Novel fluoro-substituted camptothecins: in vivo antitumor activity, reduced gastrointestinal toxicity and pharmacokinetic characterization

Prospects of Topoisomerase Inhibitors as Promising Anti-Cancer Agents

Topoisomerases are very important enzymes that regulate DNA topology and are vital for biological actions like DNA replication, transcription, and repair. The emergence and spread of cancer has been intimately associated with topoisomerase dysregulation. Topoisomerase inhibitors have consequently become potential anti-cancer medications because of their ability to obstruct the normal function of these enzymes, which leads to DNA damage and subsequently causes cell death. This review emphasizes the importance of topoisomerase inhibitors as marketed, clinical and preclinical anti-cancer medications. In the present review, various types of topoisomerase inhibitors and their mechanisms of action have been discussed. Topoisomerase I inhibitors, which include irinotecan and topotecan, are agents that interact with the DNA-topoisomerase I complex and avert resealing of the DNA. The accretion of DNA breaks leads to the inhibition of DNA replication and cell death. On the other hand, topoisomerase II inhibitors like etoposide and teniposide, function by cleaving the DNA-topoisomerase II complex thereby effectively impeding the release of double-strand DNA breaks. Moreover, the recent advances in exploring the therapeutic efficacy, toxicity, and MDR (multidrug resistance) issues of new topoisomerase inhibitors have been reviewed in the present review.

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Simultaneous determination of FL118 and W34 in rat Blood by LC-MS/MS: Application to pharmacokinetic studies

W34 is a prodrug of FL118, and it can be converted to FL118 via a hydrolysis reaction. In this report, a highly sensitive LC-MS/MS method using a C₁₈ column was validated and used for the simultaneous determination of W34 and FL118 in rat blood. A stepwise gradient elution with 0.1% formic acid in water and acetonitrile was employed. The assays were linear over a concentration range of 0.50-50.0 ng/ml for both W34 and FL118. The accuracy of the validation method ranged from 89.74 to 98.94% for W34 and from 88.61 to 94.60% for FL118. The precision was within 7.15% for W34 and 9.63% for FL118. Extraction recoveries of W34 were 94.56-100.49 and 87.67-106.32% for FL118. No significant matrix effects for both W34 and FL118 were observed in blood. The assay has been successfully applied to biological samples obtained from a stability and pharmacokinetic study of W34 and FL118.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Synthesis of novel 10,11-methylenedioxy-camptothecin glycoside derivatives and investigation of their anti-tumor effects in vivo

10,11-Methylenedioxy-camptothecin (FL118) is a novel camptothecin analogue that possesses exceptional antitumor efficacy in human tumor xenograft models. The aim of the current study was to develop novel 20-substituted FL118 derivatives coupled with glycosyl-succinic acid esters with improved antitumor efficacy. These FL118 glycoside derivatives were designed, synthesized and their cytotoxicity evaluated in three tumor cell lines (A-549, MDA-MB-231 and RM-1). All of the derivatives showed superior in vitro cytotoxic activity and were more potent than irinotecan in A549 and MDA-MB-231 cells. In mouse prostate cancer cells RM-1, 10,11-methylenedioxy-camptothecin rhamnoside 11b displayed significant activities with IC₅₀ of 48.27 nM. Western blot analysis demonstrated that 11b inhibited survivin expression and induced cancer cells apoptosis. Further cell cycle analyses clearly showed 11b induced G2/M phase cell cycle arrest. Molecule docking studies suggested that the binding mode of 11b was different from that of the crystal complex of ligand topotecan in Top1/DNA. Importantly, 11b showed high in vivo antitumor efficacy in the RM-1 mouse model with transplantation of prostate cancer (TGI = 44.9%) at dose of 9 mg kg⁻¹ without apparent toxicity.

In a RM-1 xenograft model, 11b had superior in vivo antitumor efficacy (TGI = 44.9%) at a dose of 9 mg kg⁻¹.

Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

The electronic properties and relatively small size of fluorine endow it with considerable versatility as a bioisostere and it has found application as a substitute for lone pairs of electrons, the hydrogen atom, and the methyl group while also acting as a functional mimetic of the carbonyl, carbinol, and nitrile moieties. In this context, fluorine substitution can influence the potency, conformation, metabolism, membrane permeability, and P-gp recognition of a molecule and temper inhibition of the hERG channel by basic amines. However, as a consequence of the unique properties of fluorine, it features prominently in the design of higher order structural metaphors that are more esoteric in their conception and which reflect a more sophisticated molecular construction that broadens biological mimesis. In this Perspective, applications of fluorine in the construction of bioisosteric elements designed to enhance the in vitro and in vivo properties of a molecule are summarized.

Applications of Fluorine in Medicinal Chemistry

The role of fluorine in drug design and development is expanding rapidly as we learn more about the unique properties associated with this unusual element and how to deploy it with greater sophistication. The judicious introduction of fluorine into a molecule can productively influence conformation, pKa, intrinsic potency, membrane permeability, metabolic pathways, and pharmacokinetic properties. In addition, (18)F has been established as a useful positron emitting isotope for use with in vivo imaging technology that potentially has extensive application in drug discovery and development, often limited only by convenient synthetic accessibility to labeled compounds. The wide ranging applications of fluorine in drug design are providing a strong stimulus for the development of new synthetic methodologies that allow more facile access to a wide range of fluorinated compounds. In this review, we provide an update on the effects of the strategic incorporation of fluorine in drug molecules and applications in positron emission tomography.

Antitumor activity of FL118, a survivin, Mcl-1, XIAP, and cIAP2 selective inhibitor, is highly dependent on its primary structure and steric configuration

We recently reported the identification and characterization of a novel small chemical molecule designated FL118. FL118 selectively inhibits multiple cancer survival and proliferation-associated antiapoptotic proteins (survivin, Mcl-1, XIAP, cIAP2) and eliminates small and large human tumor xenografts in animal models (Ling et al., PLoS One 2012, 7, e45571). Here, we report a follow-up study on the structure-activity relationship (SAR) of the hydroxyl group in the lactone ring of FL118. We found that the superior antitumor efficacy of FL118 heavily depends on its steric configuration through comparing the antitumor activity of FL118 with FL113 (the racemic mixture of FL118). Consistently, FL118 proved much more effective in inhibiting the expression of survivin, Mcl-1, and cIAP2, both in vitro and in vivo, compared to FL113. Additionally, Tet-on controlled induction of survivin or forced expression of Mcl-1 protects cancer cells from FL118-mediated growth inhibition and cell death. To further explore the SAR, we synthesized seven position 20-esterifiable FL118 and FL113 derivatives. Studies on these seven new compounds revealed that keeping a free hydroxyl group of FL118 is also important for high antitumor efficacy. Together, these studies confirm the superior anticancer activity of FL118 and narrow the window for further SAR studies to generate novel analogues based on FL118 core structure on its other potential chemical positions.

Cancer therapies utilizing the camptothecins: a review of the in vivo literature

This review summarizes the in vivo assessment-preliminary, preclinical, and clinical-of chemotherapeutics derived from camptothecin or a derivative. Camptothecin is a naturally occurring, pentacyclic quinoline alkaloid that possesses high cytotoxic activity in a variety of cell lines. Major limitations of the drug, including poor solubility and hydrolysis under physiological conditions, prevent full clinical utilization. Camptothecin remains at equilibrium in an active lactone form and inactive hydrolyzed carboxylate form. The active lactone binds to DNA topoisomerase I cleavage complex, believed to be the single site of activity. Binding inhibits DNA religation, resulting in apoptosis. A series of small molecule camptothecin derivatives have been developed that increase solubility, lactone stability and bioavailability to varying levels of success. A number of macromolecular agents have also been described wherein camptothecin(s) are covalently appended or noncovalently associated with the goal of improving solubility and lactone stability, while taking advantage of the tumor physiology to deliver larger doses of drug to the tumor with lower systemic toxicity. With the increasing interest in drug delivery and polymer therapeutics, additional constructs are anticipated. The goal of this review is to summarize the relevant literature for others interested in the field of camptothecin-based therapeutics, specifically in the context of biodistribution, dosing regimens, and pharmacokinetics with the desire of providing a useful source of comparative data. To this end, only constructs where in vivo data is available are reported. The review includes published reports in English through mid-2009.

Microfabricated electrospun collagen membranes for 3-D cancer models and drug screening applications

Invasive epithelial tumors form from cells that are released from their natural basement membrane and form 3-D structures that interact with each other and with the microenvironment of the stromal tissues around the tumor, which often contains collagen. Cancer cells, growing as monolayers on tissue culture plastic, do not reflect many of the properties of whole tumors. This shortcoming limits their ability to serve as models for testing of pharmacologically active compounds, including those that are being tested as antineoplastics. This work seeks to create new 3-D cellular materials possessing properties similar to those in native tissues surrounding cancers, specifically electrospun micro- and nanofibrous collagen scaffolds that support tumor growth in 3-D. We hypothesize that a 3-D culture system will provide a better replica of tumor growth in a native environment and, thus, better report the bioactivity of antineoplastic agents. In addition, we optimized conditions and identified physical characteristics that support growth of the highly invasive, prostate cancer bone metastatic cell line C4-2B on these matrices for use in anticancer drug studies. The effects of matrix porosity, fiber diameter, elasticity, and surface roughness on growth of cancer cells were evaluated. Data indicates that while cells attach and grow well on both nano- and microfibrous electrospun membranes, the microfibrous membrane represented a better approximation of the tumor microenvironment. It was also observed that C4-2B nonadherent cells migrated through the depth of two electrospun membranes and formed colonies resembling tumors on day 3. An apoptosis study revealed that cells on electrospun substrates were more resistant to both antineoplastic agents, docetaxel (DOC), and camptothecin (CAM) compared to the cells grown on standard collagen-coated tissue culture polystyrene (TCP). Growth, survival, and apoptosis were measured, as well as the differences in the apoptotic capabilities, of the two above-mentioned compounds compared to known clinical performance. We conclude that 3-D electrospun membranes are amenable to high throughput screening for cancer cell susceptibility and combination killing (Banerjee, S.; Hussain, M.; Wang, Z.; Saliganan, A.; Che, M.; Bonfil, D.; Cher, M.; Sarkar, F.H. Cancer Research, 2007, 67 (8), 3818-26).