MSI CRC Not Responding to Immunotherapy: Is it Misdiagnosis?

Alexander M. Castellino, PhD

November 15, 2018

Immunotherapy has been approved to treat cancers with microsatellite instability (MSI), where the tumor harbors defective mismatch repair (dMMR) genes.

These MSI tumors include colorectal cancer, and nivolumab (Opdivo, Bristol-Myers Squibb) was approved specifically for use in metastatic MSI CRC last year, while pembrolizumab (Keytruda, Merck) was approved more generally for use in MSI tumors.  

When patients with metastatic MSI CRC do not respond to immunotherapy, and between 10% and 46% of patients don't respond, the nonresponse is attributed to primary resistance.  

But are these cases of true primary resistance, or are they actually instances of patients' samples being initially misdiagnosed based on the tests used?

A small study suggests that it may be misdiagnosis in many cases.   

The finding comes from a post-hoc analysis of a cohort of 38 patients enrolled in clinical trials from a single center, where 3 out of 5 cases (60%) of supposed primary resistance turned out to be a misdiagnosis of MSI status from local testing, implying that the patients should never have received immunotherapy to begin with. The positive predictive value of local testing was 92.1%, with a false-positive rate of nearly 10%.

These results were first presented at as a poster during the European Society for Medical Oncology (ESMO) 2018 Congress and have now been published online on November 15 in JAMA Oncology.

Medscape Medical News caught up with principal investigator, Alex Duval, MD, PhD, from the French Institute of Health and Medical Research (INSERM), Paris, France, at the ESMO 2018 Congress.

"Patients with MSI mCRC typically have a survival of approximately 12 months and immunotherapy has changed the lives of these patients because response rates are as high as 60%, with durable clinical efficacy," Duval said. 

Duval explained that most pathology laboratories use one of two methods to identify MSI status — immunohistochemistry (IHC) or polymerase chain reaction (PCR). IHC uses antibodies against MLH1, MHS2, MHS6, and PMS2, which are associated with dMMR or MSI status. The PCR uses pentaplex markers, BAT-26, NR-22, NR-24, NR-21, and BAT-25, which are rare microsatellite polymorphisms in MMR genes.

"Our study indicates that MSI or dMMR status should be tested routinely using both immunohistochemistry and PCR methods to identify patients appropriate for immunotherapy, and the results should be confirmed in central labs in cases with discrepancies between the two methods," Duval told Medscape Medical News.

ESMO expert Rodrigo Dienstmann, MD, of the Vall d'Hebron Institute of Oncology in Barcelona, Spain, told Medscape Medical News that a misdiagnosis of about 10% is significant, but not surprising. He pointed out that this observation is consistent with discordance across literature between IHC and PCR methods.

"Cross-laboratory discordance [false-positive rate] of 10% for MSI status is within the limits reported by these researchers," he said.     

He explained that MSI status has been important in identifying patients with Lynch syndrome and oncologists managing CRC are accustomed to testing for MSI/dMMR. "The concordance is higher in CRC, but the discordance is higher in other cancers," Dienstmann said.

Study Details

For their study, Duval and colleagues identified 38 patients with mCRC who were treated with immune checkpoint inhibitors in clinical trials at the Saint-Antoine Hospital, based on local evaluation for MSI status (collaborative centers sending patients to Saint-Antoine hospital for enrollment in clinical trials). Of 38 patients, five were identified as having primary resistance to their treatment.

With intent to determine the reason for primary resistance, Duval’s lab, which is a central facility noted for its work on microsatellite markers, decided to first check the status of the five samples resistant to treatment.

Three of the samples were misdiagnosed by the local testing facility — one was a misinterpretation of the IHC results and two were due to misinterpretation of the PCR results. With 35 of the 38 samples being accurately identified as MSI, the positive predictive value of local testing was 92.1%.

Duval and colleagues determined if their conclusion could be corroborated in an independent cohort of patients. For their validation cohort, the researchers showed that in a multicenter cohort of 93 mCRC tumors, nine were misdiagnosed and were in fact microsatellite stable (MSS). In this case, the positive predictive value of MSI assessment was 90.3% — close to the 92.1% from the single-center cohort — with a false-positive rate of nearly 10%.

The researchers showed that three samples from the validation cohort showed an MSS (microsatellite stable) phenotype with the pentaplex PCR test, but a dMMR phenotype using IHC. However, if they used a test that analyzed a different microsatellite for MSI, the HSP110 T17 microsatellite, the MSI phenotype was confirmed in two of the samples.

Duval told Medscape Medical News the HSP110 T17 polymorphism, which their lab also developed, was more sensitive than the pentaplex PCR test and correctly classified samples that were uncertain from the pentaplex panel.

Lessons for Clinical Practice

Current recommendations for local assessment require pathologists (IHC) and molecular biologists (PCR) to use one of the tests. Based on their data, Duval suggested that both the IHC and the PCR tests together would provide a higher level of certainty to pathologists and molecular biologists who routinely determine MSI status of tissue samples.

"Local assessment of MSI status in mCRC is associated with a false-positive rate of nearly 10% in patients included in trials of immune checkpoint inhibitors in France," Duval said. This misdiagnosis of MSI status, therefore, biases the data of the clinical trial, he pointed out.

In clinical practice, when primary resistance to immune checkpoint inhibition is suggested based on local testing it is better to have this confirmed from a central lab, Duval suggested.

Although their results suggest HT17 be added as an additional marker for superior sensitivity, Duvall admitted that currently not all commercially available tests are equipped to do the test.

"We are learning from ongoing trials to centralize testing," Dienstmann said. "Community labs are not experienced in these procedures," he pointed out.   

Dienstmann also indicated that perhaps testing with a different technology such as next generation sequencing would be able to address the false-positive rates seen with IHC and PCR.

"MSI is not black and white," Dienstmann told Medscape Medical News. "There are levels of mismatch repair and next generation sequencing will be able to better provide a continuous scale for MSI," he added.

However, Dienstmann was more concerned with false-negative rates, which are typically low. "A false-negative test is more critical to clinical practice because these patients do not get the benefits of a well-established therapy," he said.

Duval has reported no relevant financial relationships.

JAMA Onco. Published November 15, 2018. Abstract

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