Taxol-requiring mutant of Chinese hamster ovary cells with impaired mitotic spindle assembly

Paclitaxel-Based Chemotherapy Targeting Cancer Stem Cells from Mono- to Combination Therapy

Paclitaxel (PTX) is a chemotherapeutical agent commonly used to treat several kinds of cancer. PTX is known as a microtubule-targeting agent with a primary molecular mechanism that disrupts the dynamics of microtubules and induces mitotic arrest and cell death. Simultaneously, other mechanisms have been evaluated in many studies. Since the anticancer activity of PTX was discovered, it has been used to treat many cancer patients and has become one of the most extensively used anticancer drugs. Regrettably, the resistance of cancer to PTX is considered an extensive obstacle in clinical applications and is one of the major causes of death correlated with treatment failure. Therefore, the combination of PTX with other drugs could lead to efficient therapeutic strategies. Here, we summarize the mechanisms of PTX, and the current studies focusing on PTX and review promising combinations.

Paclitaxel Activity in Anthracycline Refractory Breast Cancer Patients

Aims and background

We investigated the efficacy and tolerability of two doses of paclitaxel, 175 mg/m2 and 135 mg/m2, over a 3-hr infusion, without prophylactic G-CSF, in heavily pretreated patients with anthracycline-resistant breast cancer. Although paclitaxel may share with anthracyclines a common mechanism of drug resistance, there is evidence that the two drugs are not completely cross resistant.

Methods

From July 1994 to January 1996, 42 patients were treated every 3 weeks, for a maximum of 6 cycles; paclitaxel dose was established according to pretreatment extension.

Results

In 41 assessable patients we observed 9 partial responses, for an overall response rate of 22% (95% CI, 10–34%). There was no difference in response rate between the two dose levels. Median duration of response was 9 months, median time to progression 5 months, and median survival 9 months. The dose-limiting toxicity was neutropenia, which was grade 3–4 in 40% (135 mg/m2) and 62% (175 mg/m2) of the patients (P = 0.28); neutropenic fever occurred in 24% of the patients, without significant differences between the two dose levels. Other toxicity was mild to moderate.

Conclusions

Paclitaxel at doses of 175 mg/m2 or 135 mg/m2 is active and well tolerated in advanced breast cancer patients resistant to anthracyclines. The prophylactic use of colony-stimulating factors seems appropriate in heavily pretreated patients given the higher dose level.

PARP inhibitor combination therapy

In 2014, olaparib (Lynparza) became the first PARP (Poly(ADP-ribose) polymerase) inhibitor to be approved for the treatment of cancer. When used as single agents, PARP inhibitors can selectively target tumour cells with BRCA1 or BRCA2 tumour suppressor gene mutations through synthetic lethality. However, PARP inhibition also shows considerable promise when used together with other therapeutic agents. Here, we summarise both the pre-clinical and clinical evidence for the utility of such combinations and discuss the future prospects and challenges for PARP inhibitor combinatorial therapies.

The Upregulation of PI3K/Akt and MAP Kinase Pathways is Associated with Resistance of Microtubule-Targeting Drugs in Prostate Cancer

Resistance is a significant limitation to the effectiveness of cancer therapies. The PI3K/Akt and MAP kinase pathways play important roles in a variety of normal cellular processes and tumorigenesis. This study is designed to explore the relationship of these signaling pathways with multidrug resistance in prostate cancer (PCa). The PI3K/Akt and MAP kinase pathways were investigated utilizing paclitaxel resistant DU145-TxR PCa cells and their parental non-resistant DU145 cells to determine their relationship with resistance to paclitaxel and other anticancer drugs. Our results demonstrate that the PI3K/Akt and MAP kinase pathways are upregulated in DU145-TxR cells compared to the DU145 cells. Inactivating these pathways using the PI3K/Akt pathway inhibitor LY294002 or the MAP kinase pathway inhibitor PD98059 renders the DU145-TxR cells more sensitive to paclitaxel. We investigated the effects of these inhibitors on other anticancer drugs including docetaxel, vinblastine, doxorubicin, 10-Hydroxycamptothecin (10-HCPT) and cisplatin and find that both inhibitors induces DU145-TxR cells to be more sensitive only to the microtubule-targeting drugs (paclitaxel, docetaxel and vinblastine). Furthermore, the treatment with these inhibitors induces cleaved-PARP production in DU145-TxR cells, suggesting that apoptosis induction might be one of the mechanisms for the reversal of drug resistance. In conclusion, the PI3K/Akt and MAP kinase pathways are associated with resistance to multiple chemotherapeutic drugs. Inactivating these pathways renders these PCa cells more sensitive to microtubule-targeting drugs such as paclitaxel, docetaxel and vinblastine. Combination therapies with novel inhibitors of these two signaling pathways potentially represents a more effective treatment for drug resistant PCa.

Drug discovery targeting cell division proteins, microtubules and FtsZ

Eukaryotic cell division or cytokinesis has been a major target for anticancer drug discovery. After the huge success of paclitaxel and docetaxel, microtubule-stabilizing agents (MSAs) appear to have gained a premier status in the discovery of next-generation anticancer agents. However, the drug resistance caused by MDR, point mutations, and overexpression of tubulin subtypes, etc., is a serious issue associated with these agents. Accordingly, the discovery and development of new-generation MSAs that can obviate various drug resistances has a significant meaning. In sharp contrast, prokaryotic cell division has been largely unexploited for the discovery and development of antibacterial drugs. However, recent studies on the mechanism of bacterial cytokinesis revealed that the most abundant and highly conserved cell division protein, FtsZ, would be an excellent new target for the drug discovery of next-generation antibacterial agents that can circumvent drug-resistances to the commonly used drugs for tuberculosis, MRSA and other infections. This review describes an account of our research on these two fronts in drug discovery, targeting eukaryotic as well as prokaryotic cell division.

A phase II study of docetaxel and vinorelbine plus filgrastim for HER-2 negative, stage IV breast cancer: SWOG S0102

Docetaxel and vinorelbine have demonstrated Single-agent activity in breast cancer. Preclinical studies suggest potential synergy between these antitubulin chemotherapy agents. This study evaluates these drugs in combination in metastatic breast cancer. Taxane-naive patients with HER-2 negative, stage IV breast cancer without prior chemotherapy for metastatic disease, were eligible. Docetaxel (60 mg/m²) was given intravenously on Day 1, vinorelbine (27.5 mg/m²) intravenously on Days 8 and 15, and filgrastim on Days 2–21 of a 21-day cycle. The primary study outcome was one-year overall survival (OS), with secondary outcomes of progression-free survival (PFS), response rate (RR), and toxicity. Of 95 patients registered, 92 were eligible and received treatment. One-year OS was 74 % (95 % CI 64–82 %) with a median OS of 22.3 months (95 % CI 18.8–31.4 months). One-year PFS was 34 % (95 % CI 24–43 %) with median of 7.2 months (95 % CI 6.4–10.3). OS at 2 and 3 years were 49 % (95 % CI 38–59 %) and 30 % (95 % CI 21–40 %), respectively. OS was poorer for women with estrogen-receptor negative disease (n = 32) compared to estrogen-receptor positive (n = 60) (log-rank p = 0.031), but PFS was not significantly different (p = 0.11). RR was 59 % among the 74 patients with measurable disease. Grade 3 and 4 adverse events were 48 and 16 %, respectively. Grade 4 neutropenia was 12 % and grade 3/4 febrile neutropenia was 3 %. Common grade 3/4 nonhematologic toxicities were fatigue (14 %), pneumonitis (10 %), and dyspnea (9 %). The combination of docetaxel and vinorelbine is an active first-line chemotherapy in HER-2 nonoverexpressing, metastatic breast cancer. This combination is associated with significant hematologic and nonhematologic toxicity. The safety profile and expense of the filgrastim limit recommendations for routine use.

Random mutagenesis of β-tubulin defines a set of dispersed mutations that confer paclitaxel resistance

PURPOSE

Previous research showed that mutations in β1-tubulin are frequently involved in paclitaxel resistance but the question of whether the mutations are restricted by cell-type specific differences remains obscure.

METHODS

To circumvent cellular constraints, we randomly mutagenized β-tubulin cDNA, transfected it into CHO cells, and selected for paclitaxel resistance.

RESULTS

A total of 26 β1-tubulin mutations scattered throughout the sequence were identified and a randomly chosen subset were confirmed to confer paclitaxel resistance using site-directed mutagenesis of β-tubulin cDNA and transfection into wild-type cells. Immunofluorescence microscopy and biochemical fractionation studies indicated that cells expressing mutant tubulin had decreased microtubule polymer and frequently suffered mitotic defects that led to the formation of large multinucleated cells, suggesting a resistance mechanism that involves destabilization of the microtubule network. Consistent with this conclusion, the mutations were predominantly located in regions that are likely to be involved in lateral or longitudinal subunit interactions. Notably, fourteen of the new mutations overlapped previously reported mutations in drug resistant cells or in patients with developmental brain abnormalities.

CONCLUSIONS

A random mutagenesis approach allowed isolation of a wider array of drug resistance mutations and demonstrated that similar mutations can cause paclitaxel resistance and human neuronal abnormalities.

Design, synthesis, and biological evaluation of new-generation taxoids

Novel second-generation taxoids with systematic modifications at the C2, C10, and C3'N positions were synthesized and their structure-activity relationships studied. A number of these taxoids exhibited exceptionally high potency against multidrug-resistant cell lines, and several taxoids exhibited virtually no difference in potency against the drug-sensitive and drug-resistant cell lines. These exceptionally potent taxoids were termed "third-generation taxoids". 19 (SB-T-1214), 14g (SB-T-121303), and 14i (SB-T-1213031) exhibited excellent activity against paclitaxel-resistant ovarian cancer cell lines with mutations in beta-tubulin as well, wherein the drug resistance is mediated by the beta-tubulin mutation. These taxoids were found to possess exceptional activity in promoting tubulin assembly, forming numerous very short microtubules similar to those formed by discodermolide. Taxoids 19 and 14g also showed excellent cytotoxicity against four pancreatic cancer cell lines, expressing three to four multidrug-resistant genes. Moreover, taxoid 19 exhibited excellent in vivo efficacy against highly drug-resistant CFPAC-1 pancreatic as well as DLD-1 human colon tumor xenografts in mice.

Amino acid substitutions at proline 220 of beta-tubulin confer resistance to paclitaxel and colcemid

Chinese hamster ovary cells selected for resistance to paclitaxel have a high incidence of mutations affecting L215, L217, and L228 in the H6/H7 loop region of beta1-tubulin. To determine whether other mutations in this loop are also capable of conferring resistance to drugs that affect microtubule assembly, saturation mutagenesis of the highly conserved P220 codon in beta1-tubulin cDNA was carried out. Transfection of a mixed pool of plasmids encoding all possible amino acid substitutions at P220 followed by selection in paclitaxel produced cell lines containing P220L and P220V substitutions. Similar selections in colcemid, on the other hand, yielded cell lines with P220C, P220S, and P220T substitutions. Site-directed mutagenesis and retransfection confirmed that these mutations were responsible for drug resistance. Expression of tubulin containing the P220L and P220V mutations reduced microtubule assembly, conferred resistance to paclitaxel and epothilone A, but increased sensitivity to colcemid and vinblastine. In contrast, tubulin with the P220C, P220S, and P220T mutations increased microtubule assembly, conferred resistance to colcemid and vinblastine, but increased sensitivity to paclitaxel and epothilone A. The results are consistent with molecular modeling studies and support a drug resistance mechanism based on changes in microtubule assembly that counteract the effects of drug treatment. These studies show for the first time that different substitutions at the same amino acid residue in beta1-tubulin can confer cellular resistance to either microtubule-stabilizing or microtubule-destabilizing drugs.

Kallikrein 4 is associated with paclitaxel resistance in ovarian cancer

OBJECTIVE

Carcinoma of the ovary is the most fatal malignancy of the female genital tract in western countries. The effectiveness of chemotherapy is limited by the acquisition of drug resistance. Several members of the kallikrein (KLK) family were recently shown to be expressed in ovarian cancer and implicated in disease prognosis. One of these is KLK4, whose increased mRNA expression was identified as a poor prognostic marker in ovarian cancer. The goal of this study was to investigate if KLK4 protein (hK4) is expressed in ovarian carcinoma lesions and if it is associated with paclitaxel resistance.

METHODS

Immunohistochemistry was used to assess hK4 expression. The 126 lesions were from a cohort of 46 patients with platinum-resistant tumors, which were treated with weekly paclitaxel for recurrent disease. The overall response rate to weekly paclitaxel was 52% (24 of 46) and for patients with tumors resistant to standard three weekly paclitaxel treatment 48% (16 of 33).

RESULTS

Immunohistochemistry indicated that 85% (79 of 93) of lesions from paclitaxel-resistant patients expressed hK4, while only 61% (20 of 33) of lesions were positive for hK4 from paclitaxel sensitive patients. Statistical analysis showed that the intensity of hK4 staining was significantly associated with paclitaxel resistance (P = 0.005), whereas hK4 staining extent showed marginal significance (P = 0.05), but was significantly correlated with higher histological grade (P < 0.001).

CONCLUSION

These data show that hK4 is expressed in ovarian cancer and suggest that hK4 expression might be a predictive marker for paclitaxel resistance.

A ubiquitous beta-tubulin disrupts microtubule assembly and inhibits cell proliferation

Vertebrate tubulin is encoded by a multigene family that produces distinct gene products, or isotypes, of both the alpha- and beta-tubulin subunits. The isotype sequences are conserved across species supporting the hypothesis that different isotypes subserve different functions. To date, however, most studies have demonstrated that tubulin isotypes are freely interchangeable and coassemble into all classes of microtubules. We now report that, in contrast to other isotypes, overexpression of a mouse class V beta-tubulin cDNA in mammalian cells produces a strong, dose-dependent disruption of microtubule organization, increased microtubule fragmentation, and a concomitant reduction in cellular microtubule polymer levels. These changes also disrupt mitotic spindle assembly and block cell proliferation. Consistent with diminished microtubule assembly, there is an increased tolerance for the microtubule stabilizing drug, paclitaxel, which is able to reverse many of the effects of class V beta-tubulin overexpression. Moreover, transfected cells selected in paclitaxel exhibit increased expression of class V beta-tubulin, indicating that this isotype is responsible for the drug resistance. The results show that class V beta-tubulin is functionally distinct from other tubulin isotypes and imparts unique properties on the microtubules into which it incorporates.

Expression of class III beta-tubulin reduces microtubule assembly and confers resistance to paclitaxel

Human brain and testis specific betaIII-tubulin was amplified from a cDNA library, modified to encode a C-terminal hemagglutinin antigen epitope tag, and cloned into a vector that allows tetracycline regulated expression in mammalian cells. Immunofluorescence analysis of transfected Chinese hamster ovary cells demonstrated that expressed HA-tagged betaIII-tubulin is able to assemble with endogenous tubulin into microtubules even though betaIII-tubulin is not a normal constituent of these cells. A stable G418-resistant clone with moderate HAbetaIII-tubulin expression displayed weak (1.5-2-fold) resistance to paclitaxel. A second clone with higher HAbetaIII-tubulin expression could not grow unless tetracycline was present to repress transcription of the transfected cDNA. Analysis of cellular microtubules in each of these clones indicated that incorporation of HAbetaIII-tubulin led to a significant expression-dependent decrease in assembled tubulin. Paclitaxel resistant cells were also directly selected from the transfected cell population using a paclitaxel concentration 4 times higher than the minimum toxic dose. Few cells were able to survive the selection and they grew very slowly. Western blot analysis of these resistant cells revealed very high HAbetaIII-tubulin expression that led to almost complete replacement of endogenous beta-tubulin at steady state. Transfected betaIII-tubulin with no epitope tag behaved in a very similar fashion indicating that presence of the HA tag had no discernible functional effect. The results demonstrate that betaIII-tubulin diminishes microtubule assembly, is toxic when present at high levels, but is able to confer weak resistance to paclitaxel when expressed at moderate levels in mammalian cells.

Tubulin interacting agents: novel taxanes and epothilones

The microtubule cytoskeleton plays an important part in cell mitosis, which relies on polymerization and depolymerization of the protein tubulin. Such established drugs as the vinca alkaloids and colchacine work by inhibiting microtubule assembly, as does paclitaxel. This article describes a variety of promising novel taxanes and epothilones with similar mechanisms of action that are in various states of preclinical and clinical development.

Paclitaxel-dependent mutants have severely reduced microtubule assembly and reduced tubulin synthesis

A subset of mutant cell lines selected for resistance to the antitumor drug paclitaxel are unable to progress normally through mitosis unless the drug is present in the growth medium. Without paclitaxel the cells form defective spindles, undergo aberrant mitoses, fail to complete cell division and eventually die. Analysis of these drug-dependent cells revealed a low amount of microtubule polymer and less tubulin production than wild-type cells. Ribonuclease protection experiments indicated that the decreased tubulin protein was due to decreased tubulin mRNA. Enhancing microtubule assembly by treating the cells with paclitaxel, restored tubulin to levels comparable with those of paclitaxel-treated wild-type cells, which demonstrated that the drug-dependent cells do not have a permanent impairment in their capacity to synthesize tubulin. Paclitaxel-resistant (but not dependent) cells have a smaller reduction in microtubule polymer with little or no decrease in tubulin production, whereas colcemidresistant cells have increased microtubule assembly but also exhibit little or no change in tubulin production. Finally,a mutant cell line producing an unstable β-tubulin protein has normal growth as well as normal synthesis and polymerization of tubulin, despite an approximately 30% decrease in steady state tubulin content. These studies establish a lower limit of tubulin assembly needed for cell survival and indicate that tubulin assembly must fall below this point to trigger a significant decrease in tubulin synthesis.

Paclitaxel in cancer therapy

The last decade witnessed the introduction of exciting new chemotherapeutic agents. Among these, paclitaxel emerged as one of the most powerful compounds. Paclitaxel promotes the polymerisation of tubulin, thereby causing cell death by disrupting the normal microtubule dynamics required for cell division and vital interphase processes. Mechanisms of acquired resistance to paclitaxel include alterations of tubulin structure and the amplification of membrane phosphoglycoproteins that function as drug-efflux pumps. Toxicities associated with paclitaxel include hypersensitivity reaction, neurotoxicity and haematological toxicities. Toxicities may be both dose- and schedule-dependent. Paclitaxel has activity against a broad band of tumour types, including breast, ovarian, lung, head and neck cancers. Paclitaxel also has activity in other malignancies that are refractory to conventional chemotherapy, including previously-treated lymphoma and small cell lung cancers and oesophageal, gastric endometrial, bladder and germ cell tumours. Paclitaxel is also active against AIDS-associated Kaposi's sarcoma.

Factors determining cellular mechanisms of resistance to antimitotic drugs

With the rapidly expanding use of paclitaxel and related taxanes to treat malignant diseases, comes the realization that development of resistance to this class of agents will become an increasingly significant clinical problem. Studies have indicated that acquisition of resistance to the cytotoxic action of these drugs can occur by limiting the drug's ability to accumulate in cells, altering the stability of cellular microtubules, diminishing the drug's ability to bind tubulin, or varying the expression of specific tubulin genes. This review will critically evaluate the selection methods used to generate drug resistant mutants in tissue culture and focus on the various factors that determine which resistance mechanisms are most likely to be encountered. It is anticipated that clinical drug resistance will be complicated by pharmacokinetic considerations and variability among individuals, but that underlying genetic mechanisms will be similar to those found in culture.

A short course of induction chemotherapy followed by two cycles of high-dose chemotherapy with stem cell rescue for chemotherapy naive metastatic breast cancer

A single cycle of high-dose chemotherapy with stem cell support (HDC) in women with responsive metastatic breast cancer (BC) consistently achieves over 50% complete and near complete response (CR/nCR). This significant cytoreduction results in a median event-free survival (EFS) of 8 months, and approximately 20% 3-year and 16% 5-year EFS in selected patients. To improve long-term outcomes, new strategies to treat minimal residual tumor burden are needed. Increased total dose delivered can be achieved with two cycles of HDC. Critical design issues include shortening induction chemotherapy to avoid development of drug resistance and the use of different agents for each HDC cycle. We have determined the maximum tolerated dose (MTD) for paclitaxel combined with high-dose melphalan in the context of a double transplant and explored the impact of a short induction phase. Between June 1994 and August 1996, we enrolled 32 women with metastatic BC on to this phase I double transplant trial. Induction consisted of doxorubicin 30 mg/m2/day days 1-3 given for 2 cycles every 14 days with G-CSF 5 microg/kg s.c. days 4-12. Stem cell collection was performed by leukapheresis in each cycle when the WBC recovered to above 1000/microl. Patients with stable disease or better response to induction were eligible to proceed with HDC. HDC regimen I (TxM) included paclitaxel with dose escalation from 0 to 300 mg/m2 given on day 1 and melphalan 180 mg/m2 in two divided doses given on day 3. HDC regimen II was CTCb (cyclophosphamide 6 g/m2, thiotepa 500 mg/m2, and carboplatin 800 mg/m2 total doses) delivered by 96-h continuous infusion. At the first dose level of 150 mg/m2 paclitaxel by 3 h infusion, four of five patients developed dose-limiting toxicity consisting of diffuse skin erythema and capillary leak syndrome. Only two of these five completed the second transplant. Subsequently, paclitaxel was delivered by 24-h continuous infusion together with 96 h of dexamethasone and histamine receptor blockade. This particular toxicity was not observed again. No toxic deaths occurred and dose-limiting toxicity was not encountered. Three patients were removed from study prior to transplant: one for insurance refusal and two for disease progression. All others completed both cycles of transplant. Complete and near complete response (CR/nCR) after completion of therapy was achieved in 23 (72%) of 32 patients. The median EFS is 26 months. The median overall survival has not yet been reached. At a median follow-up of 58 months, EFS and overall survival are 41% and 53%, respectively. This double transplant approach is feasible, safe, and tolerable. Treatment duration is only 14 weeks and eliminates lengthy induction chemotherapy. The observed event-free and overall survivals are promising and are better than expected following a single transplant. Whilst selection biases may in part contribute to this effect, a much larger phase II double transplant trial is warranted in preparation for a potential randomized comparison of standard therapy vs single vs double transplant.

Risks and benefits of taxanes in breast and ovarian cancer

The taxanes are a unique class of agents with a broad spectrum of clinical activity. They act by binding to tubulin, producing unnaturally stable microtubules and subsequent cell death. The distribution and elimination of paclitaxel depend on dose and administration rate. This nonlinearity is much less evident at lower infusion rates (24-hour infusions) and more evident at high plasma concentrations (3-hour infusions). The pharmacokinetics of docetaxel also suggest the presence of nonlinear pathways, but these appear to be clinically insignificant at the current doses utilised (60 to 100 mg/m2). Both agents undergo hepatic metabolism and biliary excretion and require dose adjustment in the setting of liver dysfunction. Drug interactions are quite common with these agents, some of which are sequence-dependent and clinically significant. The optimal dose of paclitaxel is not known at this time, and controversy over possible dose- or schedule-related differences in efficacy still remain. Docetaxel is somewhat more consistent in its dose and scheduling information, but controversy remains regarding a dose-benefit relationship as well as scheduling differences (weekly vs every 3 weeks). Toxicity profiles for these agents are somewhat different. Paclitaxel is more likely to be associated with peripheral neuropathy and myalgias/arthralgias than docetaxel. Docetaxel is more likely to be associated with a cumulative fluid retention syndrome that can be dose limiting. Paclitaxel and docetaxel are both highly active agents against breast cancer, including tumours that are resistant to anthracyclines. Docetaxel tends to have higher response rates overall, but direct comparisons at maximally tolerated doses have not been completed. Combination regimens with many different agents are attempting to improve on the responses seen with single-agent taxanes. The combination of paclitaxel and a platinum compound should be utilised as first-line therapy of advanced ovarian cancer. Controversy lies in the choice of the platinum compound and the dose and administration schedule of paclitaxel. Substitution of docetaxel for paclitaxel in these platinum-containing regimens is also being investigated. The taxanes also exhibit activity against ovarian cancer in patients previously exposed to platinum agents. These agents may also be administered intraperitoneally for local therapy of metastatic ovarian cancer. Although docetaxel and paclitaxel are often considered similar in activity and tolerability, this review emphasises the fact that these agents are indeed different. Clinicians need to be familiar with the benefits and adverse events related to each agent in order to make informed, appropriate clinical decisions.

Overexpression of wild-type RhoA produces growth arrest by disrupting actin cytoskeleton and microtubules

We have investigated the role of Rho GTPase in cell growth by generating stable cells that express the wild-type RhoA (RhoA(wt)) under the control of an inducible promoter. Induction of RhoA(wt) had a biphasic effect on the actin cytoskeleton. At low levels of expression, RhoA(wt) stimulated the assembly of actin stress fibers without affecting cell growth. At high levels, there was a paradoxical disruption of the actin cytoskeleton accompanied by a growth arrest. Cell cycle analysis revealed a dual block at the G(1)/S and G(2)/M checkpoints. The G(1)/S arrest correlated with the accumulation of p21(Cip1), resulting in the inhibition of cdk2 activity, whereas the G(2)/M block correlated with the loss of microtubules. The cyclin B level and the cdc2 kinase activity, however, were increased, suggesting that the progression through mitosis rather than entry into the G(2)/M is defective when RhoA(wt) is overexpressed. Similar cell cycle defects and the loss of microtubules were observed after a cytochalasin D treatment, indicating that the ability of RhoA to regulate the integrity of actin cytoskeleton may be critical for the cell cycle transition through both the G(1)/S and M phase checkpoints.

A beta-tubulin leucine cluster involved in microtubule assembly and paclitaxel resistance

Analysis of beta-tubulin alleles from nine paclitaxel-resistant Chinese hamster ovary cell lines revealed an unexpected cluster of mutations affecting Leu-215, Leu-217, and Leu-228. Six of the mutant alleles encode a His, Arg, or Phe substitution at Leu-215; another mutant allele has an Arg substitution at Leu-217; and the final two mutant alleles have substitutions of His or Phe at Leu-228. Using plasmids that allow tetracycline regulated expression, the L215H, L217R, and L228F mutations were introduced into a hemagglutinin antigen-tagged beta-tubulin cDNA and transfected into wild-type Chinese hamster ovary cells. In all three cases, low to moderate expression of the transfected mutant gene conferred paclitaxel resistance. Higher levels of expression caused disruption of microtubule assembly, cell cycle arrest at mitosis, and failure to proliferate. Consistent with reduced microtubule stability, cells expressing mutant hemagglutinin beta-tubulin had fewer acetylated microtubules than nonexpressing cells in the same population. These data, together with previous studies showing that the paclitaxel-resistant mutant cell lines have less stable microtubules, indicate that the leucine cluster represents an important structural motif for microtubule assembly.

Paclitaxel resistance in non-small-cell lung cancer associated with beta-tubulin gene mutations

PURPOSE

The mechanisms that cause chemoresistance in non-small-cell lung cancer (NSCLC) patients have yet to be clearly elucidated. Paclitaxel is a tubulin-disrupting agent that binds preferentially to beta-tubulin. Tubulins are guanosine triphosphate (GTP)-binding proteins. Beta-tubulin is a GTPase, whereas alpha-tubulin has no enzyme activity. We reasoned that polymerase chain reaction (PCR) and DNA sequencing of the beta-tubulin gene could reveal more information regarding the connection between beta-tubulin mutations and primary paclitaxel resistance.

PATIENTS AND METHODS

Constitutional genomic DNA and paired tumor DNA were isolated from 49 biopsies from 43 Spanish and six American stage IIIB and IV NSCLC patients who had been treated with a 3-hour, 210 mg/m(2) paclitaxel infusion and a 24-hour, 200 mg/m(2) infusion, respectively. Oligonucleotides specific to beta-tubulin were designed for PCR amplification and sequencing of GTP- and paclitaxel-binding beta-tubulin domains.

RESULTS

Of 49 patients with NSCLC, 16 (33%; 95% confidence interval [CI], 20.7% to 45.3%) had beta-tubulin mutations in exons 1 (one patient) or 4 (15 patients). None of the patients with beta-tubulin mutations had an objective response, whereas 13 of 33 (39.4%; 95% CI, 22.8% to 56%; P = 0.01) patients without beta-tubulin mutations had complete or partial responses. Median survival was 3 months for the 16 patients with beta-tubulin mutations and 10 months for the 33 patients without beta-tubulin mutations (P =.0001).

CONCLUSION

We have identified beta-tubulin gene mutations as a strong predictor of response to the antitubulin drug paclitaxel; these mutations may represent a novel mechanism of resistance and should be examined prospectively in future trials of taxane-based therapy in NSCLC.

Infusional paclitaxel and weekly vinorelbine chemotherapy with concurrent filgrastim for metastatic breast cancer: high complete response rate in a phase I-II study of doxorubicin-treated patients

PURPOSE

We investigated 96-hour paclitaxel infusion combined with weekly (days eight and 15) vinorelbine as salvage therapy for metastatic breast cancer in anthracycline-exposed patients. All patients received scheduled support with granulocyte colony-stimulating factor (G-CSF; filgrastim). Tumor response, toxicity, time to progression (TTP), and survival were assessed.

PATIENTS AND METHODS

This single-center nonrandomized trial enrolled 32 patients. Anthracycline exposure and subsequent progression were common to all patients. Paclitaxel and vinorelbine were escalated over three dosing levels, stratified by liver function.

RESULTS

Seven patients (22%) achieved a complete response and nine patients achieved a partial response for an overall response rate of 50%. The median TTP was 6.1 months, and median survival time was 14.1 months. Dose-limiting toxicity was neutropenia, with dose delay or reduction in seven of 32 patients. Febrile neutropenia requiring hospitalization was uncommon (three of 32 patients; 9%). There were no treatment-related deaths. Grade 3/4 thrombocytopenia occurred in two patients (6%), and 13 patients (41%) required RBC transfusions for anemia. Grade 3 nausea and vomiting was seen in one patient, who was found to be Addisonian. Despite potentially overlapping neurologic toxicities of the two agents, only two patients (6%) were removed from the study because of progressive peripheral neuropathy.

CONCLUSION

Administration of 96-hour paclitaxel infusion and subsequent weekly vinorelbine with G-CSF support is well tolerated. The response rate, TTP, and survival data are encouraging for therapy given to anthracycline pretreated patients with metastatic breast cancer. If these results can be verified in multi-institution trials, this or a similar combination of drugs would merit investigation as first-line therapy in this patient population.

Future directions with taxane therapy

The potential biochemical and genetic mechanisms by which a cell might experience a decreased sensitivity to taxanes and other drugs of this class are discussed in the first part of this article. The use of taxanes in the current gynecologic-oncologic clinical setting is reviewed with special consideration given to the pharmacokinetics of taxane in relation to dose intensity and length of administration.

The clinical development of paclitaxel and the paclitaxel/carboplatin combination

Paclitaxel and carboplatin have nonoverlapping toxicities with a broad range of clinical activity. The combination of escalating dose paclitaxel and carboplatin dosed to a fixed area under the curve (AUC) was explored in a series of phase I studies. 76 patients were treated with paclitaxel over three hours followed by a 30 min carboplatin infusion, dosed by the Calvert formula to a target AUC of 4.0 or 4.5 mg/min/ml-1. The maximum tolerated dose of paclitaxel was 270 to 290 mg/m2, with a dose limiting toxicity of peripheral sensory neuropathy. Activity was seen in lung cancer, with a paclitaxel dose at or above 230 mg/m2. Neuropathy correlated with paclitaxel AUC due to nonlinear pharmacokinetics at higher doses. Ongoing studies include the use of amifostine as a neuroprotectant and phase II studies of the paclitaxel/carboplatin regimen in head and neck cancer, small cell lung cancer and sarcomas.

KT5720 and U-98017 inhibit MAPK and alter the cytoskeleton and cell morphology

We previously identified KT5720 and U-98017 as agents that had paclitaxel (taxol)-like activity in a Chinese hamster ovary (CHO) paclitaxel-dependent cell screen for paclitaxel mimetics. In vitro polymerization of purified brain tubulin is not affected substantially by these compounds, suggesting that, unlike paclitaxel, these agents do not directly affect tubulin. However, these compounds cause profound rearrangements of the cytoskeleton in intact cells, including an apparent alteration of microtubule length, overlapping of cells, and an increase in cell size. We show that KT5720 and U-98017 effectively inhibit mitogen-activated protein kinase (MAPK) activity in vitro. Staurosporine, a poor inhibitor of MAPK but a potent inhibitor of cAMP-dependent protein kinase A (PKA) activity, phospholipid/Ca++-dependent kinase (PKC), and cdc2, does not cause similar changes. In addition, paclitaxel-dependent cells grown in U-98017 have substantially decreased levels of stimulated MAPK. In correlation with these results, we have confirmed the presence of MAPK in isolated tubulin and microtubules in cells. We have examined the hypothesis that these compounds are working through inhibition of MAPK to alter microtubules by inhibiting the phosphorylation of microtubule-associated proteins. A MAPKK dominant negative mutation transfected in CHO cells inhibits activation of MAPK. Transfectants carrying this dominant mutant have impaired activation of MAPK and an altered cell morphology, similar in some respects to that seen with KT5720 and U-98017. These results support a role for MAPK family members in the control of microtubule dynamics and suggest that in intact cells U-98017 and KT5720 achieve their effects of altering cytoskeleton and supporting partial growth of paclitaxel-dependent cells through inhibition of kinases such as MAPK.

Comparative antitumor efficacy of docetaxel and paclitaxel in nude mice bearing human tumor xenografts that overexpress the multidrug resistance protein (MRP)

BACKGROUND

Multidrug resistance has been associated with expression of the multidrug resistance protein (MRP). Recently, MRP-expression has been detected in human tumor samples of patients with breast cancer and non-small-cell lung cancer. Since taxoids are the most active drugs in the treatment of both tumor entities, the antitumor efficacies of paclitaxel and docetaxel were compared in nude mice bearing human tumor xenografts that express MRP.

MATERIALS AND METHODS

Athymic nude mice (nu/nu) bearing tumor xenografts of parental human sarcoma HT1080 or MRP-expressing HT1080/DR4 cells (as confirmed by Northern blot analysis) were treated with the maximum tolerated doses (MTD) of doxorubicin ([Dx] 10 mg/kg i.v. push), paclitaxel ([PC] 50 mg/kg three-hour i.v. infusion), or docetaxel ([DC] 40 mg/kg three-hour i.v. infusion). In vitro, the activity of doxorubicin, paclitaxel and docetaxel was evaluated by the sulphorhodamine B (SRB) assay using the pyridine analogue PAK-104P (5 microM), a potent inhibitor of MRP-function.

RESULTS

At their MTDs both taxoids showed significant activity against MRP-negative HT1080 xenografts with response rates of 80% (40% CR) for PC and 100% (60% CR) for DC. In contrast, DC was significantly more active than PC in nude mice bearing doxorubicin resistant MRP-expressing HT1080/DR4 tumor xenografts (overall response rates: 100% (60% CR) for DC; 10% (0% CR) for PC; 0% for Dx). Since treatment of mice with the MTD of PC or DC yielded similar overall toxicity (maximum weight loss for HT1080: PC 8.6 +/- 2.2%; DC 7.5 +/- 2.2% and for HT1080/DR4: PC 11.6 +/- 3.0%; DC 7.6 +/- 1.8%, respectively), these results demonstrate the increase in the therapeutic index for docetaxel against MRP-expressing tumors. In vitro, HT1080/DR4 cells were 270-fold, 6.4-fold and 2.8-fold more resistant than parental cells to doxorubicin, PC and DC, respectively. Pyridine analogue PAK-104P completely restored drug sensitivity to PC and DC, while no effect of PAK-104P on parental HT1080 cells was observed.

CONCLUSIONS

Both taxoids, when given at their MTDs, showed significant efficacy against parental HT1080 tumor xenografts. However, docetaxel at its MTD was significantly more active against MRP-expressing tumor xenografts than paclitaxel. Furthermore, in vitro resistance of HT1080/DR4 cells was higher for PC (6.4-fold) than for DC (2.8-fold). Since PAK-104P completely restored sensitivity to both taxoids, the observed resistance appears to be related to MRP. These data suggest, that docetaxel is not as readily transported by MRP as paclitaxel leading to an increased therapeutic ratio in MRP-expressing tumors in vivo. Therefore, docetaxel may have therapeutic advantages in the clinical treatment of MRP-expressing tumors.

Microtubule-dependent multilobular organization of the nucleus in sensitive and multidrug-resistant L0 leukemia cells

The relationship between the nuclear morphology and the microtubular organization of L100 and L1000 cells, two vincristine-induced multidrug resistant human acute lymphocytic leukemia cell lines, was examined and compared to that of L0 parental cells. The L0 parental cells contained a round nucleus and the microtubules were evenly distributed throughout the cytoplasm. In contrast, the microtubules of the L100 and L1000 cells were localized between the lobular structures of a multilobulated nucleus. Disassembly of microtubules in L100 and L1000 cells by colchicine resulted in the loss of the multilobulated morphology of the nucleus. While the total cellular content of tubulin of L0 and L100 cells was similar, the content of microtubules of L100 cells was only 55% of that observed in L0 cells. Two, 28 kDa (pI 6.9) and 31 kDa (pI 4.4), microtubule-associated proteins were found to be overexpressed in L100 and L1000 cells. The results indicate that the multilobulated nuclear morphology of L100 and L1000 cells is dependent upon the unique and intact organization of the microtubules; the distinct organization of the microtubules and the multilobular nuclear morphology of the two resistant cell lines may be due to the differential expression of specific microtubule-associated proteins.

Etoposide and doxorubicin antagonize the in vitro activity of paclitaxel in human non-small cell lung cancer cell lines

GOAL

To explore in vitro interactions of paclitaxel with other agents also active against non-small cell lung cancer in the hope of identifying promising combinations for clinical evaluation.

METHODS

We measured the cytotoxic effects of paclitaxel when used alone or in combination with vinblastine, cisplatin, etoposide or doxorubicin in final concentrations covering 3-4 Logs in up to five cell lines using a 96-well plate MTT assay. Drug interactions were analyzed with the isobologram method of Steel and Peckham and bidimensional plots.

RESULTS

We detected no interactions between paclitaxel and cisplatin in two cell lines. Despite sharing a molecular site of action, there were no interactions between paclitaxel and vinblastine in two cell lines. In contrast, significant antagonism was detected between paclitaxel and etoposide or doxorubicin in 4/5 cell lines tested.

CONCLUSIONS

We failed to identify potentially synergistic paclitaxel-based combinations for the treatment of non-small cell lung cancer. Whether the observed in vitro antagonism between paclitaxel and etoposide or doxorubicin predicts for similar interaction in the clinical use of these drugs in combination is unknown.

Promising new agents in the treatment of non-small cell lung cancer

A number of new drugs and drug classes have recently become available for clinical testing which demonstrate significant antitumor activity in non-small cell lung cancer. The preclinical rationale, mechanism of action, toxicity profile and results of early trials of paclitaxel, docetaxel, edatrexate, CPT-11, topotecan, vinorelbine and gemcitabine in non-small cell lung cancer are reviewed.

New microtubular agents in pediatric oncology

The taxanes are a new group of anticancer agents with a novel mechanism of action. They promote microtubule assembly and stabilize the microtubules. Paclitaxel (Taxol), the first agent in this group in clinical trials was isolated from the Pacific yew, Taxus brevifolia in 1971. Both in preclinical and clinical studies, paclitaxel and its semisynthetic analog docetaxel exhibit significant antitumor activity. This review will provide an overview of the clinical experience with the group of anti-microtubular agents, the taxanes in pediatric oncology.

Preclinical pharmacology of docetaxel

Docetaxel is a taxoid cytotoxic agent which promotes tubulin assembly into microtubles and inhibits their depolymerisation. In vitro, docetaxel reduces murine and human tumour cell survival by 50% at concentrations of 4-35 ng/ml, with a greater cytotoxic effect on proliferating than on non-proliferating cells. In vivo, docetaxel is schedule-independent. Over 80% of murine transplantable tumours were found to be very docetaxel sensitive, with complete regression of advanced stage tumours. Activity was also observed in > 90% of advanced stage human tumour xenografts in mice. In combination therapy studies, synergism with 5-fluorouracil, cyclophosphamide and etoposide was observed in vivo. Docetaxel exhibited linear pharmacokinetics and long tumour retention in tumour-bearing mice; plasma protein binding ranged from 76 to 89%. In toxicological studies in mice and dogs, docetaxel produced haematological, gastrointestinal and neuromotor toxicity. The dog was found to be the most sensitive species to the toxic effects of docetaxel.

Paclitaxel (Taxol) and docetaxel (Taxotere): active chemotherapeutic agents in lung cancer

Paclitaxel (Taxol), the prototype of a new class of plant-derived antineoplastic compounds, is a natural product isolated from the Pacific yew. Docetaxel (Taxotere) is a hemisynthetic product derived from the European yew. These agents share a unique mechanism of cytotoxic action by promoting assembly of microtubules and rendering the microtubules resistant to depolymerization. In vitro studies suggest a possible role for radiation sensitization. In Phase I trials, the dose-limiting toxicity was neutropenia for both agents. Other toxicities include infusion-related hypersensitivity reactions, alopecia, neurotoxicity, mucositis, diarrhoea and myalgias. To prevent infusion-related reactions, routine premedication is recommended. Noncumulative cardiac toxicity has been observed with paclitaxel. Fluid retention and rash have been reported with docetaxel. In Phase II studies of paclitaxel in advanced non-small cell lung cancer, response rates of 21% and 24% were observed. In Phase II studies of docetaxel in similar patients, response rates ranging from 28-38% were reported, including patients previously treated with cisplatin. The most common toxicity in these studies was neutropenia. Combination studies with cisplatin and other agents are in progress. Paclitaxel and docetaxel are among the most active chemotherapeutic agents for non-small cell lung cancer patients. Their testing will dominate trials of new therapies in this disease for years to come.

Schedule-dependent antagonism of paclitaxel and cisplatin in human gastric and ovarian carcinoma cell lines in vitro

Paclitaxel has demonstrated broad clinical activity in a variety of malignancies both alone and in combination with other chemotherapeutic agents. The in vitro cytotoxicity of paclitaxel and cisplatin alone, in combination and in sequence, were evaluated against established human gastric and ovarian carcinoma cell lines using 2-h drug exposure. The combination of cisplatin and paclitaxel was found to be additive or even synergistic when paclitaxel was given 24 h prior to cisplatin as demonstrated by isobologram analysis. However, when both drugs were given simultaneously or when cisplatin was given prior to paclitaxel, a strong antagonistic interaction was observed. This antagonism was evident for up to 72 h after a 2-h exposure to cisplatin. Pretreatment with cisplatin caused no alteration in [3H]paclitaxel uptake in HM2 gastric carcinoma cells, but resulted in decreased intracellular retention of paclitaxel. Since cisplatin treatment led to a reduction in cellular glutathione content in these cells and reduced levels of glutathione have been associated with protection against cytotoxicity of paclitaxel, cells were pretreated with L-buthionine sulfoximine (L-BSO). However, depletion of glutathione had no influence on the activity of paclitaxel. A significant accumulation of cells in S-phase was observed 24 h after cisplatin, which resolved after 48 h and resulted in a pronounced increase of G2M phase. These data demonstrate that the interactions of paclitaxel and cisplatin are highly schedule-dependent and applications of cisplatin simultaneously with or prior to paclitaxel may result in pronounced antagonism. These findings could have implications for the design of further clinical protocols.

Stable expression of heterologous microtubule-associated proteins (MAPs) in Chinese hamster ovary cells: evidence for differing roles of MAPs in microtubule organization

To study the effects of microtubule-associated proteins (MAPs) on in vivo microtubule assembly, cDNAs containing the complete coding sequences of a Drosophila 205-kD heat stable MAP, human MAP 4, and human tau were stably transfected into CHO cells. Constitutive expression of the transfected genes was low in most cases and had no obvious effects on the viability of the transfected cell lines. High levels of expression, as judged by Western blots, immunofluorescence, and Northern blots, could be induced by treating cells with sodium butyrate. High levels of MAPs were maintained for at least 24-48 h after removal of the sodium butyrate. Immunofluorescence analysis indicated that all three MAPs bound to cellular microtubules, but only the transfected tau caused a rearrangement of microtubules into bundles. Despite high levels of expression of these exogenous MAPs and the bundling of microtubules in cells expressing tau, transfected cells had normal levels of assembled and unassembled tubulin. With the exception of the tau-induced bundles, microtubules in transfected cells showed the same sensitivity as control cells to microtubule depolymerization by Colcemid. Further, all three MAPs were ineffective in reversing the taxol-dependent phenotype of a CHO mutant cell line. The absence of a quantitative effect of any of these heterologous proteins on the assembly of tubulin suggests that these MAPs may have different roles in vivo from those inferred previously from in vitro experiments.

New chemotherapeutic agents for breast cancer

The drug discovery programs of the National Cancer Institute and the pharmaceutical industry recently have provided oncologists with a wide array of new chemotherapeutic agents that have considerable potential for breast cancer treatment. Foremost among these new agents are the taxanes, of which paclitaxel and docetaxel, the only members of this class currently in clinical use, have been associated with impressive response rates in patients with metastatic disease. Importantly, they display some evidence clinically of not being cross-resistant with the anthracyclines. Efforts now are being directed toward optimizing dose and schedule in the metastatic setting while integrating these agents into standard adjuvant regimens. Other agents that have undergone Phase II testing in breast cancer include vinorelbine, edatrexate, and losoxantrone. It remains to be determined, however, whether these drugs possess substantial advantages over other members of their class. Newer compounds, such as pyrazoloacridine, ICI D1694, topoisomerase-I inhibitors, temozolomide, penclomidine, fumagillin (TNP-470), and differentiators like the retinoids, hold substantial promise because of their unique mechanisms of action; however, Phase II testing of these agents is just beginning. Although alternative approaches to treatment, such as gene therapies, monoclonal antibodies, and growth factor inhibitors, are likely to have a positive impact, it is probable that progress will best be made by combining these strategies with chemotherapy. Therefore, continuation of the search for more effective chemotherapeutic agents should remain a high priority.

Radiation and taxol effects on synchronized human cervical carcinoma cells

PURPOSE

To evaluate the effectiveness of the plant derived chemotherapeutic agent taxol alone and in combination with ionizing radiation on synchronous and asynchronous human cervical carcinoma cells and to define the mechanistic basis for this cytotoxic response.

METHODS AND MATERIALS

Asynchronous and synchronous cells (obtained by modified mitotic shake-off) derived from carcinomas of the human uterine cervix were treated with a range of concentrations of taxol (0, 1.0, 2.5, 5.0, 10.0 and 20.0 nM) for either 8, 24 or 48 h. Synchronized cell cycling was evaluated by counting mitotic indices and by uptake of bromodeoxyuridine (BrdUrd). Cells were irradiated (137Cs gamma rays at 1.12 Gy/min) alone and after taxol treatment and plating efficiencies and radiosensitivity determined.

RESULTS

Taxol treatment resulted in a dose time dependent loss of colony forming ability with 10 nM for 24 h producing about 10% cell survival. Irradiating taxol treated cells resulted in a strictly additive response in contrast to previous supra-additive results with astrocytoma and melanoma cells. Mitotically synchronized cells rapidly moved into G1 phase with a second mitotic peak at 28 h (total cycle time). Taxol treatment resulted in a continued accumulation of mitoses, and a failure and/or delay of entry of a fraction of cells into S phase after a G1 phase of at least 10 h. That is, taxol effects cell cycling at a stage other than G2/M. Irradiating (3 Gy) synchronized cells showed a 10-fold variation in sensitivity, with mitosis as the most sensitive phase with taxol alone resulting in some cytotoxicity and combined effects additive or less than additive.

CONCLUSION

Taxol effects these cervical carcinoma cells at other stages of the cell cycle than G2/M. This may explain the failure to obtain taxol radiosensitization with these cells and it may indicate that taxol has a multiplicity of actions with differences in effectiveness likely between cells of different origins.

Future directions for paclitaxel (TAXOL) in the treatment of ovarian cancer

Ovarian cancer is the leading cause of death from gynecologic cancers in women in the United States. Although response rates to primary platinum-based therapy are high, relapse is common. The overall survival of patients with advanced-stage disease has not changed dramatically over the last 20 years, even with the advent of effective combination chemotherapy. Paclitaxel (TAXOL) is a new chemotherapeutic agent that shows promise for the treatment of patients with advanced disease as well as those with recurrent cisplatin-refractory disease. Overall response rates have been encouraging, although there are few pathological complete remissions. In contrast to other treatments, many patients achieve prolonged stable disease over multiple cycles of therapy. However, the optimal use of paclitaxel has not yet been determined. Current studies aim to evaluate dose intensity, administration schedule, multidrug regimens, sequence of drug administration, and combinations with radiation therapy. Future studies may address adjuvant therapy of early-stage disease. Results of these trials will clarify the role of paclitaxel in the primary treatment of ovarian cancer.

The taxoids: paclitaxel (Taxol) and docetaxel (Taxotere)

The taxoids, paclitaxel (Taxol) and docetaxel (Taxotere), represent a novel class of antineoplastic drugs. Paclitaxel and docetaxel share a similar mechanism of action: the promotion of microtubule assembly and inhibition of microtubule disassembly. The clinical development of paclitaxel was initially hampered by hypersensitivity reactions (HSRs). The use of premedications and prolongation of the infusion time to 24h has reduced these reactions and allowed this drug's clinical development. Although paclitaxel's clinical activity has not been fully investigated, clinical trials have demonstrated its activity against ovarian, breast, and bronchial carcinomas. Because phase I studies of docetaxel noted occasional HSRs and these observations increased with further clinical experiences, those premedications employed with paclitaxel have now been instituted in many phase II studies of docetaxel. Docetaxel is currently being investigated in ovarian, breast, and bronchial carcinomas and has shown impressive clinical activity. The dose-limiting toxicity of both these agents is neutropenia; myalgias, mucositis, neuropathies, and alopecia have also been observed with both drugs. Additionally, a fluid retention syndrome and cutaneous toxicities have been noted in patients treated with docetaxel. Future studies of the taxoids will allow further comparisons of the toxicity and efficacy of these agents.

Comparative in vitro cytotoxicity of taxol and Taxotere against cisplatin-sensitive and-resistant human ovarian carcinoma cell lines

Using the sulforhodamine B assay, we compared the cytotoxic properties of the novel microtubule agent taxol and the semi-synthetic related compound Taxotere in nine human ovarian-carcinoma cell lines, including three pairs of cell lines rendered resistant to cisplatin or carboplatin. In addition, the cytotoxicity of the commonly used anticancer drugs cisplatin and adriamycin and the topoisomerase II inhibitor etoposide was determined. The results of continuous drug exposure showed that taxol [mean concentration producing 50% growth inhibition (IC50), 1.1 x 10(-9) M; range, 2.8 x 10(-9)-5 x 10(-10) M and Taxotere (mean IC50, 5.1 x 10(-10) M; range, 7.2-3.3 x 10(-10) M) were greater than 1,000 times more cytotoxic than either cisplatin (mean IC50, 3.1 x 10(-6) M; P less than 0.05) or etoposide (mean IC50, 2.3 x 10(-6) M; P less than 0.05) and greater than 100 times more cytotoxic than Adriamycin (mean IC50, 6.9 x 10(-8) M; P less than 0.05). Taxotere was more cytotoxic than taxol; following continuous exposure, the mean difference across the cell lines was 2 orders of magnitude (range, 1.1-3.9 orders of magnitude for individual lines). Although this difference did not reach statistical significance for any individual cell line (P values ranged from 0.17 for HX/62 to 0.9 for OVCAR-3), when all IC50 values for the 96-h experiments were pooled, Taxotere was found to be significantly more potent than taxol (P = 0.05). Following 2 h exposure, the mean cytotoxicity of Taxotere was 3.9-fold greater than that of taxol across the nine lines (range, 0.75- to 10-fold; P less than 0.05 for the CH1 cell line; overall pooled IC50 data, P = 0.05). Although a 71-fold range of sensitivity to cisplatin was observed across the six parent cell lines (IC50 most resistant line/IC50 most sensitive line), this was largely abolished by treatment with taxol (5.6-fold range) and Taxotere (2.2-fold range). Following continuous exposure of the three pairs of lines exhibiting acquired resistance to platinum, no cross-resistance with either Taxotere or taxol was found (resistance factors, less than 1.5). In the 41M and 41McisR pair of lines, in which previous studies have shown resistance to be due to reduced platinum accumulation, taxol and Taxotere exhibited some collateral sensitivity (resistance factors, 0.69 and 0.66, respectively). Taxotere and, particularly, taxol showed a pronounced concentration times exposure duration (C x T) dependence as compared with cisplatin (P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Novel nucleolar and nuclear morphology in a vincristine-dependent human leukemia cell line (L100)

Acquired resistance to chemotherapeutic drugs by tumor cells is an important obstacle to effective therapy of human malignancy. We now describe a vincristine (VCR)-induced multidrug-resistant (MDR) human acute lymphatic leukemia cell line, the sustained in vitro growth of which is dependent on vincristine. The doubling time for parental drug-sensitive cells (L0) is 40.2 +/- 13.2 h and for the MDR subline (L100) 62.5 +/- 11.3 h. L100 cells have similar G2 and mitotic phase to parental cells, express the MDR phenotype, and are characterized by novel morphologic features with multilobulated nuclei and multiple small nucleoli. Compared with L0 cells which have 2-3 nucleoli per cell, L100 cells have 7-8 nucleoli per cell. Average nucleolar area is 11.3 +/- 7.3 microns 2 for L0 and 2.5 +/- 2.4 microns 2 for L100 cells determined by the laser scanning method. The striking morphologic abnormalities of L100 cells suggest a drug-induced cytoskeletal abnormality. The relationship of these abnormalities to the VCR growth dependence of L100 cells is discussed.