents with enhanced activity relative to MTX alone. Both ester- and amide-linked conjugates were synthesized starting from MGd and MTX. The ester conjugate showed greater in vitro anti-proliferative activity against the A549 lung carcinoma cell line at short incubation times than did MTX alone. Neither the amide conjugate, nor MGd, showed any observable activity under these in vitro conditions. These results are rationalized in terms of enhanced cellular uptake of both the ester and amide conjugates that is coupled with an effective rate of release (e.g., inherent or enzyme-mediated hydrolysis) in the case of the ester-linked conjugate, but not the corresponding amide system.

A genetic algorithm for structure-based de novo design

Genetic algorithms have properties which make them attractive in de novo drug design. Like other de novo design programs, genetic algorithms require a method to reduce the enormous search space of possible compounds. Most often this is done using information from known ligands. We have developed the ADAPT program, a genetic algorithm which uses molecular interactions evaluated with docking calculations as a fitness function to reduce the search space. ADAPT does not require information about known ligands. The program takes an initial set of compounds and iteratively builds new compounds based on the fitness scores of the previous set of compounds. We describe the particulars of the ADAPT algorithm and its application to three well-studied target systems. We also show that the strategies of enhanced local sampling and re-introducing diversity to the compound population during the design cycle provide better results than conventional genetic algorithm protocols.

Physicochemical parameters responsible for the affinity of methotrexate analogs for rat canalicular multispecific organic anion transporter (cMOAT/MRP2)

PURPOSE

Canalicular multispecific organic anion transporter (cMOAT/MRP2) is known to exhibit a broad substrate specificity toward amphiphatic organic anions, including methotrexate (MTX). The present study aims to identify the physicochemical properties of MTX derivatives that correlate with recognition specificity by cMOAT/MRP2.

METHODS

We examined the inhibitory effect of MTX and 24 analogs on the transport of [3H]-S-(2,4-dinitrophenyl)glutathione by cMOAT/MRP2. The affinity constants of these compounds were compared with their physicochemical parameters. The primary active transport of several compounds was also confirmed.

RESULTS

The affinity constants closely correlated with the octanol/water partition coefficient (clogP), and a linear combination of polar and nonpolar surface areas. The affinity for cMOAT/MRP2 also closely correlated with the molecular weight, which also showed a significant correlation with nonpolar surface area and clogP.

CONCLUSIONS

Recognition by cMOAT/MRP2 depends on a balance of dynamic surface properties between the polar and nonpolar regions of MTX analogs. The so-called "molecular weight threshold" for the cMOAT/MRP2 affinity of these compounds can be explained by their physicochemical parameters, especially their nonpolar surface areas.

Comparison of two independent crystal structures of human dihydrofolate reductase ternary complexes reduced with nicotinamide adenine dinucleotide phosphate and the very tight-binding inhibitor PT523

Structural data for two independent crystal forms (monoclinic, C2, and orthorhombic, P2(1)2(1)2(1)) of the ternary complex of the potent antitumor agent PT523 [N alpha-(4-amino-4-deoxypteroyl)-N delta-hemiphthaloyl-L-ornithine], reduced nicotinamide adenine dinucleotide phosphate (NADPH), and recombinant human dihydrofolate reductase (hDHFR) reveals multiple binding orientations for the hemiphthaloyl group of the inhibitor. Analysis of these data shows that PT523 binds with its pteridine ring in the same orientation observed for methotrexate (MTX) analogues. However, in each structure, the hemiphthaloyl ring occupies three alternate conformations. In the C2 lattice, the phthaloyl moiety binds in two extended conformations, A and C, with each conformer having a 180 degrees flip of the o-carboxylate group, and a third, lower occupancy conformer B, with the phthaloyl group folded within contact of the active-site pocket. In the orthorhombic lattice, PT523 also has three conformers for the phthaloyl group; however, these differ from those observed in the monoclinic lattice. Two major conformers, A and C, are displaced on either side of the extended position observed in the C2 lattice, one near the folded B conformer of the C2 lattice and the other extended. These conformers form tighter intermolecular contacts than those in the C2 lattice. Conformer B is folded back away from the active site in a unique position. There are also significant differences in the conformation of the adenine-ribose moiety of NADPH in both complexes that differ from that observed for other inhibitor-NADPH-hDHFR ternary complexes. These data suggest that the added intermolecular contacts made by the hemiphaloyl group of PT523 contribute to its tighter binding to hDHFR than MTX, which does not extend as far from the active site and cannot make these contacts. These crystallographic observations of multiple conformations for the hemiphthaloyl group are in general agreement with solution NMR data for the binding of PT523 to hDHFR [Johnson et al. (1997) Biochemistry 36, 4399-4411], which show that the hemiphthaloyl group may adopt more than one conformation. However, the crystallographic data reveal more discretely occupied positions than can be interpreted from the solution data. These results suggest that crystal packing interactions may influence their stability.

Immunolocalization of basic fibroblast growth factor (bFGF) in growing and growth-inhibited placental cells: a possible role for bFGF in placental cell development

The distribution of basic (b) fibroblast growth factor (FGF) in growing and growth-arrested human placental tumour cells, as well as normal placental villous trophoblasts, was studied by immunofluorescence microscopy with antibodies to the amino terminus of bFGF. Placenta (FAR, FEG-3), breast (MCF-7, T-47D), cervix (HeLa) and uterine (HEC-1-A) tumour cells showed the same two patterns after immunofluorescent staining with antibodies to bFGF: (i) a perinuclear pattern and (ii) an intense homogeneous staining of the nucleus and cytoplasm. The homogeneous bFGF staining pattern was associated predominantly with actively dividing cells, observed at different stages of mitosis and cytokinesis. Placental (FEG-3) cell division was inhibited with methotrexate (MTX), a chemotherapeutic agent used in the treatment of placental tumours. MTX-treated FEG-3 cells as well as 'normal' non-proliferative placental (syncytiotrophoblast) cells from term placentae, showed perinuclear staining with antibodies to bFGF and immunofluorescence microscopy. The nuclear localization of bFGF in dividing but not non-dividing placental cells, suggests a role for bFGF in cytotrophoblast proliferation in vivo.