Bioreductive Anticancer Prodrugs
Despite the advances and gains made in the past few decades
in cancer chemotherapy, many conventional anticancer drugs display
relatively poor selectivity for neoplastic cells. Solid tumor cells
are particularly resistant to radiation and chemotherapy. A number
of factors including cellular heterogeneity and physiological
properties such as inadequate blood flow are involved in the lack
of responsiveness. While there may be few useful kinetic and/or
biochemical differences between solid tumor cells and normal cells
that can be exploited, there are important micro environmental
properties unique to solid tumors, e.g., localized hypoxia,
nutrient deprivation, and low pH. There has been considerable
interest in designing drugs selective for hypoxic environments.
Much of the work has been centered on nitroheterocyclics and
quinone-containing compounds such as Mitomycin C. In my laboratory,
we have been exploring new sulfoxide-containing nitrogen mustard
prodrugs with a particular emphasis on the modulation of mustard
reactivity by the use of the S/SO/SO2 redox system to achieve
improved selectivity against neoplastic cells. (supported by NIH
Grant CA63618-01 from the National Cancer Institute).
Anticancer Prodrugs of
PDA
Cyclophosphamide (CP), a widely used anticancer and
immunosuppresive agent, is itself a prodrug ,and its activation is
initiated by hepatic microsomal mixed-function oxidase (MFO)
catalyzed C4-hydroxylation. The resulting 4-hydroxycyclophosphamide
(4-OH CP) undergoes ring opening to aldophosphamide (Aldo),
followed by generation of cytotoxic phosphoramide mustard (PDA) and
acrolein by beta-elimination. The cytotoxic activity of CP is
attributed to the aziridinium ion species derived from PDA, the
ultimate alkylating species that cross-links interstrand DNA.
Acrolein, a by-product of beta-elimination, does not play a
significant role in the anticancer activity of CP. However,
acrolein is responsible for hemorrhagic cystitis, a side effect
which is often dose-limiting in cyclophosphamide therapy. Properly
designed bioreversible prodrugs of PDA may serve as useful
antitumor drugs with improved therapeutic index, especially against
cyclophosphamide resistant cell lines. We are currently
investigating a number of bioreversible prodrugs of PDA and its
related analogs and they include “sulfonyl-containing
aldophosphamide analogs as novel anticancer prodrugs targeted
against CP-resistant tumor cell lines”: a series of
sulfonyl-group containing analogs of aldophosphamide (Aldo) were
synthesized as potential anticancer prodrugs that liberate the
cytotoxic phosphoramide mustard (PDA) species via b-elimination, a
non-enzymatic activation mechanism. Kinetic studies
demonstrated that all these compounds spontaneously liberate PDA
derivatives. Analogous to Aldo, the liberation of PDA
derivatives in all these compounds was enhanced in reconstituted
human plasma under same conditions. The compounds were more
potent than the corresponding PDA derivatives against V-79 Chinese
Hamster Lung Fibroblasts in vitro. Several compounds showed
excellent in vivo antitumor activity in CD2F1 mice against both
P388/0 (Wild) and P388/CPA (CP-resistant) tumor cell lines.
Sulfur-containing anilinoacridines related to amsarine as
potential anticancer agents: a series of sulfur-containing
9-anilineacridines related to amsacrine were synthesized and
evaluated for their anticancer potential. Among the
compounds, the diol-containing compounds were the most highly
cytotoxic among the sulfide series against V79 cells in
vitro. Among the nonalkyl substituted compounds, compounds
with electron donating substitution para to the sulfide were more
cytotoxic than the electron withdrawing nitro substituted
compound. The limited SAR suggested the importance of
hydroxyl functionality along with its location for the cytotoxicity
in the series. A preliminary anticancer screening against
P388 leukemia showed that the diol compound is highly active in
vivo as well. Topoisomerase II inhibitory activity appeared
to be involved in the cytotoxicity of the compound. The
corresponding sulfoxide compound, which is 6-7 fold less cytotoxic
than its sulfide diol, appears to be a potential bioreductive
anti-cancer prodrug on the basis of its bioreductive metabolism
findings. (supported in part by 2000ITG by the NY State
Biotechnology Grant).
1,2-Benzisoxazole Phosphorodiamidates as
Novel Anticancer Prodrugs Requiring Bioreductive
Activation
Several 1,2-benzisoxazole phosphorodiamidates have been designed
as prodrugs of phosphoramide mustard requiring bioreductive
activation. Enzymatic reduction of 1,2-benziosoxazole moiety is
expected to result in the formation of imine intermediate due to
the cleavage of the N-O bond. The imine should then be
spontaneously hydrolyzed to a ketone metabolite, thereby
facilitating base-catalyzed ‚-elimination of cytotoxic
phosphoramide mustard. As expected, the proposed prodrugs were at
least 3-5-fold more potent cytotoxins than control compounds, which
lack in the phosphoramide mustard group. Microsomal cytochrome P450
and/or P450 reductase appear to be involved in the reductive
metabolism of 1,2-benzisoxazole moiety under hypoxic as well as
oxic conditions.
Iminohydantoin Antiepileptic
Drugs
Epilepsy is a collective designation of seizure disorders which
affect over two million Americans. Currently marketed
anticonvulsants do not often provide complete control of seizures,
and are associated with a wide range of side effects. A need
currently exists for improved anticonvulsant drugs. Since the
introduction of hydantoins as antiepileptic drugs, the effect of
structural modification of the hydantoin ring on anticonvulsant
activity has been a subject of great interest and the structure
activity relationship (SAR) has been discussed. Phenytoin
(5,5-diphenylhydantoin, DPH, Dilantin*), the most active member in
this class, is one of the most widely used anticonvulsant agents,
but is associated with a variety of toxic effects and is
teratogenic. A structurally close analog of hydantoin is
2-iminohydantoin (glycocyamidine). We have been investigating a
number of iminohydantoin derivatives as potential anticonvulsant
agents. Research is in progress to define the SAR
(structure-activity relationship) of 2-iminohydantoins.
Aromatic Nitrile Metabolism
Nitriles are commercially important compounds used in the
manufacture of plastics, dyestuffs, and pharmaceuticals. Due to the
widespread use of many aliphatic nitriles, there have been numerous
investigations of their metabolic fate and toxicity. It has been
shown that free cyanide liberated during their biotransformation in
vivo plays an important role in the acute toxicity of many
aliphatic nitriles. Studies of the metabolic fate of aromatic
nitriles, in contrast to their aliphatic counterparts, have been
minimal and the molecular mechanism of their biotransformation have
been mainly investigated in microorganisms. As an ongoing project,
we are investigating the metabolism of aromatic nitriles by
mammalian enzyme systems.