Rare GU cancers: Overcoming obstacles through collaboration, novel trial design
“Patients [with rare GU tumors] would come to us saying, ‘Well, what can we do? What trial?’,” Dr. McGregor said. “But really, there was no trial to get them on.”Basket trials are therefore needed, he said, as they accelerate progress in the field and help meet patient needs.
“For some of these relatively rare diseases … there is no standard of care,” Dr. McGregor said. “And low incidence makes it challenging to conduct a dedicated clinical trial. Those patients are left with minimal therapeutic options. … We look to provide care for that unmet need.”Andrea B. Apolo, MD, described similar experiences as head of the bladder cancer section of the GU malignancies branch of the National Cancer Institute (NCI), Bethesda, Md.
“I’ve been at the NCI for the past 10 years and I’ve gotten a lot of referrals for rare tumors,” Dr. Apolo said. “[These patients] have tried all available standard of care options, and therefore are often looking for clinical trials and new drugs – any kind of therapy that may be effective for their disease.”This call for help, along with a growing scientific curiosity, motivated Dr. Apolo to design trials that would include patients who had nowhere else to go.
“I became very interested in … understanding more about the mechanism of tumorigenesis and understanding rare tumors, biologically, within the lab,” she said, “but also clinically, in terms of finding more effective therapies.”
Both Dr. McGregor and Dr. Apolo are currently conducting basket trials for patients with rare GU tumors. While Dr. McGregor is testing a combination of PD-1 inhibitor nivolumab and CTLA-4 inhibitor ipilimumab, Dr. Apolo is exploring the benefit of cabozantinib, a targeted therapy, given with either nivolumab or nivolumab plus ipilimumab.
When asked about these trials, Dr. Spiess said that “basket trials are important because they may give us an understanding of some potentially useful therapies or combinations;” however, he also pointed out their limitations, noting that they may inaccurately characterize the efficacy of given therapies over a broad array of disease entities even if they are of similar histology. As an example, he noted “very different” genomic profiles across squamous cell carcinomas of the pelvis depending on exact anatomical location, suggesting that these differences may affect responses to therapy, citing a recent study in European Urology that he conducted with Dr. Necchi.1
“[Basket trials] are probably not going to be the be-all-end-all,” Dr. Spiess said. “It really requires a global initiative to do these types of studies, which the Global Society of Rare Genitourinary Tumors will allow.”
Exploring immunotherapy combinations
Despite the potential limitations, recent basket trials involving immunotherapy regimens have been associated with overall response rates, in some subgroups, that exceed 35%.2,3
In comparison with previous trials, many of which had response rates in the single digits, or no responses at all, these results are, in Dr. McGregor’s words, “very thought provoking.”Most rare GU malignancies fall into one of four categories: bladder cancer variant histology (BCVH), adrenal tumors, penile squamous cell carcinoma (PSCC), and chemotherapy-refractory germ cell tumors (CRGCT). Among these, BCVH has the strongest evidence supporting clinical use of immunotherapy, based on U.S. approval for urothelial histology, according to Dr. McGregor.4Data supporting immunotherapy for the remaining disease subtypes are scarce. Although pembrolizumab is approved for patients with solid tumors that exhibit microsatellite instability (MSI), MSI is uncommon among patients with rare GU cancers; estimated incidence rates are less than 10%.4
“As such, clinical trials to address this unmet need are imperative,” Dr. McGregor wrote in a recent review article.4
According to Dr. McGregor, programmed death ligand 1 (PD-L1) expression in rare GU tumors may be relatively common in some disease subtypes, such as PSCC, which has a PD-L1 expression rate of up to 60%.4
But rare GU tumor trials involving a single checkpoint inhibitor have produced limited results, if any.
The largest trial for adrenocortical carcinoma (ACC), for example, which included 50 patients, showed that avelumab resulted in an objective response rate (ORR) of just 6%.5
Pembrolizumab was slightly more effective for ACC, based on a trial involving 39 patients, which returned an ORR of 23%, and another trial involving 15 patients that had a 15% ORR.6,7
Two other trials, which tested single-agent pembrolizumab or durvalumab in patients with CRGCT, resulted in no responses at all, whereas a trial testing pembrolizumab alone for penile squamous cell carcinoma was terminated in 2020, citing poor accrual.8,9 Still, the durvalumab trial for CRGCT, led by Dr. Necchi, did offer a glimpse at what might be possible with a combination of immunotherapies. Although no responses were observed among 11 patients who received durvalumab alone, an efficacy signal was observed in a second cohort of 11 patients who were given durvalumab in combination with the CTLA-4 inhibitor tremilimumab.9
Out of those 11 patients, 1 had a partial response, and another achieved stable disease.
In light of these findings, and more that have been published since then, the clinical trial landscape for rare GU tumors is shifting toward a combination immunotherapy approach, according to Dr. McGregor.4
