New-age drug meets resistance

Radiation Therapies in Cancer

A crucial element of cancer treatment is radiation therapy that is used to destroy tumors and cancer cells through radiation. Another essential component is immunotherapy that helps immune system to combat cancer. The combination of both radiation therapy and immunotherapy is being focused recently for the treatment of many tumors. Chemotherapy includes the use of some chemical agent to control the growth of cancer, whereas irradiation involves the use of radiations of high energy to kill cancer cells. The union of both became the strongest practice in cancer treatment techniques. Specific chemotherapies are combined with radiation in the treatment of cancer after proper preclinical assessment of their effectiveness. Some classes of compounds include platinum-based drugs, antimicrotubules, antimetabolites (5-Fluorouracil, Capecitabine, Gemcitabine, Pemetrexed), topoisomerase I inhibitors, alkylating agents (Temozolomide), and other agents (Mitomycin-C, Hypoxic Sensitizers, Nimorazole).

Karyotype Aberrations in Action: The Evolution of Cancer Genomes and the Tumor Microenvironment

Cancer is a disease of cellular evolution. For this cellular evolution to take place, a population of cells must contain functional heterogeneity and an assessment of this heterogeneity in the form of natural selection. Cancer cells from advanced malignancies are genomically and functionally very different compared to the healthy cells from which they evolved. Genomic alterations include aneuploidy (numerical and structural changes in chromosome content) and polyploidy (e.g., whole genome doubling), which can have considerable effects on cell physiology and phenotype. Likewise, conditions in the tumor microenvironment are spatially heterogeneous and vastly different than in healthy tissues, resulting in a number of environmental niches that play important roles in driving the evolution of tumor cells. While a number of studies have documented abnormal conditions of the tumor microenvironment and the cellular consequences of aneuploidy and polyploidy, a thorough overview of the interplay between karyotypically abnormal cells and the tissue and tumor microenvironments is not available. Here, we examine the evidence for how this interaction may unfold during tumor evolution. We describe a bidirectional interplay in which aneuploid and polyploid cells alter and shape the microenvironment in which they and their progeny reside; in turn, this microenvironment modulates the rate of genesis for new karyotype aberrations and selects for cells that are most fit under a given condition. We conclude by discussing the importance of this interaction for tumor evolution and the possibility of leveraging our understanding of this interplay for cancer therapy.

An elementary mathematical modeling of drug resistance in cancer

Targeted therapy is one of the promising strategies for the treatment of cancer. However, resistance to anticancer drug strongly limits the long-term effectiveness of treatment, which is a major obstacle for successfully treating cancer. In this paper, we analyze a linear system of ordinary differential equations for cancer multi-drug resistance induced mainly by random genetic point mutation. We investigate that the resistance generated before the beginning of the treatment is greater than that developed during-treatment. This result depends on the concentration of the drug, which holds only when the concentration of the drug reaches a lower limit. Moreover, no matter how many drugs are used in the treatment, the amount of resistance (generated at the beginning of the treatment and within a certain period of time after the treatment) always depends on the turnover rate. Using numerical simulations, we also evaluate the response of the mutant cancer cell population as a function of time under different treatment strategies. At appropriate dosages, combination therapy produces significant effects for the treatment with low-turnover rate cancer. For cancer with very high-turnover rate (close to 1), combination therapy can not significantly reduce the amount of resistant mutants compared to monotherapy, so in this case, combination therapy would not have advantage over monotherapy.

Drug resistance

Drug resistance is a fundamental problem in the treatment of cancer since cancer that becomes resistant to the available drugs may leave the patient with no therapeutic alternatives. In this chapter, we consider the dynamics of drug resistance in blood cancer and the related issue of the dynamics of cancer stem cells. After describing the main types of chemotherapeutic agents available for cancer treatment, we review the different mechanisms of drug resistance development. Various mathematical models of drug resistance found in the literature are then reviewed. Given the well-known hierarchy of the hematopoietic system, it is critical to focus on those cells that have the ability to self-renew, since these will be the only cells able to induce long-term drug resistance. Thus, a recent mathematical model taking into account the complex dynamics of the leukemic stem-like cells is described. The chapter closes with a few applications of this model to chronic myeloid leukemia.

Brain accumulation of dasatinib is restricted by P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) and can be enhanced by elacridar treatment

PURPOSE

Imatinib, a BCR-ABL tyrosine kinase inhibitor, is a substrate of the efflux transporters P-glycoprotein (P-gp; ABCB1) and ABCG2 (breast cancer resistance protein), and its brain accumulation is restricted by both transporters. For dasatinib, an inhibitor of SCR/BCR-ABL kinases, in vivo interactions with P-gp and ABCG2 are not fully established yet.

EXPERIMENTAL DESIGN

We used Abcb1a/1b(-/-), Abcg2(-/-), and Abcb1a/1b;Abcg2(-/-) mice to establish the roles of P-gp and ABCG2 in the pharmacokinetics and brain accumulation of dasatinib.

RESULTS

We found that oral uptake of dasatinib is limited by P-gp. Furthermore, relative brain accumulation, 6 hours after administration, was not affected by Abcg2 deficiency, but absence of P-gp resulted in a 3.6-fold increase after oral and 4.8-fold higher accumulation after i.p. administration. Abcb1a/1b;Abcg2(-/-) mice had the most pronounced increase in relative brain accumulation, which was 13.2-fold higher after oral and 22.7-fold increased after i.p. administration. Moreover, coadministration to wild-type mice of dasatinib with the dual P-gp and ABCG2 inhibitor elacridar resulted in a similar dasatinib brain accumulation as observed for Abcb1a/1b;Abcg2(-/-) mice.

CONCLUSIONS

Brain accumulation of dasatinib is primarily restricted by P-gp, but Abcg2 can partly take over this protective function at the blood-brain barrier. Consequently, when both transporters are absent or inhibited, brain uptake of dasatinib is highly increased. These findings might be clinically relevant for patients with central nervous system Philadelphia chromosome-positive leukemia, as coadministration of an inhibitor of P-gp and ABCG2 with dasatinib might result in better therapeutic responses in these patients.

bFGF rescues imatinib/STI571-induced apoptosis of sis-NIH3T3 fibroblasts

PDGF-B-transfected, sis-NIH3T3 fibroblasts serve as a model system for examining the role of PDGF signaling in tumors. We have found that imatinib/STI571, a tyrosine kinase inhibitor targeting PDGF receptors, induces apoptosis of sis-NIH3T3 fibroblasts cultured under serum free conditions, which was rescued by the addition of 10% newborn calf serum (NCS). Therefore, growth factors included in serum were tested with regard to their ability to rescue imatinib-induced apoptosis. While PDGF-AB, EGF, and IGF-I failed to protect imatinib-induced sis-NIH3T3 cell apoptosis, bFGF rescued it. The effects of bFGF were confirmed by both cell viability assays and Bax/Bcl-2 gene expression ratio. An FGF receptor inhibitor, PD166866, invalidated the protective effect of bFGF. However, combination of imatinib and PD166866 failed to induce cell death of sis-NIH3T3 cells when cultured in 10% NCS. These results indicate that synergistic administration of some types of tyrosine kinase inhibitors need to be tested under in vivo-like conditions to establish novel strategies in anti-cancer therapy.

Detection of BCR-ABL gene mutations in Philadelphia chromosome positive leukemia patients resistant to STI-571 cancer therapy

The ABL-BCR fusion protein is a constitutively activated tyrosine kinase thought to play a central role in chronic myeloid leukemia (CML) and Philadelphia (Ph) chromosome acute lymphoid leukemia (ALL). Targeting the tyrosine kinase activity of ABL-BCR has been shown to be a promising therapeutic strategy in treating this disorder. Among the tyrosine kinase inhibitors, STI571 is a very effective therapeutic agent when administered to CML patients in the stable chronic phase. However, it has been reported that many CML patients with blast cell crisis treated with STI571 relapsed and became resistant to STI571. In order to understand the possible molecular mechanisms underlying STI571 resistance caused by ABL gene mutations, we investigated 19 patients (18 CML patients and 1 Ph (+) ALL patient) who either relapsed after initial response or had no response to STI571 treatment. We used polymerase chain reaction followed by restriction fragment length polymorphism (PCR-RFLP) analysis, dHPLC, and direct DNA sequencing to analyze any possible mutations in exons 5 to 9 of the ABL gene. Our results showed that 5 out of 19 patients had various mutations between exons 5 and 7 of the ABL gene. The Ph (+) ALL patient had a Glu255Lys mutation in exon 5 and a Thr315Ile mutation in exon 7. The Glu255Lys substitution has a G to A change, and the Thr315Ile substitution has a C to T change in the ABL gene. The other unique mutations found in this study include: Tyr253His, Met351Thr, GAA tri-nucleotides insertion, and Leu213Pro.

The effect of P-gp (Mdr1a/1b), BCRP (Bcrp1) and P-gp/BCRP inhibitors on the in vivo absorption, distribution, metabolism and excretion of imatinib

Imatinib is transported by P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP), however, the exact impact of these transporters on absorption, distribution, metabolism and excretion (ADME) of imatinib is not fully understood due to incomplete data. We have performed a comprehensive ADME study of imatinib given as single agent or in combination with the well known BCRP/P-gp inhibitors, elacridar and pantoprazole, in wild-type and P-gp and/or BCRP knockout mice. The absence of P-gp and BCRP together resulted in a significantly higher area under the plasma concentration-time curve (AUC) after i.v. administration, whereas the AUC after oral dosing was unaltered. Both elacridar and pantoprazole significantly increased the AUC of orally administered imatinib in wild-type but also in P-gp/BCRP knockout mice. This lower clearance was not due to a (further) reduction in biliary excretion. Fecal excretion was significantly reduced in P-gp and P-gp/BCRP knockout but not in BCRP knockout mice, whereas the brain penetration was significantly higher in P-gp/BCRP knockout mice compared to single P-gp or BCRP knockout or wild-type mice. In conclusion, P-gp and BCRP have only a modest effect on the ADME of imatinib in comparison to metabolic elimination. P-gp is the most prevalent factor for systemic clearance and limiting the brain penetration. The considerable drug-drug interaction observed with elacridar or pantoprazole is only partly mediated by inhibition of P-gp and BCRP and far more by the inhibition of other elimination pathways.

Defining the steps that lead to cancer: replicative telomere erosion, aneuploidy and an epigenetic maturation arrest of tissue stem cells

Recently, an influential sequencing study found that more than 1700 genes had non-silent mutations in either a breast or colorectal cancer, out of just 11 breast and 11 colorectal tumor samples. This is not surprising given the fact that genomic instability is the hallmark of cancer cells. The plethora of genomic alterations found in every carcinoma does not obey the 'law of genotype-phenotype correlation', since the same histological subtype of cancer harbors different gene mutations and chromosomal aberrations in every patient. In an attempt to make sense out of the observed genetic and chromosomal chaos in cancer, I propose a cascade model. According to this model, tissue regeneration depends on the proliferation and serial activation of stem cells. Replicative telomere erosion limits the proliferative life span of adult stem cells and results in the Hayflick limit (M1). However, local tissue exhaustion or old age might promote the activation of M1-deficient tissue stem cells. Extended proliferation of these cells leads to telomere-driven chromosomal instability and aneuploidy (abnormal balance of chromosomes and/or chromosome material). Several of the aforementioned steps have been already described in the literature. However, in contrast to common theories, it is proposed here that the genomic damage blocks the epigenetic differentiation switch. As a result of aneuploidy, differentiation-specific genes cannot be activated by modification of methylation patterns. Consequently, the phenotype of cancer tissue is largely determined by the epigenetic maturation arrest of tissue stem cells, which in addition enables a fraction of cancer cells to proliferate, invade and metastasize, as normal adult stem cells do. The new model combines genetic and epigenetic alterations of cancer cells in one causative cascade and offers an explanation for why identical histologic cancer types harbor a confusing variety of chromosomal and gene aberrations. The Viennese Cascade, as presented here, may end the debate on if and how 'tumor-unspecific' aneuploidy leads to cancer.

Determination of imatinib mesylate and its main metabolite (CGP74588) in human plasma and murine specimens by ion-pairing reversed-phase high-performance liquid chromatography

A sensitive reversed-phase high-performance liquid chromatographic (HPLC) method has been developed and validated for the determination of imatinib, a tyrosine kinase inhibitor, and its main metabolite N-desmethyl-imatinib (CGP74588) in human plasma and relevant murine biological matrices. A simple HPLC assay for the individual quantification of imatinib and CGP74588 in murine specimens has not been reported to date. Sample pre-treatment involved liquid-liquid extraction with tert-butyl-methyl ether. Imatinib, CGP74588 (metabolite) and the internal standard 4-hydroxybenzophenone were separated using a narrow bore (2.1 x 150 mm) stainless steel Symmetry C(18) column and detected by UV at 265 nm. The mobile phase consisted of 28% (v/v) acetonitrile in 50 mM ammonium acetate buffer pH 6.8 containing 0.005 M 1-octane sulfonic acid and was delivered at 0.2 mL/min. The calibration curve was prepared in blank human plasma and was linear over the dynamic range 10 ng/mL to 10 microg/mL). The accuracy was close to 100% and the within-day and between-day precisions were within the generally accepted 15% range. The validation results showed that the assay was selective and reproducible. This method was applied to study the pharmacokinetics of imatinib and its main metabolite in human and mice.

An HSV-2-based oncolytic virus deleted in the PK domain of the ICP10 gene is a potent inducer of apoptotic death in tumor cells

The N-terminus of the ICP10 gene of type 2 herpes simplex virus (HSV-2) encodes a serine/threonine protein kinase (PK) domain that facilitates HSV-2 replication by activating the Ras/MEK/MAPK mitogenic pathway and suppressing apoptosis. We recently demonstrated that deletion of this oncogenic PK domain converts it to a potent oncolytic agent. This mutant, which we have designated FusOn-H2, preferentially replicates in and thus lyses tumor cells in which the Ras signaling pathway is constitutively activated. Here we show that FusOn-H2 exerts strong ability in inducing apoptosis in different lines of human tumor cells and in esophageal tumors growing in mice. The apoptotic effect produced by FusOn-H2 was not restricted to infected cells but extended to uninfected bystander cells, thereby increasing the lethality of the virus. These results add a novel killing mechanism to those previously assigned to FusOn-H2, rendering it an attractive candidate for clinical trials.

The fixed-size Luria-Delbruck model with a nonzero death rate

What is the expected number of mutants in a stochastically growing colony once it reaches a given size, N? This is a variant of the famous Luria-Delbruck model which studies the distribution of mutants after a given time-lapse. Instead of fixing the time-lapse, we assume that the colony size is a measurable quantity, which is the case in many in-vivo oncological and other applications. We study the mean number of mutants for an arbitrary cell death rate, and give partial results for the variance. For a restricted set of parameters we provide analytical results; we also design a very efficient computational method to calculate the mean, which works for most of the parameter values, and any colony size, no matter how large. We find that a cellular population with a higher death rate will contain a larger number of mutants than a population of equal size with a smaller death rate. Also, a very large population will contain a larger percentage of mutants; that is, irreversible mutations act like a force of selection, even though here the mutants are assumed to have no selective advantage. Finally, we investigate the applicability of the traditional, 'fixed-time' approach and find that it approximates the 'fixed-size' problem whenever stochastic effects are negligible.

Fragmentation study of imatinib and characterization of new imatinib metabolites by liquid chromatography-triple-quadrupole and linear ion trap mass spectrometers

Imatinib (Gleevec) is an anticancer drug that inhibits specific protein kinases involved in cell proliferation. Whereas this drug is considered to have opened a new era, various mechanisms of resistance have been associated with imatinib relapse. Drug disposition in cancer cells including influx, efflux and drug metabolism is one mechanism that remains to be more thoroughly investigated. Moreover, recent genomic studies have revealed that some isozymes of cytochrome P450 (CYP) are possibly associated with the treatment outcome. Therefore, this research paper investigates the role of the activity of CYP1A1, 1A2, 1B1, 3A4, 4F2 and 4F3A/B on the fate of imatinib. First, a study of imatinib fragmentation was effected using electrospray triple-quadrupole and linear ion trap tandem mass spectrometers (MSn). Accurate mass determinations were performed at enhanced mass resolution for the identification of some product ions that were not predicted by two fragmentation softwares. Whereas the quadrupole MS was not designed for accurate mass measurement, delta mass errors were below 20 ppm. Then, a biotransformation study was effected in vitro. Imatinib metabolites were produced in microsomal incubations containing CYP isozymes. Imatinib and metabolites were extracted from incubation mixtures by protein precipitation, and supernatants were injected into a liquid chromatography equipment coupled with MS(n). Hydrophobic interaction liquid chromatography resolved one demethylated-, two hydroxy- and three N-oxide metabolites. Various rates of metabolite formation were observed between CYP isozymes. Liquid chromatography with deuterium oxide-containing mobile phase (H/D exchange) or incorporation of (18)O from H(2) (18)O added in the incubations was performed to elucidate the metabolite structure. Various MS(n) product scans (n < or = 4) were acquired on the linear ion trap or on the triple-quadrupole MS. Postulated structures of new metabolites are addressed.

Strategies for Delaying or Treating In vivo Acquired Resistance to Trastuzumab in Human Breast Cancer Xenografts

PURPOSE

Acquired resistance to trastuzumab (Herceptin) is common in patients whose breast cancers show an initial response to the drug. The basis of this acquired resistance is unknown, hampering strategies to delay or treat such acquired resistance, due in part to the relative lack of appropriate in vivo tumorigenic models.

EXPERIMENTAL DESIGN

We derived an erbB-2-positive variant called 231-H2N, obtained by gene transfection from the highly tumorigenic erbB-2/HER2-negative human breast cancer cell line, MDA-MB-231. Unlike MDA-MB-231, the 231-H2N variants was sensitive to trastuzumab both in vitro and especially in vivo, thus allowing selection of variant resistant to drug treatment in the latter situation after showing an initial response.

RESULTS

The growth of established orthotopic tumors in severe combined immunodeficient mice was blocked for 1 month by trastuzumab, after which rapid growth resumed. These relapsing tumors were found to maintain resistance to trastuzumab, both in vitro and in vivo. We evaluated various therapeutic strategies for two purposes: (a) to delay such tumor relapses or (b) to treat acquired trastuzumab resistance once it has occurred. With respect to the former, a daily oral low-dose metronomic cyclophosphamide regimen was found to be particularly effective. With respect to the latter, an anti-epidermal growth factor receptor antibody (cetuximab) was effective as was the anti-vascular endothelial growth factor (anti-VEGF) antibody bevacizumab, which was likely related to elevated levels of VEGF detected in trastuzumab-resistant tumors.

CONCLUSIONS

Our results provide a possible additional rationale for combined biological therapy using drugs that target both erbB-2/HER2 and VEGF and also suggest the potential value of combining less toxic metronomic chemotherapy regimens not only with targeted antiangiogenic agents but also with other types of drug such as trastuzumab.

Stochastic modeling of drug resistance in cancer

One of the main causes of failure in the treatment of cancer is the development of drug resistance by the cancer cells. Employing multi-drug therapeutic strategies is a promising way to prevent resistance and improve the chances of treatment success. We formulate and analyse a stochastic model for multi-drug resistance and investigate the dependence of treatment outcomes on the initial tumor load, mutation rates and the turnover rate of cancerous cells. We elucidate the general principles of the emergence and evolution of resistant cells inside the tumor, before and after the start of treatment. We discover that for non-mutagenic drugs, pre-existence contributes more to resistance generation than the treatment phase; this result holds for the case where all drugs are applied simultaneously, and is not applicable for sequential therapy models. The application of mathematical modelling to aspects of adjuvant chemotherapy scheduling. J. Math. Biol. 48(4), 375-422]. Also, we find that treatment success is independent on the turnover rate for one drug, and it depends strongly on it for multi-drug therapies. For low-turnover rates, increasing the number of drugs will increase the probability of successful therapy. For very high-turnover rates, increasing the number of drugs used does not significantly increase the chances of treatment success.

Drug resistance in cancer: principles of emergence and prevention

Although targeted therapy is yielding promising results in the treatment of specific cancers, drug resistance poses a problem. We develop a mathematical framework that can be used to study the principles underlying the emergence and prevention of resistance in cancers treated with targeted small-molecule drugs. We consider a stochastic dynamical system based on measurable parameters, such as the turnover rate of tumor cells and the rate at which resistant mutants are generated. We find that resistance arises mainly before the start of treatment and, for cancers with high turnover rates, combination therapy is less likely to yield an advantage over single-drug therapy. We apply the mathematical framework to chronic myeloid leukemia. Early-stage chronic myeloid leukemia was the first case to be treated successfully with a targeted drug, imatinib (Novartis, Basel). This drug specifically inhibits the BCR-ABL oncogene, which is required for progression. Although drug resistance prevents successful treatment at later stages of the disease, our calculations suggest that, within the model assumptions, a combination of three targeted drugs with different specificities might overcome the problem of resistance.

Establishment of a human non-small cell lung cancer cell line resistant to gefitinib

The epidermal growth factor receptor (EGFR) tyrosine-kinase inhibitor gefitinib (Iressa, ZD1839) has shown promising activity preclinically and clinically. Because comparative investigations of drug-resistant sublines with their parental cells are useful approaches to identifying the mechanism of gefitinib resistance and select factors that determine sensitivity to gefitinib, we established a human non-small cell lung carcinoma subline (PC-9/ZD) that is resistant to gefitinib. PC-9/ZD cells are approximately 180-fold more resistant to gefitinib than their parental PC-9 cells and PC-9/ZD cells do not exhibit cross-resistance to conventional anticancer agents or other tyrosine kinase inhibitors, except AG-1478, a specific inhibitor of EGFR. PC-9/ZD cells also display significant resistance to gefitinib in a tumor-bearing animal model. To elucidate the mechanism of resistance, we characterized PC-9/ZD cells. The basal level of EGFR in PC-9 and PC-9/ZD cells was comparable. A deletion mutation was identified within the kinase domain of EGFR in both PC-9 and PC-9/ZD, but no difference in the sequence of EGFR cDNA was detected in either cell line. Increased EGFR/HER2 (and EGFR/HER3) heterodimer formations were demonstrated in PC-9/ZD cells by chemical cross-linking and immunoprecipitation analysis in cells unexposed to gefitinib. Exposure to gefitinib increased heterodimer formation in PC-9 cells, but not in PC-9/ZD cells. Gefitinib inhibits EGFR autophosphorylation in a dose-dependent manner in PC-9 cells but not in PC-9/ZD cells. A marked difference in inhibition of site-specific phosphorylation of EGFR was observed at Tyr1068 compared to other tyrosine residues (Tyr845, 992 and 1045). To elucidate the downstream signaling in the PC9/ZD cellular machinery, complex formation between EGFR and its adaptor proteins GRB2, SOS, and Shc was examined. A marked reduction in the GRB2-EGFR complex and absence of SOS-EGFR were observed in PC-9/ZD cells, even though the protein levels of GRB2 and SOS in PC-9 and PC-9/ZD cells were comparable. Expression of phosphorylated AKT was increased in PC-9 cells and inhibited by 0.02 microM gefitinib. But the inhibition was not significant in PC-9/ZD cells. These results suggest that alterations of adaptor-protein-mediated signal transduction from EGFR to AKT is a possible mechanism of the resistance to gefitinib in PC-9/ZD cells. These phenotypes including EGFR-SOS complex and heterodimer formation of HER family members are potential biomarkers for predicting resistance to gefitinib.

Role of Cytochrome P450 Activity in the Fate of Anticancer Agents and in Drug Resistance

Although activity of cytochrome P450 isoenzymes (CYPs) plays a major role in the fate of anticancer agents in patients, there are relatively few clinical studies that evaluate drug metabolism with therapeutic outcome. Nevertheless, many clinical reports in various non-oncology fields have shown the dramatic importance of CYP activity in therapeutic efficacy, safety and interindividual variability of drug pharmacokinetics. Moreover, variability of drug metabolism in the liver as well as in cancer cells must also be considered as a potential factor mediating cancer resistance. This review underlines the role of drug metabolism mediated by CYPs in pharmacokinetic variability, drug resistance and safety. As examples, biotransformation pathways of tamoxifen, paclitaxel and imatinib are reviewed. This review emphasises the key role of therapeutic drug monitoring as a complementary tool of investigation to in vitro data. For instance, pharmacokinetic data of anticancer agents have not often been published within subpopulations of patients who show ultra-rapid, extensive or poor metabolism (e.g. due to CYP2D6 and CYP2C19 genotypes). Besides kinetic variability in the systemic circulation, induction of CYP activity may participate in creating drug resistance by speeding up the cancer agent degradation specifically in the target cells. For one cancer agent, various mechanisms of resistance are usually identified within different cell clones. This review also tries to emphasise that drug resistance mediated by CYP activity in cancer cells should be taken into consideration to a greater degree. The unequivocal identification of the metabolising enzymes involved in clinical conditions will eventually allow improvement and individualisation of anticancer agent therapy, i.e. drug dosage and selection. In addition, a more complete understanding of the metabolism of anticancer agents will assist in the prediction of drug-drug interactions, as anticancer agent combinations are becoming more prevalent.

Simultaneous blockade of platelet-derived growth factor-receptor and epidermal growth factor-receptor signaling and systemic administration of paclitaxel as therapy for human prostate cancer metastasis in bone of nude mice

Once prostate cancer metastasizes to bone, conventional chemotherapy is largely ineffective. We hypothesized that inhibition of phosphorylation of the epidermal growth factor receptor (EGF-R) and platelet-derived growth factor receptor (PDGF-R) expressed on tumor cells and tumor-associated endothelial cells, which is associated with tumor progression, in combination with paclitaxel would inhibit experimental prostate cancer bone metastasis and preserve bone structure. We tested this hypothesis in nude mice, using human PC-3MM2 prostate cancer cells. PC-3MM2 cells growing adjacent to bone tissue and endothelial cells within these lesions expressed phosphorylated EGF-R and PDGF-R alpha and -beta on their surfaces. The percentage of positive endothelial cells and the intensity of receptor expression directly correlated with proximity to bone tissue. Oral administration of PKI166 inhibited the phosphorylation of EGF-R but not PDGF-R, whereas oral administration of STI571 inhibited the phosphorylation of PDGF-R but not EGF-R. Combination therapy using oral PKI166 and STI571 with i.p. injections of paclitaxel induced a high level of apoptosis in tumor vascular endothelial cells and tumor cells in parallel with inhibition of tumor growth in the bone, preservation of bone structure, and reduction of lymph node metastasis. Collectively, these data demonstrate that blockade of phosphorylation of EGF-R and PDGF-R coupled with administration of paclitaxel significantly suppresses experimental human prostate cancer bone metastasis.

Mutation of threonine 766 in the epidermal growth factor receptor reveals a hotspot for resistance formation against selective tyrosine kinase inhibitors

Small molecule inhibitors of protein tyrosine kinases such as STI571 represent a major new class of therapeutics for target-selective treatment of human cancer. Clinical resistance formation to the BCR-ABL inhibitor STI571 has been observed in patients with advanced chronic myeloid leukemia and was frequently caused by a C to T single nucleotide change in the Abl kinase domain, which substituted Thr-315 with isoleucine and rendered BCR-ABL resistant to STI571 inhibition. The corresponding mutation in the epidermal growth factor receptor (EGFR) tyrosine kinase replaced Thr-766 of the EGFR by methionine and dramatically reduced the sensitivity of EGFR to inhibition by selective 4-anilinoquinazoline inhibitors such as PD153035. Inhibitor-resistant EGFR exhibited the same signaling capacity as wild-type receptor in vivo and provides a useful tool for analyzing EGFR-mediated signal transduction. Our data identify Thr-766 of the EGFR as a structural determinant that bears the potential to become a relevant feature in resistance formation during cancer therapy with EGFR-specific 4-anilinoquinazoline inhibitors.

Effects of blocking platelet-derived growth factor-receptor signaling in a mouse model of experimental prostate cancer bone metastases

BACKGROUND

Expression of platelet-derived growth factor (PDGF) and activation (by autophosphorylation) of its receptor (PDGF-R), a tyrosine kinase, are associated with the growth of metastatic prostate tumor cells in the bone parenchyma. The tyrosine kinase inhibitor STI571 blocks the PDGF signaling pathway by inhibiting PDGF-R autophosphorylation. We examined the effects of STI571, given alone or with paclitaxel (Taxol), on tumor growth in a mouse model of prostate cancer metastasis.

METHODS

Human prostate cancer PC-3MM2 cells were injected into the tibias of male nude mice. Three days later the mice (20 per group) were randomly assigned to 5 weeks of treatment with oral and injected water (control), daily oral STI571, weekly injected paclitaxel, or STI571 plus paclitaxel. Lesions in bone and the surrounding muscles were then harvested and analyzed by histology, western blotting (for PDGF-R phosphorylation), immunohistochemistry (for expression of proangiogenic molecules), and double immunofluorescence (to identify endothelial cells and apoptotic tumor cells). Growth of bone lesions was monitored by digital radiography. Bone lesions from control mice were used to establish short-term cell cultures for analysis of PDGF-R phosphorylation. All statistical tests were two-sided.

RESULTS

PC-3MM2 cells cultured from bone lesions and treated in vitro with STI571 had less phosphorylated PDGF-R than untreated cells. In control mice, bone lesions expressed high levels of PDGF and activated (i.e., phosphorylated) PDGF-R, whereas lesions in the adjacent musculature did not. Activated PDGF-R was present on the surface of endothelial cells within the bone lesions but not in endothelial cells of uninjected bone. Mice treated with STI571 or STI571 plus paclitaxel had a lower tumor incidence, smaller tumors, and less bone lysis and lymph node metastasis than mice treated with water or paclitaxel alone (P<.001 for all). Mice treated with STI571 or STI571 plus paclitaxel had less phosphorylated PDGF-R on tumor cells and tumor-associated endothelial cells, less tumor cell proliferation, statistically significantly more apoptotic tumor cells (all P<.001), and fewer tumor-associated endothelial cells (P<.001) than control mice.

CONCLUSIONS

Endothelial cells appear to express phosphorylated PDGF-R when they are exposed to tumor cells that express PDGF. Using STI571 to inhibit PDGF-R phosphorylation may, especially in combination with paclitaxel, produce substantial therapeutic effects against prostate cancer bone metastasis.

A paradigm for therapy-induced microenvironmental changes in solid tumors leading to drug resistance

Intrinsic alterations in the tumor microenvironment are known to contribute to various forms of drug resistance. For example, tumor hypoxia, due to abnormal or sluggish blood flow within areas of solid tumors, can result in both microenvironment-mediated radiation and chemotherapeutic drug resistance. In contrast, acquired resistance to chemotherapy is generally considered to be the result of the gradual selection of mutant subpopulations having genetic mutations and biochemical alterations responsible for the resistant phenotype. Here we present a paradigm for therapyinduced microenvironment-mediated acquired drug resistance. It is based on the results showing that tumor cells appear to be heterogeneous in their relative dependence on adjacent tumor-associated vasculature for survival. Some tumor cells are highly vessel dependent, whereas some are significantly less so, and thus can survive in more hypoxic regions of tumors, distal from such tumor vessels. Hence, it is possible that variant tumor cells that are less vessel dependent may therefore be selected for over time by successful antiangiogenic drug therapies. This results in loss of response or attenuated responses to the therapy. Preliminary evidence is summarized in support of this hypothesis, using paired human colon cancer (HCT116) cell lines that contain two copies of either the wild-type or the disrupted p53 tumor suppressor gene. The mutant cells were found to be less responsive to antiangiogenic therapy, compared to the wild-type cells, and could be progressively selected for in mixed cell populations. Because p53 inactivation can lead to resistance to hypoxia-mediated apoptosis, the results suggest that a protracted and successful antiangiogenic therapy may create more hypoxic tumor microenvironments, thereby creating the necessary conditions to accelerate the selection of mutant tumor cells that are more adept in surviving and growing in such environments. As such, consideration might be given to the combined use of bioreductive hypoxic cell cytotoxic drugs and angiogenesis inhibitors to prolong the efficacy of antiangiogenic therapeutics.

The emergence of resistance to targeted cancer therapeutics

Drug resistance has always been a concern in cancer treatment, often blamed on the genetic complexity and instability of tumor cells. While studies of cancer cell lines have implicated an array of potential mechanisms, it has been difficult to translate these insights into clinically meaningful improvements in cancer treatment. The successful deployment of molecularly targeted therapeutics in some cancers has led to widespread optimism that this approach will become broadly applicable. Despite their early promise in the clinic, the novel therapeutics are often plagued with the age old problem of acquired drug resistance. Progress in understanding why certain patients respond and why some develop resistance can be made rapidly through studies of the drug target in tumor tissue from patient. One important lesson is that many cancers, even in the most advanced stages, continue to rely on a limited number of critical oncogenic signals for maintenance of the malignant phenotype. This article reviews the mechanisms of drug resistance to a variety of cancer therapeutics and provides an approach for how measures of drug target activity can be incorporated into clinical trial design.

NF-kappa B: arresting a major culprit in cancer

Nuclear factor-kappa B (NF-kappa B) has long been known to play a central role in the immune system by regulating the expression of key genes. Moreover, activation of this transcription factor helps a wide variety of cell types survive damage induced by pro-apoptotic stimuli. Because of its crucial role in the regulation of pro-inflammatory genes, NF-kappa B is a promising target for the discovery of anti-inflammatory drugs. More recently, NF-kappa B has also emerged as a major culprit in a variety of human cancers mainly because of its ability to protect transformed cells from apoptosis. The pharmaceutical industry should, therefore, seriously consider testing inhibitors of NF-kappa B, identified as part of their anti-inflammatory drug discovery programs, in combination with other chemotherapeutic drugs in models of cancer.

STI-571 must select for drug-resistant cells but 'no cell breathes fire out of its nostrils like a dragon'

Seemingly disappointing, the Bcr-Abl kinase inhibitor STI-571 shares an 'unfortunate' characteristic with conventional cancer drugs: the development of drug resistance. I argue that the resistance must develop even faster to STI-571 than to conventional drugs, because STI-571 is so effective. This is predictable, but is it inevitable? And how do mechanisms of resistance in relapse depend on a degree of remission. In addition to mutation rate and number of tumor cells, one additional factor determines relapse vs. 'extinction' of the leukemia cell population.

Autoinhibition of c-Abl

Despite years of investigation, the molecular mechanism responsible for regulation of the c-Abl tyrosine kinase has remained elusive. We now report inhibition of the catalytic activity of purified c-Abl in vitro, demonstrating that regulation is an intrinsic property of the molecule. We show that the interaction of the N-terminal 80 residues with the rest of the protein mediates autoregulation. This N-terminal "cap" is required to achieve and maintain inhibition, and its loss turns c-Abl into an oncogenic protein and contributes to deregulation of BCR-Abl.

Cancer gene therapy: fringe or cutting edge?

Direct targeting of cancer cells with gene therapy has the potential to treat cancer on the basis of its molecular characteristics. But although laboratory results have been extremely encouraging, many practical obstacles need to be overcome before gene therapy can fulfil its goals in the clinic. These issues are not trivial, but seem less formidable than the challenge of killing cancers selectively and rationally--a challenge that has been successfully addressed.

Origin of multidrug resistance in cells with and without multidrug resistance genes: chromosome reassortments catalyzed by aneuploidy

Cancer cells and aneuploid cell lines can acquire resistance against multiple unrelated chemotherapeutic drugs that are over 3,000-fold those of normal levels and display spontaneous resistances up to 20-fold of normal levels. Two different mechanisms were proposed for this phenotype: (i) classical mutation of drug metabolizing genes or (ii) chromosome reassortments, catalyzed by cancer- and cell line-specific aneuploidy, which generate, via new gene dosage combinations, a plethora of cancer phenotypes, including drug resistance. To distinguish between these mechanisms, we have asked whether three mouse cell lines can become drug resistant, from which two or three genes have been deleted, and on which multidrug resistance is thought to depend: Mdr1a, Mdr1b, and Mrp1. Because all three lines could acquire multidrug resistance and were aneuploid, whereas diploid mouse cells could not, we conclude that aneuploid cells become drug resistant via specific chromosome assortments, independent of putative resistance genes. We have asked further whether aneuploid drug-resistant Chinese hamster cells revert spontaneously to drug sensitivity in the absence of cytotoxic drugs at the high rates that are typical of chromosome reassortments catalyzed by aneuploidy or at the very low or zero rates (i.e., deletion) of gene mutation. We found that four drug-resistant hamster cell lines reverted to drug sensitivity at rates of about 2-3% per generation, whereas two closely related lines remained resistant under our conditions. Thus, the karyotypic instability generated by aneuploidy emerges as the common source of the various levels of drug resistance of cancer cells: minor spontaneous resistances reflect accidental chromosome assortments, the high selected resistances reflect complex specific assortments, and multidrug resistance reflects new combinations of unselected genes located on the same chromosomes as selected genes.