Immunotherapy steers focus to microenvironment

When the staining is present in significantly more than 50 percent of the tumor cells, interpretation is fairly straightforward, Dr. Bloom says. But with lower TPS scores, interpretation can be problematic. “You probably want somebody with some experience in looking at PD-L1 immunostains to read these assays. Not every stain is perfect, so before you read the slide you definitely want to ensure there is <1+ background staining. High background staining would prevent you from appropriately interpreting the slide.” (See Fig. 3.)

As in all immunohistochemistry, edge artifacts exist and need to be avoided, Dr. Bloom notes. “Necrosis can be particularly problematic, as can crush artifacts. You may have some tumor cells next to the necrosis, but you want to be sure the necrosis itself is ignored since it frequently shows strong expression.”

Dr. Bloom is pleased that his laboratory has been making findings in line with the pembrolizumab clinical trials. “When we looked at the results from the pembrolizumab trial [KEYNOTE-001], the percentage of patients demonstrating ≥50 percent expression in their tumor cells was about 22 to 23 percent overall, and to date in my laboratory patients’ samples show expression in ≥50 percent tumor cells between 25 and 30 percent of the time.”

“So it is nicely in line with where the clinical trial was. If we look at the tumor proportion score that would be called negative [< 1 percent], that represented about 40 percent of the lung cancer patients. Preliminary data on the duration of response, overall survival, and progression-free survival with pembrolizumab from the KEYNOTE-001 trial showed the greatest benefit in the ≥50 percent category, with the one to 49 percent and the < 1 percent showing about equal benefit overall.”

Since the preliminary study, data are now available for the KEYNOTE-010 study (Herbst RS, et al. Lancet. Published online ahead of print Dec. 19, 2015), which included a comparator arm. “In the KEYNOTE-010 study, two doses of pembrolizumab were compared against the standard chemotherapy, docetaxel. The recent publication reported that overall survival was improved over docetaxel with PD-L1 expressions ≥1 percent, and progression-free survival was superior at expression ≥50 percent.”

These findings leave up in the air where the cutoffs should be set. “Given the improvement in overall survival, maybe the new cutoffs are going to be one percent, not 50 percent. We’ve got to wait for the clearances to come along from the FDA.”

Nivolumab (Opdivo) studies are also proving provocative. With nivolumab, the study designs took a different approach from the original pembrolizumab research and are intriguing to pathologists, Dr. Bloom says. “One of the early nivolumab studies, called CheckMate 017, was a second-line study in stage IIIb/IV squamous cell cancers. They compared nivolumab versus docetaxel, very much as the KEYNOTE-010 did with pembrolizumab, but the KEYNOTE-010 study included all non-small cell lung cancers.”

“When we look at the benefit [Fig. 4], especially the overall survival benefit compared to chemotherapy, it was quite significant. There was a one-year median overall survival with nivolumab of 9.2 months versus 6.0 months with chemotherapy.”

Interestingly, for every category of PD-L1 expression, there was a benefit of nivolumab over docetaxel. “So it didn’t make a difference whether your tumor was PD-L1 positive or PD-L1 negative.” In its trials, Bristol-Myers Squibb used three different cutoffs for positivity—≥1 percent, ≥5 percent, and ≥10 percent—in an attempt to better define what level of PD-L1 expression predicted response, if any. PD-L1 expression didn’t make a difference in predicting response to nivolumab in squamous cell NSCLC. There was a benefit of nivolumab over docetaxel, even in PD-L1 negative patients.

That says there’s benefit, for squamous cell cancer, to receiving nivolumab over docetaxel independent of PD-L1 status. “And when we looked at the toxicity level, again, there were very few grade 3 and grade 4 toxicities or adverse events. One significant adverse event was pneumonitis. Side effect profiles are likely to become very important as oncologists consider therapy options.”

Nivolumab clearly demonstrates benefit when compared with chemotherapy. “This was a big deal. Nivolumab was the first inhibitor really demonstrating survival benefit versus the standard of care, with a 41 percent reduction in death,” Dr. Bloom says.
Taking the same drug and applying it in nonsquamous cancers yields similar but not identical results.

Findings presented last year at the American Society of Clinical Oncology annual meeting demonstrated that nivolumab doubles survival for PD-L1 expressors. “When we look at the data based on PD-L1 expression in nonsquamous NSCLC, we find that PD-L1 expression becomes important.However, the patient gets a benefit from nivolumab at even a one percent level of PD-L1 expression, versus what would be called negative expression. It just doesn’t make a significant difference whether you put the cutoff at one percent, five percent, or 10 percent benefit, but there is some minor improvement in benefit as the cutoff is raised. Survival curves of patients showing no expression of PD-L1 are basically identical whether they received docetaxel or nivolumab.”

This means the two drugs have about equal efficacy for survival in PD-L1 negative tumors. “But remember, the safety profile for nivolumab had very few grade 3 and grade 4 toxicities compared to chemotherapy. So you have similar survival in PD-L1 negative patients but a much better safety profile.” In addition, for those patients who had a response with either nivolumab or docetaxel, the duration of response was significantly higher with nivolumab—17.2 months for nivolumab versus 5.6 months for docetaxel, he says. (See Fig. 5.)

In the traditional scenario, the FDA reviews biomarker data on drug targets along with data from the clinical trial to assess if a classic companion diagnostic assay needs to accompany the drug label. All patients who were on KEYNOTE-001 pembrolizumab were assessed for PD-L1 status with the 22C3 kit. Approval was based on the overall response rate of approximately 45 percent as well as the duration of the response in patients whose tumor showed PD-L1 expression in at least 50 percent of the tumor cells. “Therefore, the approval of pembrolizumab was based on being screened as positive with the 22C3 kit, and the kit was approved as a companion diagnostic for the drug.” The 50 percent cutoff level was based on overall response rate and the duration of the response, not overall survival, he adds. But he tentatively expects the cutoff for that indication will probably reduce to one percent.

For nivolumab, the FDA introduced a different concept: complementary diagnostic. “They approved the pharmDX 28-8 kit with nivolumab for nonsquamous NSCLC but they labeled the test a complementary diagnostic, meaning testing provides additional information about how a drug might be used but testing is not essential for the safe and effective use of that drug. Tumors showing expression of PD-L1 with the pharmDX 28-8 kit were more likely to receive a significant benefit from nivolumab versus docetaxel, but even patients lacking PD-L1 expression had a benefit similar to docetaxel but with less side effects.”

Use of the pharmDX 28-8 kit is not required for patients with nonsquamous NSCLC, but PD-L1 testing with the kit provides complementary information that allows the physician to make a better-informed decision about which drug to use. “So it can be a little confusing. If a patient presents with metastatic squamous cell lung cancer and the oncologist is considering the use of nivolumab, PD-L1 testing is unnecessary since there is a clear survival advantage versus chemotherapy regardless of PD-L1 status. But if the patient presents with metastatic nonsquamous lung cancer, PD-L1 testing may be informative.”

Whether nivolumab or pembrolizumab is better alone or in combination with other drugs is another question on which the jury is still out. “Looking at first-line therapy in unresectable stage III/IV melanoma and comparing the progression-free survival of patients treated with nivolumab versus the combination of nivolumab plus Yervoy” [ipilimumab, approved by the FDA Oct. 28, 2015], “we see some interesting things.”

“The combination therapy improves PFS over nivolumab alone and is significantly better than just Yervoy. When stratified by PD-L1 expression, patients with negative tumors do significantly better with combination therapy versus either one. But patients with PD-L1 positive [≥5 percent] tumors appear to do the same whether they receive nivolumab or the combination.”

The importance of that finding again has to do with the safety profiles. “The safety of nivolumab by itself is very favorable, but patients get significantly more grade 3 and grade 4 adverse events when combination therapy is used. It might be necessary,” he says, “to do PD-L testing in the future using a different cutoff or possibly not at all.”

Anti-PD-L1 drugs are likely to be approved soon and their benefits remain to be seen as clinical trial data become available, Dr. Bloom notes. The anti-PD-L1 drugs “may have some theoretical benefit over anti-PD-1 drugs, but we will have to wait for the results of the clinical trials to see if the benefits are real.” For example, atezolizumab was engineered to remove antibody-dependent cellular cytotoxicity function, which results from natural killer cells recognizing antibodies bound to the targeted molecule and destroying the cell expressing it. In a small series of NSCLC, no cases of serious pneumonitis were noted but “we’re still waiting for more data to emerge,” Dr. Bloom says. A more significant issue related to the anti-PD-L1 drugs is that they were studied using two different PD-L1 assays with different scoring systems, both manufactured by Ventana. The SP142 assay developed as a companion to atezolizumab includes evaluation of both the tumor component and the immune component to help select patients who will benefit most from the therapies. “That means pathologists will be assessing not just the percent of tumor cells showing expression of PD-L1 but also the immune cell component, and that’s done by an area measurement. Whether the tumor component, immune component, or both need to be evaluated will depend on clinical trial results but will likely differ based on the tumor type.”

Given the complexity of this scoring, he sounds a note of caution for pathologists: Not all antibodies labeled PD-L1 stain the same. For example, three different assays reported at the IASLC annual meeting on the treatment of lung cancer with targeted therapies in February 2015 showed different levels of expression. (See Fig. 6.)

The PD-L1 IHC assays may vary in their sensitivity, he says. In addition, they may differ in how they stain tumor cells versus immune cells. For example, one assay may stain tumor cells less intensely but the immune cells will appear much darker and easier to read, while another may stain tumor cells more strongly but the immune cells much more weakly.
Until there is better understanding of the reasons for these differences, “it is probably best to use the assays that were used in the clinical trials,” he suggests. For pembrolizumab, that means it’s best to use the pharmDX 22C3 assay, and for nivolumab it’s probably best to use the pharmDX 28-8 assay.

With the plethora of phase one and phase two drugs in the immuno-oncology pipeline just in the lung trials of one company, Bristol-Myers Squibb (see Fig. 7), the real promise of immunotherapy lies ahead, Dr. Bloom says. “There are a lot of ongoing trials and there are many more indications that are coming. Some might have testing, some might have no testing, and it’s conceivable that the cutoff could be different for each one.”

The message for pathologists is clear: “This is not going away. We are now moving beyond the tumor into the microenvironment, and we’re going to need to understand how all of these things interact with each other to move forward.”

Anne Paxton is a writer in Seattle. The immunotherapy webinar featuring Dr. Bloom, “Immune Checkpoint Blockade in Cancer,” is viewable at: https://j.mp/bloom_capwebinar. Dr. Bloom is a member of the CAP Personalized Health Care Committee.