Antitumour activity of novel taxanes that act at the same time as cytotoxic agents and P-glycoprotein inhibitors

Effect of substituents at the C3´, C3´N, C10 and C2-meta-benzoate positions of taxane derivatives on their activity against resistant cancer cells

We tested the effect of substituents at the (1) C3´, C3´N, (2) C10, and (3) C2-meta-benzoate positions of taxane derivatives on their activity against sensitive versus counterpart paclitaxel-resistant breast (MCF-7) and ovarian (SK-OV-3) cancer cells. We found that (1) non-aromatic groups at both C3´ and C3´N positions, when compared with phenyl groups at the same positions of a taxane derivative, significantly reduced the resistance of ABCB1 expressing MCF-7/PacR and SK-OV-3/PacR cancer cells. This is, at least in the case of the SB-T-1216 series, accompanied by an ineffective decrease of intracellular levels in MCF-7/PacR cells. The low binding affinity of SB-T-1216 in the ABCB1 binding cavity can elucidate these effects. (2) Cyclopropanecarbonyl group at the C10 position, when compared with the H atom, seems to increase the potency and capability of the derivative in overcoming paclitaxel resistance in both models. (3) Derivatives with fluorine and methyl substituents at the C2-meta-benzoate position were variously potent against sensitive and resistant cancer cells. All C2 derivatives were less capable of overcoming acquired resistance to paclitaxel in vitro than non-substituted analogs. Notably, fluorine derivatives SB-T-121205 and 121,206 were more potent against sensitive and resistant SK-OV-3 cells, and derivatives SB-T-121405 and 121,406 were more potent against sensitive and resistant MCF-7 cells. (4) The various structure-activity relationships of SB-T derivatives observed in two cell line models known to express ABCB1 favor their complex interaction not based solely on ABCB1.

A novel taxane, difluorovinyl-ortataxel, effectively overcomes paclitaxel-resistance in breast cancer cells

Paclitaxel (PTX) is widely used to treat breast and ovarian cancers, but innate and acquired resistance often compromises its applications. The objective of this study was to screen new-generation taxanes for their efficiency against both PTX-sensitive and PTX-resistant breast cancer cells. From twelve compounds, difluorovinyl-ortataxel (DFV-OTX) displayed potent cytotoxic activities against both PTX-sensitive and PTX-resistant breast cancer cells. Moreover, DFV-OTX effectively induced tubulin/microtubule polymerization and G2/M phase arrest, leading to apoptosis in both PTX-sensitive and PTX-resistant cancer cells. Molecular docking analysis showed that DFV-OTX possesses unique hydrogen-bonding and van der Waals interactions with β-tubulin. LC-MS/MS analysis also demonstrated that the intracellular drug amount of DFV-OTX was lower than that of PTX, which would be critical to overcome PTX-resistance. Furthermore, DFV-OTX exhibited clear efficacy in the MCF-7R and MDA-MB-231R tumor xenografts in mouse models. Taken together, our results demonstrate that the novel taxane, DFV-OTX, can effectively overcome PTX-resistance in MDA-MB-231R cells, wherein the drug resistance was attributed to ABCB1/ABCG2 upregulation and a distinct mode of action in MCF-7R cells. Our results strongly indicate that DFV-OTX is a promising chemotherapeutic agent for the treatment of PTX-resistant cancers.

Design, Synthesis, and Biological Evaluations of Asymmetric Bow-Tie PAMAM Dendrimer-Based Conjugates for Tumor-Targeted Drug Delivery

A unique asymmetric bow-tie poly(amidoamine) (PAMAM) dendrimer (ABTD) scaffold was designed and developed as a well-defined macromolecular carrier for tumor-targeted drug delivery. The ABTD scaffold in this study consists of a G3-half-dendron (G3-HD) unit and a G1-half-dendron (G1-HD) unit, bearing thiol moiety in each unit and a bis(maleimide) linker unit, which undergo sequential thiol-maleimide coupling to assemble the scaffold. This assembly methodology is applicable to all other combinations of different generations of PAMAM dendrimers. In the prototype ABTD in this study, 16 biotin moieties were tethered to the G3-HD unit and 4 payloads (new-generation taxoid) to the G1-HD via a self-immolative linker to form an ABTD-tumor-targeting conjugate (ABTD-TTC-1). Two other ABTD-TTCs were synthesized, wherein the G1-HD unit was tethered to a fluorescence-labeled taxoid or to a fluorescent probe. These three ABTD-TTCs were constructed by using a common key ABTD 6 bearing a terminal acetylene group in the G1-HD unit, which was fully characterized as a single molecule by high-resolution mass spectrometry and NMR despite its high molecular weight (M_w: 12 876). Then, the click reaction was employed to couple ABTD 6 with a small-molecule payload or fluorescence probe unit bearing a terminal azide moiety. ABTD-TTC-3, as a surrogate of ABTD-TTC-2, showed substantially enhanced internalization into two cancer cell lines via receptor-mediated endocytosis, attributed to multibinding effect. ABTD-TTC-1 exhibited a remarkable selectivity to cancer cells (1400-7500 times) compared to human normal cells, which demonstrates the salient feature and bright prospect of the ABTD-based tumor-targeted drug-delivery system.

Substituents at the C3' and C3'N positions are critical for taxanes to overcome acquired resistance of cancer cells to paclitaxel

We tested the role of substituents at the C3' and C3'N positions of the taxane molecule to identify taxane derivatives capable of overcoming acquired resistance to paclitaxel. Paclitaxel-resistant sublines SK-BR-3/PacR and MCF-7/PacR as well as the original paclitaxel-sensitive breast cancer cell lines SK-BR-3 and MCF-7 were used for testing. Increased expression of the ABCB1 transporter was found to be involved in the acquired resistance. We tested three groups of taxane derivatives: (1) phenyl group at both C3' and C3'N positions, (2) one phenyl at one of the C3' and C3'N positions and a non-aromatic group at the second position, (3) a non-aromatic group at both C3' and C3'N positions. We found that the presence of phenyl groups at both C3' and C3'N positions is associated with low capability of overcoming acquired paclitaxel resistance compared to taxanes containing at least one non-aromatic substituent at the C3' and C3'N positions. The increase in the ATPase activity of ABCB1 transporter after the application of taxanes from the first group was found to be somewhat higher than after the application of taxanes from the third group. Molecular docking studies demonstrated that the docking score was the lowest, i.e. the highest binding affinity, for taxanes from the first group. It was intermediate for taxanes from the second group, and the highest for taxanes from the third group. We conclude that at least one non-aromatic group at the C3' and C3'N positions of the taxane structure, resulting in reduced affinity to the ABCB1 transporter, brings about high capability of taxane to overcome acquired resistance of breast cancer cells to paclitaxel, due to less efficient transport of the taxane compound out of the cancer cells.

Inhibit or Evade Multidrug Resistance P-Glycoprotein in Cancer Treatment

Multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy. P-glycoprotein (P-gp), a promiscuous drug efflux pump, has been extensively studied for its association with MDR due to overexpression in cancer cells. Several P-gp inhibitors or modulators have been investigated in clinical trials in hope of circumventing MDR, with only limited success. Alternative strategies are actively pursued, such as the modification of existing drugs, development of new drugs, or combination of novel drug delivery agents to evade P-gp-dependent efflux. Despite the importance and numerous studies, these efforts have mostly been undertaken without a priori knowledge of how drugs interact with P-gp at the molecular level. This review highlights and discusses progress toward and challenges impeding drug development for inhibiting or evading P-gp in the context of our improved understanding of the structural basis and mechanism of P-gp-mediated MDR.

Small and Innovative Molecules as New Strategy to Revert MDR

Multidrug resistance (MDR) is a complex phenomenon principally due to the overexpression of some transmembrane proteins belonging to the ATP binding cassette (ABC) transporter family. Among these transporters, P-glycoprotein (P-gp) is mostly involved in MDR and its overexpression is the major cause of cancer therapy failure. The classical approach used to overcome MDR is the co-administration of a P-gp inhibitor and the classic antineoplastic drugs, although the results were often unsatisfactory. Different classes of P-gp ligands have been developed and, among them, Tariquidar has been extensively studied both in vitro and in vivo. Although Tariquidar has been considered for several years as the lead compound for the development of P-gp inhibitors, recent studies demonstrated it to be a substrate and inhibitor, in a dose-dependent manner. Moreover, Tariquidar structure-activity relationship studies were difficult to carry out because of the complexity of the structure that does not allow establishing the role of each moiety for P-gp activity. For this purpose, SMALL molecules bearing different scaffolds such as tetralin, biphenyl, arylthiazole, furoxane, furazan have been developed. Many of these ligands have been tested both in in vitro assays and in in vivo PET studies. These preliminary evaluations lead to obtain a library of P-gp interacting agents useful to conjugate chemotherapeutic agents displaying reduced pharmacological activity and appropriate small molecules. These molecules could get over the limits due to the antineoplastic-P-gp inhibitor co-administration since pharmacokinetic and pharmacodynamic profiles are related to a dual innovative drug.

The synthesis of novel taxoids for oral administration

A group of novel taxoids, with modifications at C-7, C-10, C-3' and C-14 positions of paclitaxel, was synthesized in order to improve their biological profile by decreasing their affinity with P-glycoprotein (P-gp) and increasing cellular permeability. Most of the new taxoids demonstrated the similar potent cytotoxic activities in MCF-7 human tumor cell line as paclitaxel in vitro. In the permeability assay with monolayers of Caco-2 cells, most of the compounds demonstrated an increased trans-cellular transport in A-to-B direction in comparison with paclitaxel. Among them the compounds T-13, T-15 and T-26 showed the highest permeability, and with efflux ratios better than that of ortataxel. The interaction of the compounds T-13 and T-26 with P-gp was evaluated using Madin-Darby canine kidney (MDCK)-multidrug resistance-1(MDR1) and MDCK-wild-type (WT). The results indicated that T-13 and T-26 were poor substrates for P-gp and possessed inhibiting effects of P-gp mediated efflux. It was thus clear that simultaneous modifications at the C-7, C-10 and C-3' positions of paclitaxel significantly impaired its interactions with P-gp and interfered with P-gp mediated efflux.

Microtubule stabilizing agents as potential treatment for Alzheimer's disease and related neurodegenerative tauopathies

The microtubule (MT) associated protein tau, which is highly expressed in the axons of neurons, is an endogenous MT-stabilizing agent that plays an important role in axonal transport. Loss of MT-stabilizing tau function, caused by misfolding, hyperphosphorylation, and sequestration of tau into insoluble aggregates, leads to axonal transport deficits with neuropathological consequences. Several in vitro and preclinical in vivo studies have shown that MT-stabilizing drugs can be utilized to compensate for the loss of tau function and to maintain/restore effective axonal transport. These findings indicate that MT-stabilizing compounds hold considerable promise for the treatment of Alzheimer disease and related tauopathies. The present article provides a synopsis of the key findings demonstrating the therapeutic potential of MT-stabilizing drugs in the context of neurodegenerative tauopathies, as well as an overview of the different classes of MT-stabilizing compounds.

Tumor-targeting drug delivery of new-generation taxoids

A long-standing problem of conventional cancer chemotherapy is the lack of tumor specificity. Tumor-targeting drug-delivery systems have been explored to overcome this problem. These systems combine a powerful cytotoxic anticancer agent with a tumor-targeting molecule via a 'smart' linker to form highly efficacious drug conjugates. These drug conjugates can deliver potent cytotoxic drugs specifically to tumors and tumor cells with minimal systemic toxicity. This review describes our groups' research on the molecular approaches to the design and development of a novel drug-delivery system bearing highly potent new-generation taxoids for tumor-targeting chemotherapy in our laboratory.

Modulation of protein-protein interactions as a therapeutic strategy for the treatment of neurodegenerative tauopathies

The recognition that malfunction of the microtubule (MT) associated protein tau is likely to play a defining role in the onset and/or progression of a number of neurodegenerative diseases, including Alzheimer's disease, has resulted in the initiation of drug discovery programs that target this protein. Tau is an endogenous MT-stabilizing agent that is highly expressed in the axons of neurons. The MT-stabilizing function of tau is essential for the axonal transport of proteins, neurotransmitters and other cellular constituents. Under pathological conditions, tau misfolding and aggregation results in axonal transport deficits that appear to have deleterious consequences for the affected neurons, leading to synapse dysfunction and, ultimately, neuronal loss. This review focuses on both progress and unresolved issues surrounding the development of novel therapeutics for the treatment of neurodegenerative tauopathies, which are based on (A) MT-stabilizing agents to compensate for the loss of normal tau function, and (B) small molecule inhibitors of tau aggregation.

The characterization of microtubule-stabilizing drugs as possible therapeutic agents for Alzheimer's disease and related tauopathies

Tau, a protein that is enriched in neurons of the central nervous system (CNS), is thought to play a critical role in the stabilization of microtubules (MTs). Several neurodegenerative disorders referred to as tauopathies, including Alzheimer's disease and certain types of frontotemporal lobar degeneration, are characterized by the intracellular accumulation of hyperphosphorylated tau fibrils. Tau deposition into insoluble aggregates is believed to result in a loss of tau function that leads to MT destabilization, and this could cause neurodegeneration as intact MTs are required for axonal transport and normal neuron function. This tau loss-of-function hypothesis has been validated in a tau transgenic mouse model with spinal cord tau inclusions, where the MT-stabilizing agent, paclitaxel, increased spinal nerve MT density and improved motor function after drug absorption at neuromuscular junctions. Unfortunately, paclitaxel is a P-glycoprotein substrate and has poor blood-brain barrier permeability, making it unsuitable for the treatment of human tauopathies. We therefore examined several MT-stabilizing compounds from the taxane and epothilone natural product families to assess their membrane permeability and to determine whether they act as substrates or inhibitors of P-glycoprotein. Moreover, we compared brain and plasma levels of the compounds after administration to mice. Finally, we assessed whether brain-penetrant compounds could stabilize mouse CNS MTs. We found that several epothilones have significantly greater brain penetration than the taxanes. Furthermore, certain epothilones cause an increase in CNS MT stabilization, with epothilone D demonstrating a favorable pharmacokinetic and pharmacodynamic profile which suggests this agent merits further study as a potential tauopathy drug candidate.

Metabolism of new-generation taxanes in human, pig, minipig and rat liver microsomes

The novel taxanes SB-T-1102, SB-T-1214 and SB-T-1216 are up to 1000-fold more cytotoxic for resistant tumour cells than clinically used paclitaxel and docetaxel, and the current study has examined the metabolism of these new taxanes in human, rat, pig and minipig liver microsomes. Metabolites were characterized by high-performance liquid chromatography (HPLC)/tandem mass spectrometry (MS/MS) analysis. Metabolic pathways derived from their structures were confirmed by investigating subsequent metabolism of purified metabolites. SB-T-1102, SB-T-1214 and SB-T-1216 were metabolized to 14, 10 and 11 products, respectively. In contrast to docetaxel, side-chain hydroxylation did not occur at their tert-butyl group, but on the isobutyl (SB-T-1102) or isobutenyl (SB-T-1214 and SB-T-1216) chains. Species differences in their metabolism were observed. For example, human and untreated rat microsomes hydroxylated SB-T-1216 preferentially at the side-chain, whereas pig and minipig microsomes preferentially metabolized more at the taxane core. The increased formation of secondary and tertiary metabolites in rat microsomes with high expression of CYP3A1/2 compared with uninduced rats confirmed the role of CYP3A in taxane metabolism. All major products were formed by human cDNA-expressed CYP3A4 and none by CYP1A2, 1B1, 2A6, 2C9 and 2E1, indicating the principal role of CYP3A orthologues in SB-T metabolism. The knowledge of metabolic pathways of the examined agents and of their rates of formation is important due to possible metabolic inactivation of these three novel drugs with a great potential for the therapy of taxane-resistant tumours. The relatively slow metabolism of SB-T-1102 could be favourable for its antitumour efficiency in vivo.

Inhibition of P-glycoprotein-mediated docetaxel efflux sensitizes ovarian cancer cells to concomitant docetaxel and SN-38 exposure

The first-line treatment of ovarian cancer is based on cytoreductive surgery and the use of anticancer drugs. The main disadvantage in the usage of anticancer drugs is the wide capacity of cancer cells to acquire a resistance to chemotherapeutic agents and therefore new treatment strategies have to be developed and tested. In this study, the responses of seven ovarian carcinoma cell lines to docetaxel and a camptothecin derivative, SN-38, were evaluated. We further studied the expression of P-glycoprotein (P-gp), the best described mechanism of drug resistance, in these cells and the effect of treatment with a specific P-gp inhibitor (PGP-4008). Simultaneous treatment with docetaxel and SN-38 (docetaxel+SN-38) had an antagonistic growth effect that was not dependent on the administration schedule. Both drugs alone or in combination induced G2M cell cycle arrest. Docetaxel was a more potent inducer of apoptosis than SN-38, but simultaneous treatment with docetaxel+SN-38 decreased the proportion of apoptotic cells to the same level observed after exposure to SN-38 alone. SN-38 increased P-gp expression in all cell lines. PGP-4008 enhanced docetaxel-mediated growth inhibition and apoptosis, but it did not have an effect when used simultaneously with SN-38. When cells were treated with docetaxel, SN-38, and PGP-4008 simultaneously, the growth was inhibited more efficiently and the proportion of apoptotic cells was higher than that without PGP-4008. Thus, treatment of ovarian cancer cells with docetaxel+SN-38 may have antagonistic effects. The simultaneous administration of a P-gp inhibitor may prevent docetaxel efflux, thereby sensitizing cells to docetaxel and other chemotherapeutic agents.

Detection and quantification of the evolution dynamics of apoptosis using the PET voltage sensor 18F-fluorobenzyl triphenyl phosphonium

Apoptosis is a key mechanism in numerous pathologies. However, there are no effective noninvasive means available for an early detection and quantitative assessment of evolution dynamics of the apoptotic process. Here, we have characterized the ability of the novel PET voltage sensor (18)F-fluorobenzyl triphenyl phosphonium ((18)F-FBnTP) to quantify the time-dependent apoptotic action of the taxanes paclitaxel and docetaxel in vitro and in vivo.

METHODS

The duration-dependent treatment effect of paclitaxel on (18)F-FBnTP uptake was assayed in human MDA-MB-231 breast carcinoma cells. The expression of the proapoptotic Bax and antiapoptotic Bcl-2 mitochondrial proteins, release of the apoptogen cytochrome c, and activation of executioner caspase-3 were determined by Western blotting. The fraction of viable cells was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. The effect of docetaxel on (18)F-FBnTP and (18)F-FDG uptake in orthotopic prostate tumors in mice was compared.

RESULTS

(18)F-FBnTP cellular uptake in viable cells declined linearly with the increasing duration of paclitaxel treatment, from 3 to 24 h, and plateaued at 48 h. The extent of decrease of (18)F-FBnTP correlated strongly with the Bax-to-Bcl-2 ratio (R(2) = 0.83) and release of cytochrome c (R(2) = 0.92), but preceded in time the caspase-3 cleavage. The P-glycoprotein blocker verapamil did not interfere with (18)F-FBnTP cellular uptake. (18)F-FBnTP prostate tumor contrast was greater than (18)F-FDG prostate tumor contrast. Docetaxel caused a marked decrease (52.4%) of (18)F-FBnTP tumor uptake, within 48 h, whereas (18)F-FDG was much less affected (12%).

CONCLUSION

The voltage sensor (18)F-FBnTP is a viable means for quantification of paclitaxel pharmacodynamics. (18)F-FBnTP permits the detection of paclitaxel apoptotic action in vivo earlier than does (18)F-FDG. (18)F-FBnTP may afford a novel approach for early detection and quantitative assessment of the cumulative-effect kinetics of proapoptotic drugs and conditions using PET.

Paclitaxel and docetaxel resistance: molecular mechanisms and development of new generation taxanes

Taxanes represent one of the most promising classes of anticancer agents. Unfortunately, their clinical success has been limited by the insurgence of cellular resistance, mainly mediated by the expression of the MDR phenotype or by microtubule alterations. However, the remarkable relevance of paclitaxel and docetaxel in clinical oncology stimulated intensive efforts in the last decade to identify new derivatives endowed with improved activities towards resistant tumor cells, resulting in a huge number of novel natural and synthetic taxanes. Among them, several structurally different derivatives were found to exhibit a promising behavior against the MDR phenotype in terms of either MDR inhibiting properties, or enhanced cytotoxicity compared to parental drugs, or both. On the other hand, only in more recent years have the first taxanes retaining activity against resistant cancer cells bearing alterations of the tubulin/microtubule system emerged. This review describes the main molecular mechanisms of resistance to paclitaxel and docetaxel identified so far, focusing on the advances achieved in the development of new taxanes potentially useful for the treatment of resistant tumors.

In vitro and in vivo synergy of MCP compounds with mitogen-activated protein kinase pathway- and microtubule-targeting inhibitors

An important clinical task is to coherently integrate the use of protein-targeted drugs into preexisting therapeutic regimens, with the goal of improving treatment efficacy. Constitutive activation of Ras-dependent signaling is important in many tumors, and agents that inhibit this pathway might be useful in numerous therapeutic combinations. The MCP compounds were identified as inhibitors of Ras-Raf interactions and previously shown to inhibit multiple Ras-dependent transformation phenotypes when used as monoagents in cell culture analyses. In this study, we investigate the ability of the MCP110 compound to synergistically enhance the activity of other therapeutic agents. In both a defined K-Ras-transformed fibroblast model and in human tumor cell lines with mutationally activated Ras, MCP110 selectively synergizes with other agents targeting the mitogen-activated protein kinase pathway, and with multiple agents (paclitaxel, docetaxel, and vincristine) targeting the microtubule network. The synergistic activity of MCP110 and paclitaxel was further established by experiments showing that in Kaposi's sarcoma oncogenically transformed cell lines, cellular models for tumors treated with taxanes in the clinic and in which Raf-dependent signaling plays an important role, MCP110 synergizes with paclitaxel and limit growth. Finally, in vivo testing indicate that MCP110 is bioavailable, inhibits the growth of LXFA 629 lung and SW620 colon carcinoma cells in xenograft models, and again strongly synergizes with paclitaxel. Together, these findings indicate that MCP compounds have potential to be effective in combination with other anticancer agents.

Paclitaxel C-10 carbamates: potential candidates for the treatment of neurodegenerative tauopathies

A series of paclitaxel C-10 carbamates was synthesized and evaluated in a bi-directional permeability assay in comparison with paclitaxel and the blood-brain barrier-permeable C-10 ester derivative, TX-67. A number of the carbamates were found not to be substrates for Pgp. Moreover, when tested for Pgp-inhibitory potential, representative compounds proved to be devoid of Pgp interactions. Side-by-side comparison between TX-67 and the corresponding C-10 carbamate, CNDR-3, revealed a significantly longer half-life for CNDR-3 in both mouse and human plasma, suggesting that this class of derivatives is appropriate for further in vivo evaluation.

Metabolic enzyme considerations in cancer therapy

The clinical application of new antineoplastic drugs has been limited because of low therapeutic index and lack of efficacy in humans. Thus, improvement in efficacy of old and new anticancer drugs has been attempted by manipulating their pharmacokinetic properties. Four inter-related factors, which determine the pharmacokinetic behavior of a drug include absorption, distribution, metabolism and excretion. The drug-metabolizing enzymes have been classified in two major groups: phase I and phase II enzymes. Phase I enzymes comprise the oxidases, dehydrogenases, deaminases, hydrolases. Phase II enzymes include primarily UDP-glucuronosyltransferases (UGTs), glutathionetransferases (GSTs), sulfotransferases (SULTs), N-acetyl transferases (NATs), methyltransferases and aminoacid transferases that conjugate products of phase I reactions and parent compounds with appropriate functional groups to generate more water soluble compounds which are more readily eliminated. The importance of these enzymes in the metabolism of specific drugs varies according to the chemical nature of the drug, Drug metabolism is modulated by factors that change among species and even among individuals in a population. Such factors can be environmental or genetic in origin, and influence how a drug is metabolized and to what extent. An awareness of these variables is invaluable when the safety and efficacy of new anticancer drugs are evaluated (1).

Transport and cytotoxicity of paclitaxel, docetaxel, and novel taxanes in human breast cancer cells

The resistance of tumors to classic taxanes (paclitaxel and docetaxel) presents problems in chemotherapy. Thus, new taxanes with higher antitumor activity in resistant tumors are synthesized. This study compared cytotoxicity and transport of paclitaxel and docetaxel with novel taxanes SB-T-1103, SB-T-1214, and SB-T-1216 in adriamycin-sensitive (MDA-MB-435) and -resistant (NCI/ADR-RES) human breast cancer cells. The cell lines examined differ in adriamycin transport, suggesting different expression of ABC membrane transporters. Reverse transcription-polymerase chain reaction revealed that NCI/ADR-RES cells expressed high levels of P-glycoprotein mRNA, which was absent in MDA-MB-435 cells, while the opposite was true for MRP2 mRNA. Both cell lines shared or differently expressed eight other ABC transporters and LRP. NCI/ADR-RES cells were 1,000-fold more resistant to paclitaxel and 600-fold more resistant to docetaxel in MTT assay than MDA-MB-435 cells, but almost equally sensitive to SB-T-1103, SB-T-1214, and SB-T-1216. This complied with the fact that NCI/ADR-RES cells absorbed almost 20-fold less [14C]paclitaxel, about 7-fold less docetaxel, and almost equal amounts of SB-T-1103, SB-T-1214, and SB-T-1216 as the MDA-MB-435 cells. Verapamil increased uptake of [14C]paclitaxel by NCI/ADR-RES cells 7-fold and decreased its efflux 2.5-fold; in contrast, it weakly influenced uptake and increased the efflux in MDA-MB-435 cells. SB-T-1103 and SB-T-1216 did not influence transport of paclitaxel, but SB-T-1214 decreased [14C]paclitaxel uptake in both cell lines indicating inhibition of uptake. This suggests that the novel taxanes are not inhibitors of P-glycoprotein. However, novel taxanes exert much higher activity on resistant tumor cells than classic taxanes and seem to be potential drugs for therapy in taxane-resistant tumors.

The discovery of BMS-275183: an orally efficacious novel taxane

The evolution of 2, a C-4-methylcarbonate analogue of paclitaxel with minimal oral bioavailability and oral efficacy, into its C-3'-t-butyl-3'-N-t-butyloxycarbonyl analogue (15i), a novel taxane with oral efficacy in preclinical models that is comparable to iv administered paclitaxel, is described.

Novel plant triterpenoid drug amooranin overcomes multidrug resistance in human leukemia and colon carcinoma cell lines

Amooranin (AMR), a plant terpenoid, isolated from Amoora rohituka, was investigated for its ability to overcome multidrug resistance in human leukemia and colon carcinoma cell lines. AMR IC(50) values of multidrug-resistant leukemia (CEM/VLB) and colon carcinoma (SW620/Ad-300) cell lines were higher (1.9- and 6-fold) than parental sensitive cell lines (CEM and SW620). AMR induced G(2)+M phase-arrest during cell cycle traverse in leukemia and colon carcinoma cell lines and the percentage of cells in G(2)+M phase increased in a dose-dependent manner. Coincubation of tumor cells with both DOX and AMR reversed DOX resistance in 104-fold DOX-resistant CEM/VLB and 111-fold DOX-resistant SW620/Ad-300 cell lines with a dose modification factor of 50.9 and 99.6, respectively. Flow cytometric assay showed that AMR causes enhanced cellular DOX accumulation in a dose-dependent manner. AMR inhibits photolabeling of P-glycoprotein (P-gp) with [(3)H]-azidopine and the blocking effect enhanced with increasing concentrations of AMR. Our results show that AMR competitively inhibits P-gp-mediated DOX efflux, suggestive of a mechanism underlying the enhanced DOX accumulation and reversal of multidrug resistance by AMR.

Nonlinear accumulation in the brain of the new taxoid TXD258 following saturation of P-glycoprotein at the blood-brain barrier in mice and rats

1. TXD258, a new taxoid antitumor agent, is a poor substrate for the P-glycoprotein (P-gp) in Caco-2 cells. In this study, we investigated the amount of drug accumulating in the brains of rats and mice under a variety of conditions (dose and infusion time, species and plasma concentration) using conventional in vivo pharmacokinetic techniques and in situ brain perfusion. 2. Mice were infused with radiolabeled TXD258 at 15, 30, 45 and 90 mg m(-2) for 45 s or 1 h and rats were infused with 15 and 60 mg m(-2) over 2.3 min. The radioactivity in the plasma and brains was measured. The brain concentrations of TXD258 in mice and rats were maximal from 2 min to 1 h postinfusion and radioactivity was still detectable at 168 h. While the plasma concentration of TXD258 increased linearly in mice with the infused dose, the brain content increased more than proportionally with the dose between 15 and 90 mg m(-2). This nonlinear uptake of TXD258 also occurred in the plasma and brain of the rat. 3. These findings suggest that the protein-mediated efflux across the blood-brain barrier (BBB) becomes saturated. In situ brain perfusion studies confirmed that TXD258 is a P-gp substrate at the BBB of mice and rats. The P-gp of both species was saturated at the half-inhibitory concentration ( approximately 13 micro M) produced by i.v. infusion. 4 Thus, the observed nonlinear accumulation of TXD258 in the brain seems to occur by saturation of the P-gp at the rodent BBB. This saturation could have several advantages, such as overcoming a P-gp-mediated efflux, but the nonlinear pharmacokinetics could increase the risk of toxicity.

Subcellular daunorubicin distribution and its relation to multidrug resistance phenotype in drug-resistant cell line SMMC-7721/R

AIM: To investigate the correlation between subcellular daunorubicin distribution and the multidrug resistance phenotype in drug-resistant cell line SMMC-7721/R.

METHODS: The multidrug resistant cell line SMMC-7721/R, a human hepatocellular carcinoma cell line, was established. Antisense oligonucleotides (AS-ODN) were used to obtain different multidrug resistance phenotypes by inhibiting the expression of mdr1 gene and/or multidrug resistance-related protein gene (mrp) using Lipofectamine as delivery agent. Expression of mdr1 and mrp genes was evaluated by RT-PCR and Western blotting. Intracellular daunorubicin (DNR) concentration was measured by flow cytometry. Subcellular DNR distribution was analyzed by confocal laser scanning microscopy. Adriamycin (ADM) and DNR sensitivity was examined by MTT method.

RESULTS: Low level expression of mdr1 and mrp mRNAs and no expression of P-Glycoprotein (P-gp) and multidrug resistance-related protein (P₁₉₀) were detected in parental sensitive cells SMMC-7721/S, but over-expression of these two genes was observed in drug-resistant cell SMMC-7721/R. The expression of mdr1 and mrp genes in SMMC-7721/R cells was down-regulated to the level in the SMMC-7721/S cells by AS-ODN. Intracellular DNR concentration in SMMC-7721/S cells was 10 times higher than that in SMMC-7721/R cells. In SMMC7721/S cells intracellular DNR distributed evenly in the nucleus and cytoplasm, while in SMMC-7721/R cells DNR distributed in a punctate pattern in the cytoplasm and was reduced in the nucleus. DNR concentration in SMMC-7721/R cells co-transfected with AS-ODNs targeting to mdr1 and mrp mRNAs recovered to 25 percent of that in SMMC7721/S cells. Intracellular DNR distribution pattern in drug-resistant cells treated by AS-ODN was similar to drug-sensitive cell, and the cells resistance index (RI) to DNR and ADM decreased at most from 88.0 and 116.0 to 4.0 and 2.3, respectively. Co-Transfection of two AS-ODNs showed a stronger synergistic effect than separate transfection.

CONCLUSIONS: P-gp and P₁₉₀ are two members mediating MDR in cell line SMMC7721/R. Intracellular drug concentration increase and subcellular distribution change are two important factors in multidrug resistance (MDR) formation. The second factor, drugs transport by P-gp and P₁₉₀ from cell nucleus to organell in cytoplasm, may play a more important role.

Chemistry and chemical biology of taxane anticancer agents

Taxol (paclitaxel) and Taxotere (docetaxel) are currently considered to be among the most important anticancer drugs in cancer chemotherapy. The anticancer activity of these drugs is ascribed to their unique mechanism of action, i.e., causing mitotic arrest in cancer cells, leading to apoptosis through inhibition of the depolymerization of microtubules. Although both paclitaxel and docetaxel possess potent antitumor activity, treatment with these drugs often results in a number of undesirable side effects, as well as multidrug resistance (MDR). Therefore, it has become essential to develop new anticancer agents with superior pharmacological properties, improved activity against various classes of tumors, and fewer side effects. This paper describes an account of our research on the chemistry of paclitaxel and taxoid anticancer agents at the biomedical interface, including: 1. The structure-activity relationship (SAR) study of taxoids leading to the development of the "second-generation" taxoids, which possess exceptional activity against drug-resistant cancer cells expressing the MDR phenotype. 2. Development of fluorinated taxoids to study the bioactive conformation of paclitaxel and photoaffinity labeling taxoids for mapping of the drug-binding domain on both microtubules and P-glycoprotein. 3. The synthesis of novel macrocyclic taxoids for the investigation into the common pharmacophore for microtubule stabilizing anticancer agents.

Effects of novel taxanes SB-T-1213 and IDN5109 on tubulin polymerization and mitosis

SB-T-1213 and IDN5109 are semisynthetic, orally available taxanes that are up to 400-fold more active than paclitaxel against drug-resistant cells. IDN5109 is in clinical trials. We investigated the primary target for SB-T-1213 and IDN5109 and whether the compounds interact with microtubules differently than paclitaxel. Unlike paclitaxel, at 1-10 microM both novel taxanes initiate microtubule polymerization in vitro with no lag. They enhance polymerization equally or more potently than paclitaxel. SB-T-1213 induces unusual microtubules with attached extra protofilaments or open sheets, and IDN5109 induces large protofilamentous sheets. Both inhibit HeLa cell proliferation, block mitosis at the metaphase/anaphase transition, bundle microtubules at high drug concentrations, and induce abnormal metaphase spindles and apoptosis. They target microtubules but alter their polymerization and structure differently than paclitaxel. These differences may play a role in their enhanced cytotoxicity and efficacy.

Combination chemotherapy of the taxanes and antimetabolites: its use and limitations

In an effort to improve response rates of chemotherapy, taxanes have been combined with other cytotoxic agents such as antimetabolites. However, the use of some of these combinations in patients has been restricted by severe toxicity. The significance of the sequence of drug administration in combining methotrexate (MTX) and taxanes was recognised in in vitro studies, showing synergistic effects for the sequence of MTX followed by paclitaxel, and antagonism for exposure in the reverse order. A possible explanation might be an MTX-induced synchronisation of cells in the S phase of the cell cycle, after which cells are more susceptible for the cytotoxic action of taxanes. Clinical studies using this sequence were hampered by severe neutropenia and mucositis at relatively low doses of both drugs. As no pharmacokinetic interactions were observed, the excess of toxicity may have been due to sequence-dependent synergistic actions on bone marrow and mucosa. In contrast, and confusingly, in vitro studies on 5-fluorouracil (5-FU) and taxanes indicate that 5-FU preceeding or simultaneously given to paclitaxel impairs cytotoxicity as compared with paclitaxel monotherapy, while the reverse sequence results in additive or synergistic cytotoxicity. While almost all clinical studies have used the sequence of a taxane followed by 5-FU, various schedules appeared feasible and effective. The combination of a 5-FU analogue, capecitabine and taxanes was supported by in vitro data. A large phase III trial confirmed the feasibility and superior efficacy of this combination in breast cancer patients relapsing after an anthracycline. Conflicting results exist on the benefit of combining gemcitabine and taxanes in tumour cell lines. Although the accumulation of gemcitabine triphosphate (dFdCTP) in mononuclear cells was significantly higher with an increasing dose of paclitaxel, no pharmacokinetic interactions for both agents were noticed. A pharmacokinetic analysis of the gemcitabine-docetaxel combination therapy has not been published in detail. Despite numerous trials, so far no optimum schedule has been established. Regarding data on actually delivered dose intensities, a 2- or 3-weekly cycle seems favourable and feasible. However, possible severe pulmonary toxicity warrants cautious monitoring of patients treated with this combination. Different outcomes of preclinical and clinical studies reveal that combining two chemotherapeutic agents is not simply a matter of putting antitumour activities together. Drug interaction may result in synergism, not only of efficacy but also of toxic side-effects. Adding two drugs may also implicate antagonism in drug efficacy due to unwanted interference in cytotoxicity or pharmacokinetics. For agents acting at a specific phase of the cell cycle, the sequence of administration may determine the efficacy and toxicity of a combination therapy. Because of an observed discrepancy between in vitro data and clinical studies, we would like to emphasise the need for adequate dose-finding clinical trials together with pharmacokinetic data analysis before examining any new combination chemotherapy in more detail in phase II studies.