Tumor physiology and resistance to chemotherapy: repopulation and drug penetration

Synthetic engineered bacteria for cancer therapy

INTRODUCTION

Cancer mortality worldwide highlights the urgency for advanced therapeutic methods to fill the gaps in conventional cancer therapies. Bacteriotherapy is showing great potential in tumor regression due to the motility and colonization tendencies of bacteria. However, the complicated in vivo environment and tumor pathogenesis hamper the therapeutic outcomes. Synthetic engineering methods endow bacteria with flexible abilities both at the extracellular and intracellular levels to meet treatment requirements. In this review, we introduce synthetic engineering methods for bacterial modifications. We highlight the recent progress in engineered bacteria and explore how these synthetic methods endow bacteria with superior abilities in cancer therapy. The current clinical translations are further discussed. Overall, this review may shed light on the advancement of engineered bacteria for cancer therapy.

AREAS COVERED

Recent progress in synthetic methods for bacterial engineering and specific examples of their applications in cancer therapy are discussed in this review.

EXPERT OPINION

Bacteriotherapy bridges the gaps of conventional cancer therapies through the natural motility and colonization tendency of bacteria, as well as their synthetic engineering. Nevertheless, to fulfill the bacteriotherapy potential and move into clinical trials, more research focusing on its safety concerns should be conducted.

Microbial cancer therapeutics: A promising approach

The success of conventional cancer therapeutics is hindered by associated dreadful side-effects of antibiotic resistance and the dearth of antitumor drugs' selectivity and specificity. Hence, the conceptual evolution of anti-cancerous therapeutic agents that selectively target cancer cells without impacting the healthy cells or tissues, has led to a new wave of scientific interest in microbial-derived bioactive molecules. Such strategic solutions may pave the way to surmount the shortcomings of conventional therapies and raise the potential and hope for the cure of wide range of cancer in a selective manner. This review aims to provide a comprehensive summary of anti-carcinogenic properties and underlying mechanisms of bioactive molecules of microbial origin, and discuss the current challenges and effective therapeutic application of combinatorial strategies to attain minimal systemic side-effects.

Drug penetration in pediatric brain tumors: Challenges and opportunities

Larger clinical trial enrollments and a greater understanding of biological heterogeneity have led to improved survival rates for children diagnosed with brain tumors in the last 50 years. However, reducing long-term morbidities and improving survival rates of high-risk tumors remain major challenges. Chemotherapy can reduce tumor burden, but effective drug penetration at the tumor site is limited by barriers in the route of drug administration and within the tumor microenvironment. Bioavailability of drugs is impeded by the blood-brain barrier, plasma protein binding, and structural components by the tumor including the matrix and vasculature contributing to increased interstitial fluid pressure, hypoxia, and acidity. Designing drug delivery systems to circumvent these barriers could lead to improved drug penetration at the tumor site and reduce adverse systemic side effects. In this review, we expand on how systemic and local barriers limit drug penetration and present potential methods to enhance drug penetration in pediatric brain tumors.

Therapeutic bacteria to combat cancer; current advances, challenges, and opportunities

Successful treatment of cancer remains a challenge, due to the unique pathophysiology of solid tumors, and the predictable emergence of resistance. Traditional methods for cancer therapy including radiotherapy, chemotherapy, and immunotherapy all have their own limitations. A novel approach is bacteriotherapy, either used alone, or in combination with conventional methods, has shown a positive effect on regression of tumors and inhibition of metastasis. Bacteria-assisted tumor-targeted therapy used as therapeutic/gene/drug delivery vehicles has great promise in the treatment of tumors. The use of bacteria only, or in combination with conventional methods was found to be effective in some experimental models of cancer (tumor regression and increased survival rate). In this article, we reviewed the major advantages, challenges, and prospective directions for combinations of bacteria with conventional methods for tumor therapy.

Targeted inactivation of Salmonella Agona metabolic genes by group II introns and in vivo assessment of pathogenicity and anti-tumour activity in mouse model

The fight against cancer has been a never-ending battle. Limitations of conventional therapies include lack of selectivity, poor penetration and highly toxic to the host. Using genetically modified bacteria as a tumour therapy agent has gained the interest of scientist from the past few decades. Low virulence and highly tolerability of Salmonella spp. in animals and humans make it as the most studied pathogen with regards to anti-tumour therapy. The present study aims to construct a genetically modified S. Agona auxotroph as an anti-tumour agent. LeuB and ArgD metabolic genes in ΔSopBΔSopD double knockout S. Agona were successfully knocked out using a Targetron gene knockout system. The knockout was confirmed by colony PCR and the strains were characterized in vitro and in vivo. The knockout of metabolic genes causes significant growth defect in M9 minimal media. Quadruple knockout ΔSopBΔSopDΔLeuBΔArgD (BDLA) exhibited lowest virulence among all of the strains in all parameters including bacterial load, immunity profile and histopathology studies. In vivo anti-tumour study on colorectal tumour bearing-BALB/c mice revealed that all strains of S. Agona were able to suppress the growth of the large solid tumour as compared with negative control and ΔLeuBΔArgD (LA) and BDLA auxotroph showed better efficacy. Interestingly, higher level of tumour growth suppression was noticed in large tumour. However, multiple administration of bacteria dosage did not increase the tumour suppression efficacy. In this study, the virulence of BDLA knockout strain was slightly reduced and tumour growth suppression efficacy was successfully enhanced, which provide a valuable starting point for the development of S. Agona as anti-tumour agent.

Relationship between the SER treatment period and prognosis of patients with small cell lung cancer

PURPOSE

To explore the relationship between SER (time between the start of any treatment and the end of radiation therapy) and the survival of patients with limited-stage small cell lung cancer.

MATERIALS AND METHODS

Between 2008 and 2013, 135 cases of limited-stage small cell lung cancer (LS-SCLC) treated with consecutively curative chemoradiotherapy were included in this retrospective analysis. In terms of SER, patients were divided into early radiotherapy group (SER<30 days, n=76) and late radiotherapy group (SER≥30 days, n=59) with a cut- off of SER 30 days. Outcomes of the two groups were compared for overall survival.

RESULTS

For all analyzable patients, median follow-up time was 23.8 months and median overall survival time was 16.8 months. Although there was no significant differences in distant metastasis free survival between the two groups, patients in early radiotherapy group had a significantly better PFS (p=0.003) and OS (p=0.000).

CONCLUSIONS

A short SER may be a good prognostic factor for LD-SCLC patients treated with concurrent chemoradiotherapy.

Role of difucosylated Lewis Y antigen in outcome of locally advanced cervical squamous cell carcinoma treated with cisplatin regimen

BACKGROUND

Several mechanisms are involved in the development of resistance to therapy in locally advanced cervical squamous cell carcinoma (LACSCC). Studies have shown that CD44 and Lewis Y antigen (LeY) form a complex that is associated with chemoresistance, tumor invasion and metastasis. We assessed the role of CD44 and LeY in the outcome of LACSCC patients treated with different chemotherapy regimens.

METHODS

126 LACSCC patients at FIGO stages IIB-IVA were selected from the GOCS database: 74 patients included in 3 different prospective phase II trials in the neoadjuvant setting (vinorelbine, docetaxel, ifosfamide-vinorelbine-cisplatin) and 52 patients treated with standard radiochemotherapy based on cisplatin (RCBC). Clinical data at baseline, disease-free survival (DFS) and overall survival (OS) were recorded. Univariate and multivariate Cox models were employed.

RESULTS

Median age was 45.6 years (range: 24.9-80.5). Sixty-three and 47 tumors were CD44+ and LeY+, respectively. Tumors with expansive growth showed higher grade (p = 0.0024), mitotic index (p = 0.0505), tumor necrosis (p = 0.0191), LeY+ (p = 0.0034) and CD44+/LeY+ coexpression (p = 0.0334). CD44+ cells were present in 91.3% of patients with local recurrence (p = 0.0317). Advanced stage was associated with LeY+ tumors. Patients treated with RCBC had worse DFS and OS when their tumors expressed LeY (p = 0.0083 and p = 0.0137, respectively). Pre-treatment hemoglobin level, FIGO stage and tumor response remained the most significant prognostic factors in Cox regression.

CONCLUSIONS

In our cohort of LACSCC patients, the coexpression of CD44 and LeY was not associated with worse outcome. However, in the subgroup of patients receiving RCBC, LeY expression was correlated with shorter DFS and OS.

Intermittent tri-weekly docetaxel plus bicalutamide in patients with castration-resistant prostate cancer: a single-arm prospective study using a historical control for comparison

Whether continuous docetaxel (DTX) chemotherapy offers an advantage over intermittent therapy for castration-resistant prostate cancer (CRPC) is unknown. In this study, we evaluated the efficacy, toxicity and quality of life (QoL) of intermittent tri-weekly DTX with bicalutamide in CRPC. Forty-two patients (group A) with CRPC were enrolled. The patients received intravenous DTX (75 mg m(-2)) once tri-weekly with oral bicalutamide (50 mg) once daily. Patients had a DTX holiday when the prostate-specific antigen (PSA) level declined ≥50%. DTX was restarted in patients with a PSA increase ≥25%. Sixty patients (group B) who had matching characteristics and had continuously received DTX without bicalutamide for 10-12 cycles were also enrolled. There were no statistically significant differences in progression-free survival (8 months vs. 9 months, P=0.866) or overall survival (19 months vs. 21 months, P=0.753) between groups A and B; however, the proportions of patients in group A with all grades of neutropenia (33% vs. 58%, P=0.013) and nausea/vomiting (11% vs. 29%, P=0.024) were significantly less compared to group B. A significant improvement in the global health and fatigue scores was recorded for group A post-chemotherapy compared to pre-chemotherapy (P<0.05). The fatigue, nausea/vomiting and appetite loss scores in group B were increased post-chemotherapy compared to pre-chemotherapy (P<0.05). In conclusion, intermittent tri-weekly DTX plus bicalutamide is well tolerated and has the potential to achieve comparable disease control with an improvement in QoL for patients with CRPC.

Repopulation of ovarian cancer cells after chemotherapy

The high mortality rate caused by ovarian cancer has not changed for the past thirty years. Although most patients diagnosed with this disease respond to cytoreductive surgery and platinum-based chemotherapy and undergo remission, foci of cells almost always escape therapy, manage to survive, and acquire the capacity to repopulate the tumor. Repopulation of ovarian cancer cells that escape front-line chemotherapy, however, is a poorly understood phenomenon. Here I analyze cancer-initiating cells, transitory senescence, reverse ploidy, and cellular dormancy as putative players in ovarian cancer cell repopulation. As part of the standard of care, ovarian cancer patients do not receive treatment between primary cytotoxic therapy and clinical relapse. Understanding the mechanisms driving cellular escape from chemotherapy should lead to the development of low toxicity, chronic treatment approaches that can be initiated right after primary therapy to interrupt cell repopulation and disease relapse by keeping it dormant and, therefore, subclinical.

Motility is critical for effective distribution and accumulation of bacteria in tumor tissue

Motile bacteria can overcome the penetration limitations of cancer chemotherapeutics because they can actively migrate into solid tumors. Although several genera of bacteria have been shown to accumulate preferentially in tumors, the spatiotemporal dynamics of bacterial tumor colonization and their dependence on bacterial motility are not clear. For effective tumor regression, bacteria must penetrate and distribute uniformly throughout tumors. To measure these dynamics, we used an in vitro model of continuously perfused tumor tissue to mimic the delivery and systemic clearance of Salmonella typhimurium strains SL1344 and VNP20009, and Escherichia coli strains K12 and DH5α. Tissues were treated for 1 hour with 10(5) or 10(7) CFU ml(-1) suspensions of each strain and the location and extent of bacterial accumulation were observed for 30 hours. Salmonella had 14.5 times greater average swimming speed than E. coli and colonized tissues at 100 times lower doses than E. coli. Bacterial motility strongly correlated (R(2) = 99.3%) with the extent of tissue accumulation. When inoculated at 10(5) CFU ml(-1), motile Salmonella formed colonies denser than 10(10) CFU/(g-tissue) and less motile E. coli showed no detectable colonization. Based on spatiotemporal profiles and a mathematical model of motility and growth, bacterial dispersion was found to be necessary for deep penetration into tissue. Bacterial colonization caused apoptosis in tumors and apoptosis levels correlated (R(2) = 98.6%) with colonization density. These results show that motility is critical for effective distribution of bacteria in tumors and is essential for designing cancer therapies that can overcome the barrier of limited tumor penetration.

Modulating pharmacokinetics, tumor uptake and biodistribution by engineered nanoparticles

Background

Inorganic nanoparticles provide promising tools for biomedical applications including detection, diagnosis and therapy. While surface properties such as charge are expected to play an important role in their in vivo behavior, very little is known how the surface chemistry of nanoparticles influences their pharmacokinetics, tumor uptake, and biodistribution.

Method/Principal Findings

Using a family of structurally homologous nanoparticles we have investigated how pharmacological properties including tumor uptake and biodistribution are influenced by surface charge using neutral (TEGOH), zwitterionic (Tzwit), negative (TCOOH) and positive (TTMA) nanoparticles. Nanoparticles were injected into mice (normal and athymic) either in the tail vein or into the peritoneum.

Conclusion

Neutral and zwitterionic nanoparticles demonstrated longer circulation time via both IP and IV administration, whereas negatively and positively charged nanoparticles possessed relatively short half-lives. These pharmacological characteristics were reflected on the tumor uptake and biodistribution of the respective nanoparticles, with enhanced tumor uptake by neutral and zwitterionic nanoparticles via passive targeting.

Nanoscale Drug Delivery and Hyperthermia: The Materials Design and Preclinical and Clinical Testing of Low Temperature-Sensitive Liposomes Used in Combination with Mild Hyperthermia in the Treatment of Local Cancer

The overall objective of liposomal drug delivery is to selectively target drug delivery to diseased tissue, while minimizing drug delivery to critical normal tissues. The purpose of this review is to provide an overview of temperature-sensitive liposomes in general and the Low Temperature-Sensitive Liposome (LTSL) in particular. We give a brief description of the material design of LTSL and highlight the likely mechanism behind temperature-triggered drug release. A complete review of the progress and results of the latest preclinical and clinical studies that demonstrate enhanced drug delivery with the combined treatment of hyperthermia and liposomes is provided as well as a clinical perspective on cancers that would benefit from hyperthermia as an adjuvant treatment for temperature-triggered chemotherapeutics. This review discusses the ideas, goals, and processes behind temperature-sensitive liposome development in the laboratory to the current use in preclinical and clinical settings.

Bacterial therapies: completing the cancer treatment toolbox

Current cancer therapies have limited efficacy because they are highly toxic, ineffectively target tumors, and poorly penetrate tumor tissue. Engineered bacteria have the unique potential to overcome these limitations by actively targeting all tumor regions and delivering therapeutic payloads. Examples of transport mechanisms include specific chemotaxis, preferred growth, and hypoxic germination. Deleting the ribose/galactose chemoreceptor has been shown to cause bacterial accumulation in therapeutically resistant tumor regions. Recent advances in engineered therapeutic delivery include temporal control of cytotoxin release, enzymatic activation of pro-drugs, and secretion of physiologically active biomolecules. Bacteria have been engineered to express tumor-necrosis-factor-alpha, hypoxia-inducible-factor-1-alpha antibodies, interleukin-2, and cytosine deaminase. Combining these emerging targeting and therapeutic delivery mechanisms will yield a complete treatment toolbox and increase patient survival.

iNOS as a therapeutic target for treatment of human tumors

Nitric oxide synthase (NOS) has been shown to be overexpressed in a number of human tumors compared to normal tissues and therefore potentially represents an exploitable target in future anticancer therapies. To achieve this, there will be a need to profile tumors to identify those expressing high levels of NOS; alternatively, endogenous (low) levels of NOS could be modulated by induction or through gene therapy approaches. NOS consists of a reductase domain which shares a high degree of sequence homology with P450 reductase and this domain supplies reducing equivalents to a haem containing oxygenase domain that is responsible for the production of nitric oxide. Thus, there are a number of routes of exploitation. Firstly, to take advantage of the reductase domain to activate bioreductive drugs as has been exemplified with tirapazamine and now extended to AQ4N (1,4-bis{2-(dimethylamino-N-oxide)ethylamino}5,8-dihydroxy-anthracene-9,10-dione). Secondly, to take advantage of nitric oxide production for its ability to increase the sensitivity of resistant hypoxic cells to radiation. Lastly, to utilize inhibition of HIF-1 to amplify NO based therapies. In this review we provide examples/evidence of how these objectives can be achieved.

The effect of the histological properties of tumors on transfection efficiency of electrically assisted gene delivery to solid tumors in mice

Uniform DNA distribution in tumors is a prerequisite step for high transfection efficiency in solid tumors. To improve the transfection efficiency of electrically assisted gene delivery to solid tumors in vivo, we explored how tumor histological properties affected transfection efficiency. In four different tumor types (B16F1, EAT, SA-1 and LPB), proteoglycan and collagen content was morphometrically analyzed, and cell size and cell density were determined in paraffin-embedded tumor sections under a transmission microscope. To demonstrate the influence of the histological properties of solid tumors on electrically assisted gene delivery, the correlation between histological properties and transfection efficiency with regard to the time interval between DNA injection and electroporation was determined. Our data demonstrate that soft tumors with larger spherical cells, low proteoglycan and collagen content, and low cell density are more effectively transfected (B16F1 and EAT) than rigid tumors with high proteoglycan and collagen content, small spindle-shaped cells and high cell density (LPB and SA-1). Furthermore, an optimal time interval for increased transfection exists only in soft tumors, this being in the range of 5-15 min. Therefore, knowledge about the histology of tumors is important in planning electrogene therapy with respect to the time interval between DNA injection and electroporation.

Timing of chest radiotherapy in patients with limited stage small cell lung cancer: a systematic review and meta-analysis of randomised controlled trials

BACKGROUND

We undertook a systematic review and literature-based meta-analysis to determine whether the timing of chest radiotherapy may influence the survival of patients with limited stage small cell lung cancer (LS-SCLC).

OBJECTIVES

To establish the most effective way of combining chest radiotherapy with chemotherapy for patients with limited-stage small cell lung cancer in order to improve long-term survival.

MATERIALS

Eligible studies were identified according to the Cochrane Collaboration Guidelines and were randomised controlled clinical trials comparing different timing of chest radiotherapy in patients with LS-SCLC. Early chest irradiation was defined as beginning within 30 days after the start of chemotherapy.

RESULTS

Seven randomised trials were eligible. The overall survival at 2 years or at 5 years was not significantly different between early or late chest radiotherapy. When only trials were considered that used platinum chemotherapy concurrent with chest radiotherapy, significantly higher 2 and 5-year survival rates were observed when chest radiotherapy (RT) was started within 30 days after the start of chemotherapy (2-year survival: HR: 0.73, 95% CI 0.57-0.94, p=0.01; 5-year survival: HR: 0.65, 95% CI 0.45-0.93, p=0.02). This was even more pronounced when the overall treatment time of chest radiotherapy was less than 30 days. In studies that did not show a survival advantage by early chest radiation, a lower dose-intensity of chemotherapy in the early vs. late arm was observed.

CONCLUSIONS

When platinum-based chemotherapy concurrently with chest RT is used, the 2- and 5-year survival rates of patients with LS-SCLC may be in favour of early chest radiotherapy, with a significant difference if the overall treatment time of chest radiation is less than 30 days.

Systematic review and meta-analysis of randomised, controlled trials of the timing of chest radiotherapy in patients with limited-stage, small-cell lung cancer

BACKGROUND

We undertook a systematic review and literature-based meta-analysis to determine whether the timing of chest radiotherapy may influence the survival of patients with limited-stage small-cell lung cancer (LS-SCLC).

MATERIALS

Eligible randomised controlled clinical trials were identified according to the Cochrane Collaboration Guidelines, comparing different timing of chest radiotherapy in patients with LS-SCLC. Early chest irradiation was defined as beginning within 30 days after the start of chemotherapy.

RESULTS

Considering all seven eligible trials, the overall survival at 2 or 5 years was not significantly different between early or late chest radiotherapy. When only trials were considered that used platinum chemotherapy concurrent with chest radiotherapy, a significantly higher 5-year survival was observed when chest radiotherapy was started within 30 days after the start of chemotherapy (2-year survival: OR: 0.73, 95% CI 0.51-1.03, P = 0.07; 5-year survival: OR: 0.64, 95% CI 0.44-0.92, P = 0.02). This was even more pronounced when the overall treatment time of chest radiotherapy was less than 30 days.

CONCLUSIONS

There are indications that the 5-year survival rates of patients with LS-SCLC are in favour of early chest radiotherapy, with a significant difference if the overall treatment time of chest radiation is less than 30 days.

Time between the first day of chemotherapy and the last day of chest radiation is the most important predictor of survival in limited-disease small-cell lung cancer

PURPOSE

To identify time factors for combined chemotherapy and radiotherapy predictive for long-term survival of patients with limited-disease small-cell lung cancer (LD-SCLC).

METHODS

A systematic overview identified suitable phase III trials. Using meta-analysis methodology to compare results within trials, the influence of the timing of chest radiation and the start of any treatment until the end of radiotherapy (SER) on local tumor control, survival, and esophagitis was analyzed. For comparison between studies, the equivalent radiation dose in 2-Gy fractions, corrected for the overall treatment time of chest radiotherapy, was analyzed.

RESULTS

The SER was the most important predictor of outcome. There was a significantly higher 5-year survival rate in the shorter SER arms (relative risk [RR] = 0.62; 95% CI, 0.49 to 0.80; P = .0003), which was more than 20% when the SER was less than 30 days (upper bound of 95% CI, 90 days). A low SER was associated with a higher incidence of severe esophagitis (RR = 0.55; 95% CI, 0.42 to 073; P < .0001). Each week of extension of the SER beyond that of the study arm with the shortest SER resulted in an overall absolute decrease in the 5-year survival rate of 1.83% +/- 0.18% (95% CI).

CONCLUSION

A low time between the first day of chemotherapy and the last day of chest radiotherapy is associated with improved survival in LD-SCLC patients. The novel parameter SER, which takes into account accelerated proliferation of tumor clonogens during both radiotherapy and chemotherapy, may facilitate a more rational design of combined-modality treatment in rapidly proliferating tumors.

Salmonella typhimurium specifically chemotax and proliferate in heterogeneous tumor tissue in vitro

Multi-drug resistance greatly limits the efficacy of conventional blood-born chemotherapeutics, which have limited ability to penetrate tumor tissue and are ineffective at killing quiescent cells far from tumor vasculature. Nonpathogenic, motile bacteria can overcome both of theses limitations. We hypothesize that the accumulation of S. typhimurium in tumors is controlled by two mechanisms: (1) chemotaxis towards compounds produced by quiescent cancer cells and (2) preferential growth within tumor tissue. We tested this hypothesis by quantifying the relative contributions of these mechanisms using the tumor cylindroid model, which mimics the microenvironments of in vivo tumors. Time-lapse fluorescence microscopy was used to measure the accumulation of GFP-labeled S. typhimurium into cylindroids of different size. Cylindroids larger than 500 microm in diameter contain quiescent cells, whereas cylindroids smaller than 500 microm do not. Spatio-temporal profiles of bacterial concentration were fit to a mathematical model to calculate two parameters that describe bacterial interaction with tumors: intratumoral bacterial growth, M, and intratumoral bacterial chemoattraction, K. It was observed that S. typhimurium is attracted to cylindroids and accumulate at long time points in the central region of large cylindroids. Both intratumoral bacterial growth and chemotaxis were significantly greater in large cylindroids, suggesting that quiescent cells secrete bacterial chemoattractants and the presence of necrotic and quiescent cells enable S. typhimurium to replicate in tumor tissue. In this study, several mechanisms of S. typhimurium accumulation in solid tumors have been quantified, which we believe is an important step in the development of bacterial-based therapeutics to target tumor quiescence.

Targeted therapies in breast cancer

Targeted therapeutic agents in breast cancer are representing a larger proportion of new drugs entering clinical testing. Carcinogenesis is a multistep process characterized by genetic alterations that influence key cellular pathways involved in growth and development. Therefore, there are numerous opportunities for pharmacologic targeting. Hormonal therapy is the prototype of a treatment targeting hormone receptors, and this class of drugs still provides the greatest overall impact on outcome. Even though chemotherapy is considered a cytotoxic and nonspecific therapy, it does modulate many key cellular pathways and therefore shares characteristics of biologic drugs. It is clear that targeted therapies are going to play a greater role in improving survival and quality of life in advanced breast cancer, with trastuzumab (Herceptin) serving as a successful model that is a relatively nontoxic agent associated with survival benefits. However, several challenges to the successful identification and application of therapeutic targets remain. These include the dissection of complicated and interacting biologic pathways and the limitations of preclinical models that will allow for a better prioritization of which drugs and combinations will succeed best in the clinic. Better methods for selecting ideal candidates for therapy need to be based on known modes of action. Mechanisms of intrinsic and acquired resistance need further exploration in order to refine drug design. Toxicities that might result from modulation of the targeted pathway must be expected and fully characterized. Some biologic strategies may need to be tested in less refractory cases, or even in early stages, even though this may be more costly and could raise safety concerns. Fortunately progress in all of these areas is expected with the availability of new technologies and a growing infrastructure for preclinical and clinical testing.