Novel Antitumor Compounds Targeting Folate Receptors

Human cells require folates and B vitamin, which are
necessary for cell growth and function. Cells cannot
produce the folates needed for DNA de novo synthesis and
require an external source. Anticancer agents which block
folate uptake can be cytotoxic. Reduced Folate Carrier
(RFC) is the primary transport system for folate uptake
and is ubiquitously expressed. Therefore, RFC is the target
for current clinically relevant antifolate drugs. The use of
chemotherapeutic agents has been linked to loss of RFC
levels or function resulting in resistance to antifolates.

Other receptors such as FR alpha and FR beta are
expressed to a lesser extent in normal cells while over
expressed in certain tumor cells. FR alpha is known to be over expressed in ovarian, endometrial, kidney, lung,
mesothelioma, breast and brain tumors while FR beta is
expressed in acute myeloid leukemia. Antifolates that
selectively target FRs over RFC, such as the pyrrolo
pyrimidines illustrated below, confer greater selectivity.

Conjugates of folic acid have been used to selectively deliver
chemotherapeutic agents to FR-expressing tumors. The
problem with folic acid conjugates is that they require
cleavage from the folic acid moiety, which is difficult to
achieve. Antitumor activity is reduced if the cytotoxic agent is not released from the conjugate.

The cyclic pyrimidines act as the cytotoxic agent without a
prior step of cleavage. The collection of compounds
developed by Duquesne University selectively target FR
alpha and FR beta of cancerous cells. This reduces the cytotoxicity associated with conventional antifolates while
maintaining chemotherapeutic (cytotoxic) activity for the
targeted tumor cells as shown by activity against
carboplatin and paclitaxel resistant cell cultures. In mouse
models of ovarian cancer tumor growth was inhibited for
both early and late stage tumors without corresponding
toxicity to normal tissues as evidenced by lack of weight
loss and effect in major organs. Inhibition of GARFTase
occurred at nanomolar quantities and FR alpha based
inhibition occurred at and IC50 of 2.5 nM.

There is a considerable amount of evidence that at high
extracellular concentrations these compounds (3 and 4
Carbon bridge structure) have increased inhibitory potency
against CCRF-CEM cell growth in culture compared to
compounds with a shorter carbon bridge such as
pemetrexed making them potentially more effective and
less toxic. It has been demonstrated that these compounds act on the Proton Coupled Folate Transport system (PCFT) in target cells thereby reducing GTP levels (50%) and ATP pools (75%) with human PCFT4 cells with an IC50 value of 0.97 nM. This compares to IC50 values of 7.3 nm for pemetrexed with the same cell line. This human PCFT selectivity is preserved under low pH conditions
which commonly occur in certain solid tumors.

File Number: PCT 60/816931 

Web site: http://www.research.duq.edu

Disease: Cancer