Laboratory Section - In Vitro Chemoresistance and Chemosensitivity Assays

IMPORTANT REMINDER

This Medical Policy has been developed through consideration of medical necessity, generally accepted standards of medical practice, and review of medical literature and government approval status.

Benefit determinations should be based in all cases on the applicable contract language. To the extent there are any conflicts between these guidelines and the contract language, the contract language will control.

The purpose of medical policy is to provide a guide to coverage. Medical Policy is not intended to dictate to providers how to practice medicine. Providers are expected to exercise their medical judgment in providing the most appropriate care.

Description

In vitro chemoresistance and chemosensitivity assays have been investigated as a means of predicting the tumor response to various chemotherapies. Thus these assays have been used by oncologists to select chemotherapy regimens for an individual patient. A variety of assays have been developed that differ in their processing and in the technique used to measure the sensitivity or resistance. However, all involve the same four basic steps:

1. Isolation of cells
2. Incubation of cells with drugs
3. Assessment of cell survival
4. Interpretation of the results

A variety of techniques have been evaluated to assess cell survival, including DiSC assay (differential staining cytotoxicity assay), the thymidine incorporation assay, fluorescence (cytoprint) assays, and the MTT assay. In the MTT assay, single tumor cell suspensions are exposed to the chemical MTT. If the cell is metabolically active, blue crystals are produced. In the DiSC assay and fluorescence assay, viable cells selectively uptake hematoxylin and eosin or fluorescein, respectively. In the thymidine incorporation assay, tritiated thymidine is added to the cell culture after 72 hours of incubation. After additional incubation, the radiolabeled material is collected and counted by liquid scintillation. The histoculture drug response assay is a type of chemosensitivity assay commercially available from AntiCancer, Inc. (San Diego, CA).

Results may be reported as drug sensitive, drug resistant, or intermediate. Drugs identified as drug sensitive are thought to be potentially effective in vivo chemotherapies, while drugs identified as resistant are thought to be potentially ineffective chemotherapies. Chemoresistance assays are most commonly used. Kern and colleagues have further refined the technique into an assay that predicts extreme drug resistance (EDR). In this assay, tumor cells from an individual patient are cultured in soft agar and then exposed to high concentrations of selected chemotherapeutic agents for prolonged periods of time, far exceeding the exposure anticipated in vivo. Cell lines that survive this exposure are characterized as showing extreme drug resistance. The negative predictive value associated with this assay has been reported as greater than 99%, such that potentially ineffective drugs can be eliminated from a patient's drug regimen. The extreme drug resistance assay is offered commercially by Oncotech, Inc.

Policy/Criteria

In vitro chemosensitivity assays, including but not limited to the histoculture drug response assay or a fluorescent cytoprint assay, are considered investigational.

In vitro chemoresistance assays, including but not limited to extreme drug resistance assays, are considered investigational.

Scientific Background

Chemoresistance Assays

This policy is based in part on a 1995 BlueCross BlueShield Association Technology Evaluation Center (TEC) assessment of chemoresistance assays that pointed out that the clinical utility of these assays will depend on the prior probability of response to a given chemotherapy. (2) Since chemoresistance assays are used to deselect potential chemotherapies, the negative predictive value (NPV) is the key statistical measure. NPV relates to the likelihood that chemoresistance as measured in vitro will correspond to a lack of clinical effect. Unless the negative predictive value is high, there is a chance that clinical decision-making based on a chemoresistance assay could inappropriately exclude an effective therapy. The negative predictive value will vary according to the prior probability of chemoresistance. For example, the negative predictive value in testicular cancer, typically a very chemosensitive tumor, will be lower than that associated with malignant melanoma, a very chemoresistant tumor. The TEC assessment concluded that chemoresistance assays have the highest clinical relevance in tumors with a low probability of response. However, it is still unclear how this information will affect clinical decision-making and whether health outcomes are improved as a result.

The extreme drug resistance (EDR) assay was specifically designed to produce a very high negative predictive value (>99%), such that the possibility of inappropriately excluding effective chemotherapy is remote in all clinical situations. (3) However, there are still inadequate clinical data to determine whether the use of EDR assays to deselect ineffective chemotherapies results in improved health benefits. While the relevant clinical outcome in chemosensitivity assays focuses on improved survival, the relevant outcome associated with chemoresistance assays is more controversial. Advocates of the EDR assay point out that avoidance of the toxicity of ineffective drugs is the relevant outcome, while others point out that this represents an intermediate outcome and that improved patient survival is the relevant outcome for chemoresistance assays. (4) For example, in clinical practice, deselection of one chemotherapy implies positive selection of another drug that did not show chemoresistance. Therefore, the toxicity and effectiveness of the drugs that are selected as a result of the EDR assay are relevant outcomes. Finally, a related clinical outcome is the extent to which an in vitro assay can improve on the empirical performance of the physician. For example, chemoresistance typically can be predicted without the use of an EDR assay in heavily pretreated patients with refractory tumors.

The bulk of the literature regarding extreme drug resistance assays have focused on correlation studies that correlate results from predictive in vitro assays with observed outcomes of chemotherapy. (6-11) However, in these studies, the patients do not receive assay-guided chemotherapy regimens. As discussed in a 2000 TEC Assessment (5), correlational studies are inadequate for several reasons. First, such studies often aggregate patients with different tumor types, disease characteristics, chemotherapy options, and probabilities of response. This process is problematic since the accuracy of each assay used to predict in vivo response probably varies across different malignancies and patient characteristics. Second, the method by which assay results are translated into treatment decisions is not standardized. Without knowing the rules for converting assay findings into treatment choices, it is impossible to determine the effects of assay-guided treatment on health outcomes. Third, it is important to consider not only response, but also survival and adverse effects. The overall value of assay-guided therapy depends on the net balance of all health outcomes observed after treatment for all patients subjected to testing, regardless of the assay results or the accuracy of its predication for response.  A literature search found no prospective studies focusing on the use of the EDR.

Chemosensitivity Assays

The enthusiasm for chemosensitivity assays, in general, has diminished over the years, due to the poor positive predictive values (PPV).  PPV relates to the likelihood that drugs shown to be effective in vitro will produce a positive clinical response. For example, a meta-analysis by Von Hoff of 54 retrospective studies reported a positive predictive value of only 69%. (12) The poor positive predictive value may, in part, be related to a variety of host factors, such as tumor vascularity. Several prospective trials have also reported technical challenges and inconclusive results, which further dampened enthusiasm. (13-20) Interpretation of other trials is flawed by methodologic issues.  For example, using a chemosensitivity assay, Xu and colleagues compared outcomes for an assay-guided treatment group with outcomes for a group given contemporaneous empiric therapy. (16) The patient sample consisted of 156 patients with advanced breast cancer. The article stated that choice of regimen in the assay-guided group was based on assay results, but no specific decision rules were reported. Patients whose EDR results suggested resistant disease were given empiric regimens and were excluded from the analysis of outcome results. This violated principles of intention-to-treat analysis. An intention-to-treat analysis is required to assess chemotherapy sensitivity and resistance assays since resources are consumed for all tested patients. Furthermore, it permits investigators to calculate the number of patients needed to test to identify one patient whose outcomes could be improved by use of assay-guided rather than empiric therapy.

Kurbacher and colleagues studied 55 patients with ovarian cancer, whose tumors were evaluated with a chemosensitivity assay. (17) This report stated that patients treated with assay guidance received the optimal protocol indicated by assay. However, the report did not include specific rules for translating assay results into treatment decisions. Patients in the empiric group were contemporaneous to the assay-guided group. However, some patients subjected to testing were added to the control group when assay results proved unevaluable or when clinicians preferred a different regimen than identified by assay results. This method of defining groups violated the intention-to-treat principle, and potentially biases outcomes estimated for assay-guided management.

In the only prospective, randomized study, Cree and colleagues reported on a chemosensitivity assay-directed chemotherapy versus physician’s choice in patients with recurrent platinum-resistant ovarian cancer. (21) Response rate and progression-free survival were studied in 180 patients randomized to either ATP-based tumor chemosensitivity assay-directed therapy (n=94) or physician's-choice chemotherapy (n=86). Median follow-up at analysis was 18 months; response was assessable in 147 (82%) patients: 31.5% achieved a partial or complete response in the physician's-choice group compared with 40.5% in the assay-directed group (26% vs. 31% by intention-to-treat analysis, respectively). Intention-to-treat analysis showed a median progression-free survival of 93 days in the physician's-choice group and 104 days in the assay-directed group (hazard ratio 0.8, not significant). No difference was seen in overall survival between the groups, although 12/39 (41%) of patients who crossed over from the physician's-choice arm obtained a response. Increased use of combination therapy was seen in the physician's-choice arm during the study as a result of the observed effects of assay-directed therapy in patients. The authors concluded that this small randomized, clinical trial documented a trend toward improved response and progression-free survival for assay-directed treatment and that chemosensitivity testing might provide useful information in some patients with ovarian cancer. They also noted that the ATP-based tumor chemosensitivity assay remains an investigational method in this condition.

2000 TEC Assessment

A 2000 TEC assessment reviewed both chemosensitivity and chemoresistance assays. (5)  This TEC assessment provides a detailed discussion on what type of data would be required to validate the clinical use of chemoresistance and chemosensitivity assays and considered the following methods:

• Correlation studies based on in vitro prediction of in vivo response

  A variety of studies have reported a correlation between in vitro prediction or response and clinical response. While these studies may have internal validity, they cannot answer the question of whether patients given assay-guided therapy or empiric therapy have different outcomes. For example, suppose that one group of patients is treated based on assay results and demonstrates an overall response rate of 75%. It is possible that a similar group of patients matched for important prognostic factors and given a uniform empiric chemotherapy regimen, could achieve the same overall response rate. However, if the response rates are the same for the two groups, the assay-guided group may experience more adverse effects from treatment or may have lower overall survival. To determine whether assay-guided treatment results in overall different outcomes than empiric treatment, it is important to take into account response rates, survival, and adverse effects. These effects may be assessed by decision analysis or comparative trials.

• Decision analysis

  While decision analysis is a useful tool, it may be limited when the decision tree is so complex that it is not possible to obtain evidence-based estimates for many of the probabilities in the tree. For this reason, the TEC assessment concluded that decision analysis would not be a useful tool for assessing the relative effectiveness of assay-guided and empiric treatment.

• Assessment based on direct evidence

  Given the limitations in the above two techniques, the TEC assessment focused on direct evidence that compared outcomes for patients treated either by assay-guided therapy or contemporaneous empiric therapy. A total of 7 studies were identified, none of which provided strong evidence to validate the clinical role of chemosensitivity or chemoresistance assays.

The BCBSA TEC Assessment was updated in 2002. (22) No studies were identified that would address the limitations noted in the above discussion. Specifically, no studies were identified that provided direct evidence comparing outcomes for patients treated either by assay-guided therapy or contemporaneous empiric therapy.

In 2004, the American Society of Clinical Oncology (ASCO) published a technology assessment of chemotherapy sensitivity and resistance assays (CSRA) (23) along with a systematic review of the literature. (24) The assessment concluded that “review of the literature does not identify any CSRAs for which the evidence base is sufficient to support use in oncology practice.” 

References

1. BlueCross BlueShield Association Medical Policy Reference Manual, Policy No. 2.03.01
2. TEC Assessments 1995; Nonclonogenic Cytotoxic Drug Resistance Assay
3. Kern DH, Weisenthal LM. Highly specific prediction of antineoplastic drug resistance with an in vitro assay using supra-pharmacologic drug exposures. J Natl Cancer Inst 1990;4:582-88
4. Brown E, Markman M. Tumor chemosensitivity and chemoresistance assays. Cancer 1996;77:1020-5
5. 2000 TEC Assessment; Chemotherapy Sensitivity and Resistance Assays, 2000. BlueCross BlueShield Association Technology Evaluation Center. Vol 15, Tab 11
6. Eltabbakh GH, Piver MS, Hempling RE et al. Correlation between extreme drug resistance and response to primary paclitaxel and cisplatin in patients with epithelial ovarian cancer. Gynecol Oncol 1998;70:392-7
7. Eltabbakh GH. Extreme drug resistance assay and response to chemotherapy in patients with primary peritoneal carcinoma. J Surg Oncol 2000;73(3):148-52
8. Mehta RS, Bornstein R, Yu IR. Breast cancer survival and in vitro tumor response in the extreme drug resistance assay. Breast Cancer Res Treat 2001;66(3):225-37
9. Holloway RW, Mehta RS, Finkler NJ et al. Association between in vitro platinum resistance in the EDR assay and clinical outcomes for ovarian cancer patients. Gynecol Oncol 2002;87(1):8-16
10. Ellis RJ, Fabian CJ, Kimler BF et al. Factors associated with success of the extreme drug resistance assay in primary breast cancer specimens. Breast Cancer Res Treat 2002;71(2): 95-102
11. Parker RJ, Fruehauf JP, Mehta R et al. A prospective blinded study of the predictive value of an extreme drug resistance assay in patients receiving CPT-111 for recurrent glioma. J Neurooncol 2004; 66(3):365-75
12. Von Hoff DD. He's not going to talk about in vitro predictive assays again, is he? J Natl Cancer Inst 1990;82:96-101
13. Von Hoff DD, Sandbach JF, Clark GM et al. Selection of cancer chemotherapy for a patient by an in vitro assay vs. a clinician. J Natl Cancer Inst 1990;82:110-16
14. Gazdar AF, Steinberg SM, Russell EK et al. Correlation of in vitro drug sensitivity testing results with response to chemotherapy and survival in extensive stage small cell lung cancer. A prospective clinical trial. J Natl Cancer Inst 1990;82:117-24
15. Maenpaa JU, Heinonen E, Hinkaa SM et al. The subrenal capsule assay in selecting chemotherapy for ovarian cancer. A prospective randomized trial. Gynecol Oncol 1995;57:294-8
16. Xu JM, Song ST, Tang ZM et al. Predictive chemotherapy of advanced breast cancer directed by MTT assay in vitro. Breast Cancer Res Treat 1999;53(1):77-85
17. Kurbacher CM, Cree IA, Bruckner HW et al. Use of an ex vivo ATP luminescence assay to direct chemotherapy for recurrent ovarian cancer. Anticancer Drugs 1998;9(1):51-7
18. Staib P, Staltmeier E, Neurohr K et al. Prediction of individual response to chemotherapy in patients with acute myeloid leukaemia using the chemosensitivity index C_i. Br J Haematol. 2005 Mar;128(6):783-91
19. Iwahashi M, Nakamori M, Nakamura M et al. Individualized adjuvant chemotherapy guided by chemosensitivity test sequential to extended surgery for advanced gastric cancer. Anticancer Res 2005;25(5):3453-9
20. Ugurel S, Schadendorf D, Pfohler C et al. In vitro drug sensitivity predicts response and survival after individualized sensitivity-directed chemotherapy in metastatic melanomas: A multicenter phase II trial of the Dermatologic Cooperative Oncology Group. Clin Cancer Res 2006;12(18):5454-5463
21. Cree IA, Kurbacher CM, Lamont A et al. A prospective randomized controlled trial of tumour chemosensitivity assay directed chemotherapy versus physician's choice in patients with recurrent platinum-resistant ovarian cancer. Anticancer Drugs 2007;18(9):1093-101
22. TEC Assessments: Chemotherapy Sensitivity and Resistance Assays, 2002; BlueCross BlueShield Association Technology Evaluation Center. Vol 17, Tab 12
23. Schrag D, Garewal HS, Burstein HJ et al. American Society of Clinical Oncology Technology Assessment: Chemotherapy sensitivity and resistance assays
24. Samson DJ, Seidenfeld J, Zeigler K et al. Chemotherapy sensitivity and resistance assays: A systematic review. J Clin Oncol 2004;22(17):3618-30

Cross References

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