Novel microtubule-targeting agents – the epothilones

History

Drugs that target tubulin are active in human malignant disease and are an essential component of medical treatment of these diseases (Budman 2005, Pellegrini and Budman 2005). Of this class of agents, a variety of active drug structures have been identified, with the vinca alkaloids, the taxanes, and the epothilones achieving regulatory approval for cancer treatment. As a result, pharmaceutical research on compounds that interfere with tubulin function has concentrated on agents which might have enhanced efficacy or reduced toxicity. A current Medline search for “tubulin binding drugs” yielded 430 references. An analogous search of the National Institutes of Health Clinical Trials Registry, http://clinicaltrials.gov, using the phrase “epothilone” lists 108 clinical studies in patients with malignant disease.

The epothilones were originally discovered in 1987 from the fermentation of soil bacteria found on the banks of the Zambesi River in Africa (Gerth et al 1996; Lee et al 2008). A myxobacterium, Sorangium cellulosum, was noted to produce biologically active substances. One chemical isolate, epothilone A, was noted to be highly cytotoxic (inhibitory concentration at 50% activity, IC₅₀ = 0.05 μM) in vitro when applied to the human T-24 bladder carcinoma cell line (Gerth et al 1996).

Natural products are a well recognized source of novel agents for anticancer development and such drugs are usually selected on the basis of functional screening (Shah and Kaye 2003). Epothilone, a macrolide compound, was selected for further development on the basis of demonstrated paclitaxel-like properties during preclinical screening of compounds binding to tubulin. Bollag et al (1995) used a novel microtubule filtration assay to rapidly screen large numbers of compounds to select agents which induced tubulin polymerization. Electron microscopy differentiated true polymerization induced by the screened compounds from nonspecific protein aggregation. Of 7000 test samples screened, 6% of plant extracts and 0.5% of other sources (marine, insect, microbial) screened positive, one of them being epothilone. Although other natural products with similar in vitro taxane-like activity were isolated, epothilone’s successful development can be credited to its easy synthesis in bulk from fermentation and its structure being amenable to modification (Bollag et al 1995). The original structures were epothilone A and B. Epothilones derived from fermentation are classified as either epoxides on the presence of an epoxide structure at C-12–C-13 (epothilones A, B, E, F) or olefins (epothilone C, D) (Fumoleau et al 2007). Semi-synthetic derivatives have subsequently been developed to enhance pharmacologic properties. Epothilones demonstrated greater cytotoxicity than paclitaxel with an IC₅₀ in the low nanomolar range in a variety of tumor cell lines (Bollag et al 1995; Chou et al 1998; Lee et al 2001; Fumoleau et al 2007). The six agents that have reached clinical trials in man are discussed below (Table 1).

Chemistry

The National Institutes of Health maintains a web site, PubChem (http://pubchem.ncbi.nlm.nih.gov) which is invaluable for information on chemical, preclinical activity, and toxicology of drugs. Twelve epothilones are listed on this web site. Epothilones are structurally less complex and have a ring structure unrelated to that of paclitaxel. The critical structure is the macrolide ring with a thiazole-containing side chain and a ketone (Figure 1). The only structural difference between epothilone A and B is that epothilone B (patupilone) has a methyl group at position C12 (Figure 1). Patupilone is more hydrophilic than epothilone A (XlogP, the partition distribution coefficient between water and octanol, is 2.1 vs 1.7 respectively). This small modification increases microtubule stabilization such that patupilone is twice as potent as epothilone A (Goodin et al 2004; Cortes and Baselga 2007). Epothilone A has poor pharmacologic properties with loss of structural stability in animal plasma which led to the agent being dropped from further studies (Lee et al 2008).

Structural modifications have also been made to improve drug solubility and in some cases to negate the need for solubilization in vehicles that may cause hypersensitivity reactions. Ixabepilone is a second-generation patupilone derivative with an azide group in place of oxygen on position 16 of the macrolide ring (Figure 1). Ixabepilone is more resistant to degradation by carboxylesterase than the parent compound (Goodin et al 2004). BMS-310705 is a water-soluble derivative of patupilone with a substitution of the hydroxyl group with an amino group in the C21 position of the methythiazole side chain. An analog of patupilone, ABJ879 (2–desmethyl-20-methylsulfanyl-Epo-B), is highly active in a taxane-resistant cervical cancer model and did reach phase I studies. No more information is available to date (Altmann and Memmert 2008). Modifications at specific positions on the macrolide ring enhance the in vitro activity. Sagopilone (ZK-EPO, ZK-219447) is the only fully synthetic third generation analog of patupilone, assembled from three building blocks (Alexander et al 2008). This drug was selected from a variety of analogs for further development because of its high potency in preclinical tumor models and its solubility in aqueous solutions (Klar et al 2006).

Desoxyepothilone B (KOS-862, epothilone D) lacks the C12–13 epoxide and is more potent than epothilone A in preclinical models utilizing ovarian cell lines that are resistant to taxanes (Nicolaou et al 1997; Goodin et al 2004). Hence, the epoxide ring is not a requirement for anticancer effect (Altmann 2005). A second generation epothilone D analog, KOS 1584, has also been shown to have greater in vitro potency than patupilone or epothilone D (Fumoleau et al 2007).

Structure/function activity

Analogous to paclitaxel, epothilones bind to a common binding site on β-tubulin. A preclinical study in resistant cell lines also noted that both taxanes and epothilones share a common tertiary structure for binding to tubulin even though their chemical structures are distinct (Giannakakou et al 2000). The binding affinity of epothilone A to tubulin is of the same order of magnitude as the binding affinity of paclitaxel to tubulin based on competition assays. The 50% inhibitory concentration for displacement of 100 nM of (³H) paclitaxel from the tubulin binding site was 3.6 μM for paclitaxel, 2.3 μM for epothilone A, and 3.3 μM for patupilone (Nettles et al 2004). These studies indicate that taxanes and epothilones bind at or near the same site (Kowalski et al 1997).

However, other studies have suggested that the interactions by the pharmacophore of each agent within the binding pocket are not identical (Nettles et al 2004). These differences may reflect the differences in experimental conditions used to elucidate drug function (Reese et al 2006). In vitro studies of epothilone A bound to α,β tubulin in zinc-stabilized sheets demonstrate that epothilone A and paclitaxel share only one polar contact at C-7-OH while the thiazole side chain of epothilone binds to a different region of β-tubulin not occupied by paclitaxel (Nettles et al 2004; Lee and Swain 2008). In addition, the methyl group at C12 seems to enhance a hydrophobic interaction between patupilone and β tubulin that allows patupilone to be more potent than epothilone A (Kowalski et al 1997).

Nuclear magnetic resonance spectroscopy, electron crystallography, and molecular modeling have revealed further binding interactions of the epothilones with tubulin. The 16 member ring and part of the side chain reside within a hydrophobic region of β-tubulin. In the case of epothilone A, hydrogen bonds bind C1=O, C3-OH, C5=O, and C7-OH to tubulin at threonine 274, arginine 278, and arginine 282 residues of the M-loop. Hydrophobic interactions occur at the C3 to C11 portion of epothilone A (Nettles et al 2004). As previously noted (Altmann 2005), although initial studies indicated that the C12-13 epoxide was required for localizing the epothilone onto its tubulin target, more recent findings demonstrate that this moiety is not necessary as epothilone D is highly active as a tubulin binder (Nicolaou et al 1997).

Preclinical functional activity

Microtubules are present in all eukaryotes and are essential for the intracellular structure, cell division, and intracellular transport (Esteve et al 2007). These processes are regulated by polymerization and disassembly of the microtubule polymers. Disruption of these processes targets the cell for G2/M phase arrest resulting in apoptosis (Bhat and Setaluri 2007). Epothilones promote tubulin polymerization in vitro in the absence of microtubule associated proteins or guanosine triphosphate. These agents also prevent depolymerization of the microtubulin structure in the presence of calcium (Bollag et al 1995; Altmann et al 2004). Epothilones can also induce apoptosis at concentrations that do not cause mitotic cell cycle arrest. This cytotoxic effect is believed to be caused by disruption in the dynamic equilibrium between the intracellular pool of basic tubulin components (α, β heterodimers) and the microtubule polymer (Chen and Horwitz 2002; Goodin et al 2004).

Using human MDA-MB-435 cells and fluorescence microscopy, investigators have noted a biologic difference in the spindle pole effects of epothilone A and of patupilone (Sakaushi et al 2008). These results suggest that small changes in the epothilone structure may have major differences in biological effect. Patupilone has recently been noted to cause mitochrondrial collapse with release of reactive oxygen species in sensitive cells thus leading to apoptosis (Khawaja et al 2008). Hence, these drugs have potent apoptotic promoting effects.

In vitro data demonstrate that patupilone at a concentration of 1 nmol/L has antiangiogenic properties when assayed using a human umbilical vein endothelial cell culture (Bijman et al 2006). The antiangiogenic activity has been related to interruption of microtubule dependent control of HIF-1α (hypoxia inducible factor 1α) translation (Escuin et al 2005). This anti-angiogenesis data suggests that these substances could potentially kill malignant cells at subtoxic concentrations in vivo by an alternative mechanism.

Attractive features of epothilones include their potency relative to taxanes and their activity in both taxane sensitive and in resistant cell lines (Kowalski et al 1997; Nicolaou et al 1997; Chou et al 1998; Goodin et al 2004; Mok et al 2006; Lee and Swain 2008) (Table 2). Epothilones have shown activity in ovarian, breast, lung, colon, prostate, squamous cell, fibroblast, and leukemia cell lines (Goodin et al 2004). Ixabepilone demonstrated significant activity against the pediatric tumors – brain tumors, neuroblastoma, osteosarcoma, rhabdomyosarcoma, and Wilm’s tumor – when grown in a murine model (Peterson et al 2005).

Patupilone has 3- to 20-fold higher in vitro cytotoxic potency compared with paclitaxel. In hepatocellular carcinoma cell lines, patupilone was 4 to 19 times more potent than docetaxel or paclitaxel (Table 2) (Mok et al 2006). Ixabepilone, a patupilone analog, has been shown to be 2.5 times more potent than paclitaxel in some cell lines (Table 2). In addition, ixabepilone has enhanced metabolic stability compared to the natural compounds (Lee et al 2001). Sagopilone, a synthetic patupilone analogue, acts in a taxane-like manner but is more cytotoxic than other epothilone derivatives in development (Hoffmann et al 2008). This analog demonstrates activity in taxane-resistant cell lines (Hoffmann et al 2008). Recent in vitro studies have noted that sagopilone is rapidly taken up into A549 epidermoid nonsmall cell lung cancer cells with localization of drug into the cytoskeleton and accelerated tubulin polymerization compared with paclitaxel or patupilone (Hoffmann et al 2008).

P-glycoprotein (also known as MDR) is an active drug efflux pump that results in cellular resistance to many cytotoxic agents. Epothilones are poor substrates for P-glycoprotein and are cytotoxic in cell lines that express high levels of P-glycoprotein (Lehne 2000). The epothilones can demonstrate an order of magnitude more cytotoxic effect at a given dose compared to paclitaxel (Table 2) (Chou et al 1998). As recent genomic data has determined that P-glycoprotein is over expressed in clinical samples of human Her2/neu+ breast cancer, these agents may be of particular value in this subset of patients (O'Brien et al 2008). The different epothilones demonstrate varying susceptibility to P-glycoprotein mediated resistance. ZK-EPO is not a substrate for this pump resistance protein (Stupp et al 2008). This drug is also highly active in cell lines expressing P-glycoprotein efflux pumps (Hoffmann et al 2008). Epothilone D is highly active in both taxane sensitive and taxane resistant cell lines (Lee and Swain 2008), while ixabepilone (BMS-247550) appears to be the most affected by P-glycoprotein expression but less so than paclitaxel (Table 2). Ixabepilone is 108 to 529 times more potent than taxanes or doxorubicin in the P-glycoprotein expressing hepatocellular carcinoma cell line SNU-449 (Mok et al 2006). While in vitro studies show patupilone to be potent, animal xenograft models have shown increased toxicity and a narrower therapeutic index of B compared with epothilone D, suggesting a disassociation between the findings in cell lines and data from animal models (Chou et al 1998).

Through molecular modeling, microtubulin stabilizing agents have been found to have different affinities to β-tubulin isoforms. Paclitaxel has more affinity to βI than to the βIII isoform (Magnani et al 2006). Increased βIII isoform expression is one mechanism of increased resistance to paclitaxel (Magnani et al 2006; Seve et al 2005, 2008). In contrast, epothilone A has the same affinity for both isoforms (Magnani et al 2006).

Tubulin mutations are one cause of drug resistance in cultured cell lines as the target of these drugs is altered. Single point mutations in β-tubulin near the M loop at the nucleotide binding site and at the COOH terminus have been noted in some cancer cell lines. These sites are critical for the stabilization of microtubules. Point mutations in β-tubulin that confer resistance to epothilones are also associated with resistance to paclitaxel (Goodin et al 2004). One clinical study of 49 patients with nonsmall cell lung cancer suggested that paclitaxel resistance is associated with β-tubulin mutations (Monzo et al 1999). The significance of this finding is unclear, as tubulin mutations seem to be a rare occurrence in most human tumors (Berrieman et al 2004). In addition, tubulin mutations detected in non-small cell lung cancer samples isolated from patients were determined to be due to contamination of the genomic assay by amplification of pseudogenes which share homology. These resistance mutations therefore were not present in clinical specimens (Noguchi 2006).

Tubulin binding agents can activate the steroid and xenobiotic receptor, also known as the human pregnane X receptor. Pregnane X receptor is known to be a master switch in drug resistance. Activation of this pathway leads to enhanced expression of cytochrome CYP3A4 in both tumor cell lines and in mice which enhances metabolic degradation of drugs thus resulting in drug resistance (Mani et al 2005). This effect has been seen with epothilones as well as with taxanes (Mani et al 2005).

Pharmacokinetics

Phase I data have evaluated pharmacokinetic parameters using various dosing schedules (Table 3). The C_pmax (maximal concentration of drug in the plasma) of these agents reaches the microgram per milliliter level in some of the schedules. A high volume of distribution is also commonly seen, suggesting high tissue binding. The half-lives of the various analogs varies from 7 to 89 hours and seems to differ depending upon the compound studied (Table 3). Apart from differences in the drug structures, the pharmacokinetic and pharmacodynamic effects in man may be partly attributed to the solvent required for clinical administration of drug. Some of these agents are poorly soluble in aqueous solutions and therefore need a co-solvent (Table 1) which may perturb drug distribution and elimination.

BMS-310705 is a water soluble patupilone derivative and thus has the advantage of not requiring premedications to prevent reactions to solvent (Table 1) (Sessa et al 2007). Sagopilone, is also water soluble (Alexander et al 2008). Other formulations, such as ixabepilone and epothilone D, are formulated in polyoxyethylated castor oil and require prophylactic histamine-1 and histamine-2 blockers in order to avert hypersensitivity reactions induced by the solvents (Table 1). These adverse effects were previously well described for paclitaxel (Markman 2003).

Both patupilone and ixabepilone are metabolized by the liver. Excretion studies of radiolabeled ixabepilone found 52% of label excreted in the feces and 25% in the urine (Beumer et al 2007). Therefore liver dysfunction may affect clearance of the active drug. There are interpatient variabilities in drug disposition, especially with ixabepilone which is a substrate of cytochrome CYP3A4 (Goel et al 2008). Incubation of ixabepilone in human liver microsomes with different cytochrome inhibitors demonstrated that metabolism of ixabepilone was inhibited by 90% with a potent CYP3A4 inhibitor (ketoconazole). In humans, co-administration of ketoconazole with ixabepilone increased plasma AUC_0–∞ (total drug exposure in the plasma) by 79% with a small increase in C_pmax. In this dose ranging study, the maximum tolerated dose was decreased from 30 mg/m² in patients with normal hepatic function to 25 mg/m² in the presence of ketoconazole with dose-limiting toxicities of fatigue, neutropenia, mucositis, diarrhea, and febrile neutropenia (Goel et al 2008). In vitro studies indicate that patupilone is a weak inhibitor of CYP2C9, but a phase I study with warfarin did not show significant drug-drug interaction (Takimoto et al 2008).

Pharmacodynamics

At present, there is no way to predict the effectiveness of these drugs in clinical medicine. Surrogate markers of efficacy and toxicity are needed. There are ongoing studies to correlate clinical response with microtubule bundle formation, tumor expression of microtubule stabilizing proteins, and post-translational changes in α tubulin (Pusztai 2007). Since epothilones have effects at concentrations lower than needed for microtubule bundle formation, the presence of microtubule formation may not serve as an effective surrogate marker.

In humans, microtubule bundle formation in peripheral blood mononuclear cells increased 1 hour after ixabepilone administration. Of note, there were different time courses of microtubule bundle formation in peripheral blood mono-nuclear cells compared with tumor cells, suggesting differential uptake and clearance (Mani et al 2004; Mani et al 2007; Lee and Swain 2008). Preliminary data did not indicate any change in efficacy in a mouse model with βIII isoform expression (Pusztai 2007). Increased expression of the βIII isoform has previously been implicated as a mechanism of paclitaxel resistance (Kamath et al 2005; Pusztai 2007).

While the ability to measure post translational changes (detyrosination and acetylation) to α tubulin has been correlated with microtubule stabilization and has increased after ixabepilone administration, this measurement does not correlate with clinical effectiveness of the drug (Zhuang et al 2007). The investigators have suggested several potential explanations: 1) microtubule engagement is insufficient to cause cell death, 2) death pathways must be activated, 3) the timing of post-treatment biopsies, or 4) threshold level of tubulin stabilization had not been reached. Tau protein expression, which is directly associated with estrogen expression, competes with tubulin-stabilizing agents for the same binding site and may confer resistance (Pusztai 2007).

Human phase I studies

Phase I data are available for: patupilone and its analogs (BMS-310705, sagopilone, ixabepilone), and epothilone D and its derivative (KOS-1584) (Table 4). The maximum tolerated dosages of the agents varies with the structure as do the toxicities. A variety of dosing schedules has been reported. Both dose and schedule are important in the generation of toxicities seen in man with all the epothilones studied to date.

Various schedules for the epothilones have been studied in order to modify adverse effects with varying degrees of success. The most distressing adverse effect is neurotoxicity, which occurs with the epothilone B analogs, but is not dose limiting with the epothilone D analogs (Table 4) (Fumoleau et al 2007). This problem is often dose limiting with taxane treatment (Markman 2003). Most patients in the published epothilone trials were previously treated with a taxane or platinum agent and thus may have had pre-existing clinical or subclinical neuropathy. Past studies have indicated that C_pmax of drugs may correlate with toxicities such as neuro-toxicity or cardiotoxicity (Abraham et al 2003). Therefore, schedules that distribute the drug over time, such as a weekly or daily regimens, or prolonged infusion times have been investigated (Awada et al 2001; Burris et al 2002; Hao et al 2002; Abraham et al 2003; Spriggs et al 2003; Rubin et al 2005; Shimizu et al 2008).

Phase II data

The available phase II data are summarized by tumor type in Table 5. The largest number of trials reported involved the administration of ixabepilone either as a single agent or in combination with other established agents.

Ixabepilone has been widely studied for metastatic breast cancer with or without prior taxane exposure using a variety of schedules (Table 5). The response rates for patients without prior taxane exposure range from 41.5% to 57% (Denduluri et al 2007; Roche et al 2007). The response rates were lower for relapsed or refractory patients who had failed to respond to taxane treatment (11.5% to 22%) (Low et al 2005; Perez et al 2007; Thomas et al 2007a).

Response rates increased in the taxane resistant patients with the addition of capecitabine to ixabepilone, or the addition of trastuzumab with ixabepilone for Her2+ breast cancer patients (Moulder et al 2007; Thomas et al 2007b). These studies suggest that this agent can be combined with known effective drugs commonly used with taxane therapy.

One single arm phase II trial, initially presented in abstract form (Perez et al 2007), has led to US Food and Drug Administration (FDA) approval for commercial use in heavily treated breast cancer patients who had failed to respond to anthracyclines, taxanes, and capecitabine (Lechleider et al 2008). In the updated report, 126 patients from 36 centers with prior drug resistance, and at least one evaluable lesion by imaging, Karnofsky performance status ≥ 70, and normal organ function were treated with ixabepilone at 40 mg/m² administered as an intravenous infusion once every 3 weeks. Independent review committee response evaluation of the 113 study patients determined a 12.4% partial response rate with a median time to response of 6.1 weeks and an average duration of response of 6 months (5 to 7.6 months, 95% confidence limits) (Lechleider et al 2008).

One ongoing study is evaluating epothilone B in the setting of metastatic breast cancer patients with brain metastases, progressing after whole brain radiation therapy, with 1 out of 12 patients demonstrating a partial response (Conlin et al 2008). The grade 3–4 toxicities reported were mainly gastrointestinal.

Patupilone has been evaluated in patients with metastatic carcinoid and related neuroendocrine tumors (Table 5). Preliminary data demonstrate disease stabilization with grade 3–4 gastrointestinal side effects (Anthony et al 2003).

Advanced colon cancer patients refractory to at least one prior chemotherapy containing 5-fluorouracil, irinotecan, and leucovorin (IFL) did not respond to ixabepilone (Eng et al 2004). Patupilone treatment in 47 patients with colon cancer on a once weekly for three weeks schedule every 28 days noted only 1 partial response but was associated with significant toxicity (Table 5) (Poplin et al 2003).

A study of metastatic gastric cancer patients treated with ixabepilone at 50 mg/m² administered once every 21 days resulted in a 9% response rate (2 out of 23 patients) (Ajani et al 2006). No response was seen in patients who received ixabepilone 6 mg/m² for 5 days, possibly related to low plasma drug concentrations with this divided dosing schedule. Grade 3 to 4 neuropathies in the 50 mg/m² cohort and 6 mg/m² cohort were statistically different (8% vs 4%) (Ajani et al 2006). In another study of locally advanced or metastatic gastric cancer patients, patupilone demonstrated a partial response rate of 9% of patients and a combined response and disease stabilization rate of 36% (Hsin et al 2006).

Ixabepilone was evaluated in hepatobiliary cancer and while there were partial responses and disease stabilization, the magnitude of the anticancer effects was deemed to be not clinically significant (Singh et al 2006).

Ixabepilone has been a disappointment in metastatic germ cell tumors refractory to cisplatin therapy. Only 12 patients were entered into the study with 1 partial response in a taxane naive patient. The time to progression was 2 weeks, and the study was terminated (Table 5) (Feldman et al 2007).

Sagopilone was believed to be a good candidate for evaluation in human brain tumors based upon preclinical data and absence of perturbation of in vitro efficacy by the P-glycoprotein resistance protein (Alexander et al 2008; Hoffmann et al 2008). However, in phase II trials for recurrent malignant gliomas and recurrent glioblastomas, this agent demonstrated disease stability of only 6 to 13 weeks duration (Table 5) (Silvani et al 2008; Stupp et al 2008).

For gynecologic cancers, preliminary data in patients with advanced ovarian cancer who did not respond to prior platinum drugs demonstrated a 10% response rate (Smit et al 2005). Ixabepilone has also shown responses in ovarian and endometrial cancer patients previously treated with taxanes (Table 5) (Chen et al 2004).

In head and neck cancer, ixabepilone was studied in two regimens: 1) treatment on days 1 to 5 repeated every 21 days, or 2) weekly dosing. Burtness et al (2006) also stratified patients with metastatic or recurrent squamous cell cancer of the head and neck on the basis of being taxane naïve or having prior taxane exposure. Partial responses were seen in the taxane-naïve patient group using the weekly regimen (Table 5). Lower rates of sensory and motor neuropathy were noted in the 5-day dosing regimen compared with the weekly regimen in both taxane-naïve and exposed groups (Burtness et al 2006).

Preliminary data show activity of ixabepilone in indolent, mantle cell lymphoma, and aggressive non-Hodgkin’s lymphoma (O’Connor et al 2005; Smith et al 2005). Ixabepilone is not an effective treatment in Stage IV melanoma while sagopilone demonstrated responses in 2 out of 14 patients who had previously failed dacarbazine or temozolamide (Pavlick et al 2004; Wenk et al 2008). Ixabepilone and KOS-862 have shown responses in non-small cell lung cancer that failed prior platinum (Yee et al 2005; Vansteenkiste et al 2007). Ixabepilone did not show significant responses in children and adults with refractory pediatric tumors (Jacobs et al 2008). Ixabepilone has also been noted to have activity in soft tissue sarcoma (Okuno et al 2005).

In patients with prostate cancer that was castrate resistant and taxane refractory, patupilone, ixabepilone, and KOS-862 have all decreased prostate specific antigen levels (Beer et al 2007; Rosenberg et al 2007; Chi et al 2008). In patients with castrate refractory metastatic prostate cancer tumors, but not chemotherapy-resistant tumors, patupilone, ixabepilone, and sagopilone induced prostate-specific antigen declines (Hussain et al 2004, 2005; Graff et al 2008).Ixabepilone has been studied in different dosing regimens with the once every 21 days dosing schedule compared with days 1, 8, 15 dosing every 28 days. Higher rates of grade 3–4 sensory neuropathy and neutropenia occurred in the days 1, 8, 15 regimen (Hussain et al 2005; Wilding et al 2008). The addition of estramustine to ixabepilone increased the response rate compared with ixabepilone alone (response rate 69% vs 48% respectively) (Galsky et al 2005).

Unlike most ixabepilone dosing regimens demonstrating better responses with the once every 21 day regimen, metastatic renal cell carcinoma treated with this schedule had stable disease as the best response (Posadas et al 2007). In this tumor, ixabepilone was administered on days 1 to 5 every 21 days or on days 1, 8, 15 with a cycle length of 28 days. Both regimens demonstrated responses (Fojo et al 2005; Burtness et al 2006; Huang et al 2008). Patupilone has also been shown to have activity in metastatic renal cell carcinoma (Thompson et al 2003). Ixabepilone also has activity in advanced urothelial cancer (Dreicer et al 2007).

Conclusions

Epothilones have potent preclinical and clinical activity in human malignancies. As such, they represent new entities that need to be integrated into cancer treatment. Early studies indicate that drugs of this class have broad activity in many of the human tumor types. These agents have a preclinical rationale to offer an advance over taxane therapy and in early human phase I–II trials have noted responses in patients believed to be taxane resistant. However, their distinction from the more commonly used taxane drugs in the treatment of clinical disease will require a randomized phase III trial of an epothilone vs. taxane as primary therapy for metastatic disease. No such trial has been reported. This concern is not a trivial issue as the US Medicare reimbursement for a single treatment with paclitaxel at 175 mg/m² for a patient of 1.7 m² is US$570 per treatment and for ixabepilone at 40 mg/m² for the same patient is US$17,615.

The role of P-glycoprotein resistance protein as a primary mechanism of clinical refractoriness of tumors to anticancer therapy is a controversial area. The major difference between epothilones and taxanes is the ability of epothilones to be unaffected by the P-glycoprotein resistance pump. As previously mentioned, specific subgroups of solid tumors such as the Her2 positive breast cancers may be a particularly important target for this class of agents. Other mechanisms of resistance are also thought to be important in the management of solid tumors.

The epothilones such as ixabepilone and sagopilone, analogous to taxanes, exhibit neurologic toxicity. Although changes in the dosing schedule, decreasing the maximum plasma concentration, and prolonging infusion to lessen C_pmax may diminish neurological adverse effects, such treatments have shifted the dose limiting toxicity from neurological to gastrointestinal adverse events. These toxicities in part may reflect the heavily pretreated population exposed in the initial studies to these agents. Data on the efficacy and toxicity of the various epothilone analogs as primary therapy in humans is urgently needed as the toxicities in this group of patients may be markedly less.

The epothilone structure does allow semisynthetic and fully synthetic production of the agent. In addition, the chemical structure is more amenable to modification than is paclitaxel. As a consequence, there are a plethora of analogs which are undergoing evaluation (Altmann and Memmert 2008). The ability to modify the chemical structure also allows the solubility and stability of the agent to be adjusted for human use. In addition, the structure allows modifications that significantly differ from the parent compound and yet allow the resulting compounds to remain tubulin binders (Feyen et al 2008). At present, epothilone D is not being actively clinically investigated.

Ixabepilone is the most clinically developed of the analogs and has received commercial approval in the US for treatment of refractory breast cancer. This approval is on the basis of efficacy in clinically taxane resistant breast cancer as a single agent and in combination with capecitabine. Combination of epothilones with either classical cytotoxics or with biologics are currently in progress and will serve to further define the role of these drugs in the treatment of cancer.