Christin L. Melton, ELS

Timothy A. Yap, MBBS, PhD, MRCP(UK), BSc(Hons)

The early development of PARP inhibitors in 2003 focused on their use in combination with cytotoxic chemotherapy agents, but this was eventually abandoned because of excess toxicity. Approximately 2 years later, data emerged showing synthetic lethality between PARP inhibitors and

mutations in ovarian cancer, leading the way to treatment of

Investigators have since determined that PARP inhibitors are active across a range of tumor types harboring

mutations and can also evoke responses in subsets of

According to Timothy A. Yap, MBBS, PhD, MRCP(UK), BSc(Hons), associate professor in the Department of Investigational Cancer Therapeutics and associate director of translational research at the Khalifa Institute for Personalized Cancer Therapy, both at The University of Texas MD Anderson Cancer Center in Houston, PARP inhibitors show synthetic lethality when combined with germline abnormalities in many DNA damage response (DDR) pathway components, such as mutations in the

genes. Some of these mutations may eventually be shown to &ldquo;predict robustly for antitumor response,&rdquo; he said.

Yap, who is also medical director of the Institute for Applied Cancer Science at MD Anderson, is investigating whether PARP inhibitors demonstrate similar synthetic lethality when combined with other targeted DDR therapies or different classes of anticancer drugs. He said he is particularly excited about efforts to combine PARP inhibitors &ldquo;with immunotherapies such as PD-1 or PD-L1 inhibitors, other DDR agents such as ATR inhibitors, and other molecularly targeted agents that may be used to induce a state of chemical &lsquo;BRCAness&rsquo; in HR [homologous recombination]proficient cancers, such as antiangiogenic agents.&rdquo; Yap said studies have already provided &ldquo;promising early results with rational combinations and responses beyond

The FDA has approved 4 PARP inhibitors as single-agent therapies for breast or ovarian cancer (TABLE 1): olaparib (Lynparza), rucaparib (Rubraca), niraparib (Zejula), and talazoparib (Talzenna).

PARP inhibitors are a family of enzymes essential for many cellular processes in addition to DNA repair, including chromatin remodeling, transcription, messenger RNA processing, and replication fork stabilization.

PARP inhibitors primarily target the PARP1 and PARP2 isoforms, which detect and promote repair of DNA single- or double-strand breaks (DSBs) to maintain genomic stability.

Tumors harboring DDR pathway aberrations that prevent homologous recombination repair of DSBs depend on PARP to repair single-strand DNA breaks and prevent them from progressing to irreparable, synthetically lethal DSBs.

Investigators are still elucidating the complex mechanisms of PARP inhibitors, but, in addition to inhibiting PARP catalytic activity, these agents are known to trap PARP at sites of damaged DNA. The resulting PARP-DNA complexes are highly efficient at preventing DNA repair, resulting in greater accumulation of DSBs; replication fork collapse; and, ultimately, cell death.

&ldquo;The antitumor efficacy of the PARP-trapping PARP inhibitors is probably not dissimilar,&rdquo; Yap said. Veliparib, an investigational PARP inhibitor in late-stage clinical trial development, appears to be a weak PARP trapper, which may explain its less robust single-agent activity in preclinical trials.

Yap noted that PARP inhibitors mainly differ in their toxicities: &ldquo;For example, alopecia with talazoparib, transient elevations in liver transaminases with rucaparib, and hypertension with niraparib.&rdquo;

Despite recent treatment advances, metastatic castration-resistant prostate cancer (mCRPC) remains incurable.

An estimated 20% to 25% of men with metastatic prostate cancer harbor a homologous recombination pathway mutation,10 and 1 study found that 11% of prostate cancer specimens had pathogenic

Homologous recombination deficiency (HRD) mutations are associated with more aggressive disease and worse prognosis.13 Ongoing clinical trials are exploring PARP inhibitors as monotherapy or in combination with other therapies for prostate cancer (TABLE 2).

Several trials have evaluated single-agent olaparib in mCRPC featuring a DDR alteration, and findings suggest that olaparib is more active in patients with

mutations than in those with mutations in other DDR genes, such as

The open-label phase III PROfound trial (NCT02987543) enrolled 245 patients with

mutations (cohort A) and 142 with other DDR mutations (cohort B) whose dis- ease had progressed after abiraterone acetate (Zytiga) or enzalutamide (Xtandi).33 Patients were randomly assigned 2:1 to olaparib or physician&rsquo;s choice of enzalutamide or abiraterone.

Preliminary data presented at the European Society for Medical Oncology (ESMO) 2019 Congress associated olaparib with significant improvement in 12-month progression-free survival (PFS) compared with treatment of physician&rsquo;s choice (TPC) in cohort A (7.39 vs 3.55 months, respectively; HR, 0.34; 95% CI, 0.25-0.47;

In cohort A, olaparib also produced significant improvements in objective response rate (ORR) and median time to pain progression. Although the overall survival (OS) data for cohort A showed a favorable trend, the difference did not achieve a predefined boundary for statistical significance, possibly because of the high crossover rate from the TPC arm.

Olaparib did not significantly improve outcomes in cohort B.

The most common treatment-related adverse events (TRAEs) in the olaparib cohort were anemia (46%), nausea (41%), decreased appetite (30%), and fatigue (26%).

Data from the phase II TOPARP-B trial (NCT01682772) presented at the 2019 American Society of Clinical Oncology (ASCO) Annual Meeting confirmed the activity of olaparib in progressive mCRPC harboring a DDR mutation.

Patients were randomly assigned to 300 or 400 mg of olaparib twice daily. The overall response rate in the 400-mg group was 54% (95% CI, 39%-69%) compared with 37% (95% CI, 23%-53%) in the 300-mg group.

The highest ORRs occurred in patients with

mutations (80% and 57%, respectively). Overall median PFS was 5.4 months.

The ongoing phase II open-label Galahad trial (NCT02854436) is assessing niraparib in patients with previously treated mCRPC and a DDR mutation, approximately half of whom have a

An interim analysis showed that pronounced clinical activity was largely confined to the

cohort, which had an ORR of 41%, a composite response rate of 63%, and median radiographic PFS (rPFS) of 8.2 months.

Common grade 3/4 TRAEs were anemia (29%), thrombo- cytopenia (15%), and neutropenia (7%).

In 2018, rucaparib received breakthrough therapy designation for

-mutated mCRPC based on promising efficacy in the phase II TRITON2 trial (NCT02952534).

Updated data presented at ESMO 2019 Congress confirmed rucaparib&rsquo;s activity.

mutation, the prostate-specific antigen response rate was 52% and the ORR was 44%, with a 5% complete response (CR) rate.

Responses were durable, persisting &ge;24 weeks in 60% of patients. Common grade &ge;3 TRAEs included anemia (11%) and asthenia/ fatigue (7%).

The phase III TRITON3 trial (NCT02975934) will compare rucaparib with TPC in progressive mCRPC with a

The phase III TALAPRO-2 trial (NCT03395197) is evaluating talazoparib with enzalutamide in unselected patients with mCRPC.

Preliminary data indicated promising activity and a manageable safety profile. Interim results from an ongoing phase I/II study of olaparib plus the PD-L1 inhibitor durvalumab (Imfinzi) in mCRPC also showed promising activity, particularly in patients with a DDR alteration.

Other combination trials are QUEST (NCT03431350), which is evaluating niraparib plus a novel antiPD-1 monoclonal antibody13; MAGNITUDE (NCT03748641), which combines niraparib with abiraterone and prednisone

; and PROPEL (NCT03732820), which is investigating first-line therapy with olaparib plus abiraterone.

Between 4% and 7% of patients with pancreatic cancer have a germline

mutation. Treatment options after progression of advanced pancreatic ductal adenocarcinoma (PDAC) are limited, and the presence of homologous recombination defects provides a rationale for evaluating PARP inhibitors in this disease.

Interim results from the phase III POLO trial (NCT02184195) suggest that olaparib holds promise as a maintenance therapy for patients with metastatic PDAC who have a germline

mutation and no progression after platinum-based chemotherapy.

At data cutoff, median PFS was significantly longer in patients randomly assigned to olaparib versus those given placebo (7.4 vs 3.8 months; HR, 0.53; 95% CI, 0.35-0.82;

= .004).40 However, the ORR, median OS, and health-related quality of life were not significantly different between the 2 arms.

Approximately one-fourth of patients treated with olaparib experienced a serious adverse event (AE).

Although the phase II RUCAPANC trial (NCT02042378) of rucaparib monotherapy for previously treated advanced PDAC harboring a

mutation was terminated early because of poor response, investigators noted that the 16% of patients who responded had durable, clinically significant responses.

Efficacy was enhanced in platinum-sensitive patients.

A recent interim analysis of a phase II trial (NCT03140670) of maintenance rucaparib showed greater success in platinum-sensitive patients with advanced PDAC and a

The ORR for the 19 evaluable patients was 37%, which included 6 partial responses (PRs) and 1 CR. Responses were confined to patients with a

mutation. The disease control rate (DCR) was 90% for &ge;8 weeks, and median PFS was 9.1 months.

Common AEs possibly related to treatment were nausea, dysgeusia, and fatigue.

An ongoing phase I dose-escalation trial (NCT01286987) is assessing the single-agent activity of talazoparib in advanced solid tumors that harbor a germline

Preliminary data for 13 patients with pancreatic cancer revealed an ORR of 20% (all PRs) and a clinical benefit rate of 30%.

Just 1 patient remained on talazoparib at data cutoff.

Final data from a phase II trial combining veliparib with FOLFOX (folinic acid, fluorouracil, and oxaliplatin) chemotherapy showed an ORR of 50%, median PFS of 7.2 months, and median OS of 11.1 months in patients with metastatic PDAC and a DDR mutation.

Outcomes were slightly better in platinum-na&iuml;ve patients with a family history of pancreatic cancer and a DDR mutation.

Additional trials are evaluating veliparib combined with gemcitabine and cisplatin (NCT01585805) and with FOLFIRI (folinic acid, fluorouracil, and irinotecan) chemotherapy (NCT02890355). In the phase II PARPVAX trial (NCT03404960), niraparib is being explored with an immune checkpoint inhibitor (nivolumab [Opdivo] or ipilimumab [Yervoy]) for advanced platinum-sensitive PDAC.

Immune checkpoint inhibitors have shown limited activity in pancreatic cancer, prompting investigators to look for synergistic combinations.

Olaparib and talazoparib are the only PARP inhibitors approved for breast cancer.

Although the single-agent activity of PARP inhibitors in BRCA1/2-mutated breast cancer is well established, interest in expanding their use to other settings is burgeoning (TABLE 2).

Regulatory approval of olaparib for germline

-mutated, HER2-negative metastatic breast cancer (mBC) was based on results of the randomized phase III OlympiAD trial, showing significant improvement in median PFS with olaparib versus physician&rsquo;s choice of chemotherapy (7.0 vs 4.2 months, respectively; HR, 0.58; 95% CI, 0.43-0.80;

Follow-up data showed no significant improvement in OS with olaparib, however.46 The randomized phase III OlympiA trial (NCT02032823) is assessing single-agent olaparib as adjuvant therapy in high-risk

-mutated, HER2-negative nonmetastatic breast cancer.

Talazoparib received FDA approval based on conclusions from the randomized phase III EMBRACA trial, which compared talazoparib with TPC in BRCA1/2-mutated advanced breast cancer.48 Median PFS was significantly longer in the talazoparib arm versus the TPC arm (8.6 vs 5.6 months; HR, 0.54; 95% CI, 0.41-0.71;

<.001), but median OS did not differ significantly between the groups.

Subsequently, a small trial suggested that neoadjuvant talazoparib could reduce tumor volume in early-stage breast cancer, leading to a larger trial (NCT03499353) to investigate the regimen&rsquo;s efficacy in triple-negative breast cancer (TNBC) with a germline

Talazoparib has also been studied in platinum-sensitive advanced HER2-negative breast cancer with homologous recombination pathway alterations besides

, producing an ORR of 25% in 12 evaluable patients.

Although the phase III BRAVO trial assessing single-agent niraparib in locally advanced or metastatic, HER2-negative,

-mutated breast cancer ended prematurely because of poor patient adherence,

ongoing studies continue to evaluate niraparib for breast cancer. The phase II ABC Study (NCT02826512) is testing niraparib in advanced, locally recurrent, BRCA-like, HER2-negative breast cancer, and a phase I trial (NCT03329937) is evaluating neoadjuvant niraparib in a similar population.

In the MEDIOLA trial (NCT02734004), olaparib was administered with durvalumab to women with germline

The ORR was 63%, median PFS was 8.2 months, and median OS was 20.5 months. The same combination is being studied in another phase II trial (NCT03801369) but for metastatic BRCA wild-type TNBC.

The ongoing VIOLETTE trial (NCT03330847) is combining olaparib with an investigational ATR inhibitor in previously treated advanced TNBC.

The phase II TOPACIO/KEYNOTE-162 trial (NCT02657889) investigated the combination of niraparib plus the antiPD-1 monoclonal antibody pembrolizumab (Keytruda).

The ORR was 21%, which included 5 CRs and 5 PRs, and the DCR was 49%.

- mutated tumors, producing an ORR of 47% and a DCR of 80%.

Common TRAEs were anemia (18%), thrombocytopenia (15%), and fatigue (7%); 15% of patients experienced immune-related AES.

Veliparib combinations have been evaluated in multiple trials. The phase III BrighTNess trial (NCT02032277) randomly assigned patients with TNBC to receive paclitaxel alone; paclitaxel plus carboplatin; or paclitaxel, carboplatin, and veliparib.

Veliparib did not enhance the efficacy of paclitaxel/carboplatin, nor did it substantially increase toxicity.

The BROCADE3 trial (NCT02163694) assessed the same triplet regimen but in patients with HER2-negative, advanced or metastatic, germline

This time, adding veliparib to carboplatin and paclitaxel significantly prolonged PFS versus carboplatin and paclitaxel alone (14.5 vs 12.6 months, respectively; HR, 0.71; 95% CI, 0.57-0.88;

= .002) but failed to significantly improve OS or ORR.

The 3-year PFS rate was more than doubled in the veliparib arm.

Olaparib, rucaparib, and niraparib are all indicated for ovarian, fallopian tube, and primary peritoneal cancers.

Niraparib was first approved as maintenance therapy for recurrent platinum-sensitive disease and received an expanded indication in October 2019 for advanced disease.

Data were presented at the 2019 ASCO meeting from the randomized phase III SOLO3 trial (NCT02282020), which compared single-agent olaparib with TPC in germline BRCA1/2-mutated platinum-sensitive relapsed ovarian cancer.

Outcomes significantly favored olaparib over TPC, with ORRs of 72% vs 51% (HR, 2.53; 95% CI, 1.40-4.58;

= .002) and median PFS of 13.4 vs 9.2 months (HR, 0.62; 95% CI, 0.43- 0.91;

SOLO1 (NCT01844986) is an earlier trial of maintenance olaparib in newly diagnosed advanced ovarian cancer with a confirmed

mutation, and which showed a 70% reduction in disease progression or death versus placebo (HR, 0.30; 95% CI, 0.23-0.41;

Approval of niraparib for advanced ovarian cancer in patients with HRD was based on results of QUADRA, a single-arm phase II trial (NCT02354586) in heavily pretreated patients.

The ORR in a subset of patients with HRD and platinum sensitivity was 28% (95% CI, 15.6%-42.6%; 1-sided

The most common grade &ge;3 TRAEs were small intestinal obstruction (7%), thrombocytopenia (7%), and vomiting (6%). The phase III PRIMA study (NCT02655016) compared niraparib with placebo in newly diagnosed, advanced, platinum-sensitive ovarian cancer regardless of BRCA mutation status.55 Niraparib was associated with significantly longer PFS than placebo overall and in a subset of patients with HRD.

Combinations featuring niraparib or olaparib are also being investigated in ovarian cancer. TOPACIO/KEYNOTE-162 (NCT02657889) included 62 women with ovarian cancer, regardless of BRCA mutation status, and administered a combination of niraparib and pembrolizumab.26 The ORR was 18% (90% CI, 11%-29%), median PFS was 3.4 months (95% CI, 2.1-5.1 months), and the DCR was 65% (90% CI, 54%-75%), which included 3 CRs and 8 PRs.

Responses occurred even in patients without HRD.

The phase III PAOLA-1/ENGOT-ov25 trial (NCT02477644) assessed the addition of olaparib to bevacizumab (Avastin) maintenance therapy in unselected patients with advanced ovarian cancer who had a stable CR or PR after first-line platinum-based therapy plus bevacizumab. Overall median PFS was significantly longer with the doublet versus bevacizumab monotherapy (22.1 vs 16.6 months, respectively; HR, 0.59; 95% CI, 0.49-0.72;

mutation or HRD achieved even longer PFS.

Several ongoing trials are evaluating olaparib in combination with the antiangiogenic agent cediranib in various ovarian cancer settings (Table).28 Recently published findings from a randomized phase II trial in women with relapsed platinum-sensitive ovarian cancer associated the combination of olaparib plus cediranib with significantly longer median PFS and OS than single-agent olaparib, primarily in patients without a BRCA1/2 mutation.

Studies continue to reveal the promising antitumor activity of PARP inhibitors beyond breast and ovarian cancers. Beyond the expansion of PARP inhibitors to prostate and pancreatic cancers, investigators are also now evaluating these agents in solid tumors such as lung and gastric cancers, as well as in hematologic malignancies. Combining PARP inhibition with agents that target other DDR pathways or have complementary mechanisms, such as immune checkpoint inhibitors and DNA-damaging chemotherapeutics, may provide the best opportunity to deliver the clinical benefits of PARP inhibition to more patients. In the future, biomarkers will likely play an even greater role in identifying those patients most likely to respond to PARP inhibitors.

Yap TA, Plummer R, Azad NS, Helleday T. The DNA damaging revolution: PARP inhibitors and beyond.

Morales J, Li L, Fattah FJ, et al. Review of poly (ADP-ribose) polymerase (PARP) mechanisms of action and rationale for targeting in cancer and other diseases.

Lynparza [package insert]. Wilmington, DE: AstraZeneca Pharmaceuticals LP, 2019.

Rubraca [package insert]. Boulder, CO: Clovis Oncology, 2018.

Zejula [package insert]. Waltham, MA: Tesaro, Inc, 2019.

Talzenna [package insert]. New York, NY: Pfizer Labs, 2019.

Murthy P, Muggia F. PARP inhibitors: clinical development, emerging differences, and the current therapeutic issues.

2019;2:665-679. doi: 10.20517/cdr.2019.002.

Michelena J, Lezaja A, Teloni F, Schmid T, Imhof R, Altmeyer M. Analysis of PARP inhibitor toxicity by multidimensional fluorescence microscopy reveals mechanisms of sensitivity and resistance.

2018;9(1):267 doi: 10.1038/s41467-018-05031-9.

Noordermeer SM, van Attikum H. PARP inhibitor resistance: a tug-of-war in BRCA-mutated cells.

2019;29(10):820-834. doi: 10.1016/j.tcb.20107.008.

Mateo J, Lord CJ, Serra V, et al. A decade of clinical development of PARP inhibitors in perspective.

2019;30(9):1437-1447. doi: 1093/annonc/mdz192.

Lang SH, Swift SL, White H, Misso K, Kleijnen J, Quek RGW. A systematic review of the prevalence of DNA damage response gene mutations in prostate cancer.

2019;55(3):597-616. doi: 10.3892/ijo.2019.4842.

Agarwal N, Sokol ES, Lara P Jr., et al. Pan-cancer assessment of BRCA1/2 genomic alterations (GAs) by comprehensive genomic profiling (CGP) of tissue and circulating tumor DNA (ctDNA).

2018;29(suppl 8). doi: 10.1093/annonc/mdy269.049.

Prostate cancer. OncologyPro website. oncologypro.esmo.org/Oncology-in-Practice/Anti-Cancer-Agents-and-Biological-Therapy/PARP-inhibition-and-DNA-Damage-Response-DDR/PARP-inhibitors/Clinical-Activity/Prostate-Cancer. Updated July 25, 2019. Accessed October 26, 2019.

A Study of Niraparib in Combination With Abiraterone Acetate and Prednisone Versus Abiraterone Acetate and Prednisone for Treatment of Participants With Metastatic Prostate Cancer (MAGNITUDE). clinicaltrials.gov/ct2/show/NCT03748641. Updated October 17, 2019. Accessed October 30, 2019.

Clarke NW, Armstrong AJ, Thiery-Vuillemin A, et al. PROPEL: a randomized, phase III trial evaluating the efficacy and safety of olaparib combined with abiraterone as first-line therapy in patients with metastatic castration-resistant prostate cancer (mCRPC).

2019;37(suppl 7; abstr TPS340). doi: 10.1200/JCO.2019.37.7_suppl.TPS340.

Articles

The early development of PARP inhibitors in 2003 focused on their use in combination with cytotoxic chemotherapy agents, but this was eventually abandoned because of excess toxicity.

Recent Studies Point to a Greater Role for PARP Inhibitors in Solid Tumors

Approximately one-third to one-half of patients with advanced melanoma respond to PD-1 inhibitor therapy, which is superior to response rates observed with systemic chemotherapy or even single-agent ipilimumab (Yervoy)a cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) blocker and the first checkpoint inhibitor approved for melanoma.

Nevertheless, a significant proportion of patients have primary, or innate, resistance to PD-1 inhibition and derive no benefit from these therapies that have become the standard of care for advanced melanoma.

In addition, several years have passed since the first patients received PD-1 inhibitors in clinical trials, and it has become clear that although most responses are durable and last several years, one-fourth to one-third of patients relapse despite ongoing treatment.

These patients are categorized as having secondary, or acquired, resistance.

The lack of adequate therapies for advanced melanoma that is resistant to immune checkpoint inhibitors (ICIs) remains an important unmet need in this tumor type. To address this problem, investigators across the globe have expended considerable effort attempting to identify mechanisms of response and resistance to ICIs. The general conclusion is that each patient&rsquo;s response to immunotherapy is likely complex and highly individualized.

Numerous pathways and patient characteristics have been implicated in poor response to PD-1 inhibition, with more discovered every year.

They are broadly divided between tumor-extrinsic, or host, and tumor-intrinsic factors.

Potential host factors include the gut microbiome, diet, the patient&rsquo;s health and immune status, and other medications or treatments.

Tumor-intrinsic mechanisms include histology, genetic and epigenetic changes in the tumor or its microenvironment (TME), and tumor mutational burden.

The wealth of information about resistance mechanisms has inspired the development of many different treatment strategies for preventing or overcoming PD-1 inhibitor resistance, which are being evaluated in ongoing clinical trials.

Jennifer A. Wargo, MD, MMSc, associate professor in the department of Surgical Oncology and Genomic Medicine at The University of Texas MD Anderson Cancer Center in Houston, runs a research laboratory, and is coleader of MD Anderson&rsquo;s Melanoma Moon Shot Program. She said the goal of the almost 7-year-old program is to &ldquo;help end pain and suffering for patients with cancer over the next decade,&rdquo; and it has supported her extensive research into mechanisms of resistance to ICIs and new treatment approaches. &ldquo;We&rsquo;ve made some major discoveries and have been able to run some novel clinical trials that have profoundly improved outcomes for patients with cancer,&rdquo; she said in an interview with Targeted Therapies in Oncology.

Wargo is particularly excited about her group&rsquo;s efforts to untangle the relationship between the gut microbiome and responses to ICIs. She explained that the human body contains more than 100 trillion microbes, which is almost 10 times the number of human cells.

&ldquo;A large proportion of those microbes reside in the gut, which has a huge surface area. On one side, you have all these microbes, and on the other side, you have a dense network of immune cells,&rdquo; she said. Microbial metabolites facilitate cross talk between microbes and immune cells in the gastrointestinal tract; they also influence immune system functions locally and then systemically as immune cells migrate from the intestines throughout the body via the lymphatic and circulatory systems.

Wargo said the sheer quantities of microbes and immune cells in the gut and their constant interactions convinced her there is a &ldquo;tight link between the gut microbiome and response to cancer immunotherapy.&rdquo; Bolstered by findings from murine and other preclinical studies,

Wargo and colleagues decided to test the theory in humans with metastatic melanoma. They prospectively collected and analyzed samples of the gut microbiome from 112 patients about to start PD-1 inhibitor therapy.

Later, they assessed whether bacterial diversity at baseline corresponded with treatment outcomes. According to Wargo, &ldquo;Patients who had low diversity of bacteria within the microbiome and a higher abundance of Bacteroidales were far less likely to respond than patients who had what we would characterize as a favorable microbiome, with high diversity and a higher abundance of specific taxa such as Ruminococcaceae or Clostridiales.&rdquo; Also, immune profiling of the TME revealed that enrichment of these favorable taxa in the microbiome of responders was associated with an increase in the CD8-positive T-cell infiltrate in the tumor.

Next, they transplanted stool from the treated patients into germ-free mice, which were subsequently injected with melanoma cells and dosed with an antiPD-1 ligand 1 (PD-L1) inhibitor. Mice transplanted with fecal microbiota from the PD-1 responders had significantly greater tumor shrinkage, better responses to the anti&ndash;PD-L1 regimen, and a more favorable microbiome than mice that received stool from the nonresponders.

&ldquo;We have now gone on to ask, &lsquo;Can we modulate the microbiome and make patients respond better?&rsquo;&rdquo; Wargo said. Investigators from MD Anderson and The Angeles Clinic, in collaboration with Seres Therapeutics and the Parker Institute for Cancer Immunotherapy, are currently recruiting patients with advanced melanoma for an ongoing phase Ib clinical trial of therapies designed to alter the microbiome before PD-1 inhibitor therapy (NCT03817125). One group of patients will receive SER-401, an oral capsule that contains fecal bacterial spores consistent with the microbiome signature of responders to anti PD-1 therapy. Both groups will be treated with nivolumab (Opdivo), a PD-1 inhibitor approved for advanced melanoma.

Wargo said she was encouraged by preliminary data presented at the 2019 American Association for Cancer Research (AACR) Annual Meeting from 2 clinical trials of fecal transplant in patients with melanoma.

She described one study that &ldquo;used a fecal transplant from a complete responder donor, gave that back to patients who had progressed on antiPD-1, and reintroduced checkpoint blockade.&rdquo; Wargo said the patients responded to retreatment and that &ldquo;the fecal transplant was associated with an increased infiltrate of favorable immune cells within the tumor microenvironment.&rdquo;

Another study led by Wargo examined whether certain lifestyle factors influence the microbiome in patients with melanoma. Data were presented at the 2019 AACR Annual Meeting that showed &ldquo;patients who had a high-fiber diet were about 5 times more likely to respond to treatment with antiPD-1 therapy.&rdquo;

Wargo&rsquo;s group also examined the relationship among baseline use of over-the-counter probiotics, the microbiome signature, and treatment response.

&ldquo;About 42% of our patients reported taking a probiotic, and strikingly, patients who took a probiotic had lower diversity of their microbiome and were less likely to respond,&rdquo; Wargo said. The findings suggest that clinicians may want to discuss probiotic use with patients starting immunotherapy.

Overcoming Tumor-Intrinsic Resistance to PD-1 Inhibition

In cancer, chronic antigen exposure leads to exhaustion of CD8-positive tumor-infiltrating lymphocytes (TILs) and upregulation of multiple coinhibitory factors, including PD-1 and PD-L1.13 The success of checkpoint blockade with PD-1/ PD-L1 inhibitors largely depends on their ability to mobilize, reactivate, and expand antigen-specific cytotoxic CD8-positive T lymphocytes.5 Achieving long-term disease control depends on generating a persistent population of effector memory T cells. Defects that lead to loss of T-cell function, limit the expression or recognition of tumor neoantigens, promote an immunosuppressive TME, or facilitate immune escape contribute to treatment failure.5,7 Various strategies to overcome PD-1 resistance or optimize antitumor responses are being evaluated in clinical trials for melanoma and other solid tumors. Several strategies take a combinatorial approach, using a PD-1/ PD-L1 inhibitor as a backbone and adding a novel or existing therapeutic approach (TABLE).

The FDA has already approved dual checkpoint blockade combining a PD-1 inhibitor (nivolumab) with a CTLA-4 inhibitor (ipilimumab) as a first-line strategy for unresectable or metastatic melanoma based on the pivotal CheckMate 067 trial.

Data from the trial showed that the combination produced a significantly better objective response rate (ORR), median progression-free survival (PFS), and median overall survival (OS) than ipilimumab monotherapy.

After preclinical data suggested that ICI therapy might be more effective in the neoadjuvant setting versus the adjuvant setting, Wargo and colleagues initiated a phase II trial to evaluate neoadjuvant use of the ipilimumab/nivolumab combination in patients with high-risk melanoma. The combined approach produced a higher RECIST overall response rate than nivolumab monotherapy (73% vs 25%, respectively) and a higher rate of pathologic complete response (45% vs 25%, respectively). However, the study ended prematurely because of the 73% rate of grade 3 toxicity in the combination arm.

Wargo said despite the disappointing outcome, they succeeded in identifying a possible biomarker of response. &ldquo;Everyone is looking for the holy grail of biomarkers, and a lot of the focus has been on cytotoxic T cells,&rdquo; she said, adding, &ldquo;What we found were major differences in B cells between responders and nonresponders to immune checkpoint blockade.&rdquo;

Responders had higher proportions of class-switched memory B cells and plasma-like cells, whereas nonresponders had a higher proportion of na&iuml;ve B cells.

Switched B cells have been activated to produce a specific class of antibodies, whereas na&iuml;ve B cells have not yet been activated. Wargo said, &ldquo;We found that these B cells were actually forming little lymph nodes within the patient&rsquo;s tumor.&rdquo; She said that although they determined that activated effector B-cell phenotypes contributed to response, other studies have reported that B cells were detrimental to response. Wargo said she hoped more research into biomarkers of ICI response would look beyond T cells to include B cells and other immune cell subtypes.

Wargo said an important goal of investigators is to improve the safety of regimens that combine ICIs. She noted that the OpACIN-neo trial also evaluated neoadjuvant ipilimumab/nivolumab in melanoma and found that &ldquo;you can reduce the dose of the CTLA-4 inhibitor and increase the dose of the PD-1 inhibitor to lower the toxicity while preserving the response.&rdquo;20 She praised the multinational study as an example of what global cooperation can achieve and stressed the importance of &ldquo;breaking down silos&rdquo; between investigators in melanoma. Recently, Wargo and other melanoma experts around the world formed the International Neoadjuvant Melanoma Consortium to advance neoadjuvant melanoma research. They plan to meet in June at the 2019 American Society of Clinical Oncology Annual Meeting.

Lymphocyte-activation gene 3 (LAG-3) is another coinhibitor expressed on various immune cells in the TME, including on the surface of CD8-positive and CD4-positive T cells. LAG-3 is often coexpressed with PD-1 and exacerbates T-cell exhaustion.

Preclinical and early-phase trial data suggest that inhibitors of LAG-3 and PD-1 work synergistically to reactivate T cells.

A phase I/II study evaluated combinatorial therapy with the LAG-3 inhibitor relatlimab (BMS-986016) and nivolumab in heavily pretreated patients with advanced melanoma. The ORR was 11.5%, with patients with &ge;1% of tumor cells positive for LAG-3 expression significantly more likely to respond.

The regimen was well tolerated and is now being studied in a phase II/III trial as first-line therapy for advanced melanoma (NCT03470922). Several ongoing clinical trials are evaluating regimens that combine PD-1 inhibitors with novel ICIs that target other coinhibitory receptors.

Immune-Stimulating Agents Plus PD-1 Inhibition

Instead of blocking pathways that inhibit immune function, immune-stimulating agents amplify immune responses and may act synergistically with PD-1/PD-L1 inhibitors.

Multiple clinical trials are investigating regimens that combine an ICI with a stimulatory checkpoint agonist. Stimulatory checkpoints are molecules that enhance innate and adaptive antitumor immune responses by improving immunologic memory; promoting T-cell activation, differentiation, and proliferation; and boosting T-cell effector function.

Oncolytic viruses, toll-like receptor (TLR) agonists, and anticancer vaccines are other types of immune-stimulating agents being paired with ICIs in clinical trials.

The monoclonal antibody APX005M is a CD40 agonist being investigated with nivolumab for advanced melanoma that has progressed during antiPD-1 therapy, and data from a phase I/II clinical trial of the doublet were presented at the 2019 AACR Annual Meeting. Only 1 of 5 patients with melanoma in the phase I trial responded, achieving a partial response (PR). In the phase II trial, which included 10 patients, investigators documented 2 PRs and no complete responses (CRs). The regimen was well tolerated, and recruitment for the phase II trial is ongoing.

In a phase Ib/II trial, the TLR9 agonist SD-101 was administered with pembrolizumab (Keytruda) to patients with advanced melanoma who were na&iuml;ve to PD-1 inhibitors. The ORR for the 25 participants who completed at least 15 weeks of therapy was 60% (CR, 12%), and the disease control rate (DCR) was 76%. The combination was well tolerated and did not increase the risk of immune-related adverse events (irAEs).

The oncolytic virus T-VEC (talimogene laherparepvec [Imlygic]), a modified herpes simplex virus type 1 approved for advanced melanoma, expresses granulocyte-macrophage colony-stimulating factor to increase tumor-specific antigen presentation by dendritic cells at the injection site, thus creating antigen-specific CD8-positive T cells for the PD-1 inhibitor to reactivate and proliferate. In the phase Ib portion of the MASTERKEY-265 trial, 21 patients with advanced melanoma received single-agent T-VEC followed by a combination of T-VEC and pembrolizumab.

The ORR was 62%, and the CR rate was 33%. Even some patients with a low density of CD8- positive TILs or no IFN-&gamma; gene signature at baseline achieved responses, suggesting that T-VEC turned cold, or nonT-cell inflamed, tumors into hot tumors.

Biopsies after the T-VEC run-in phase showed increases over baseline in the density of infiltrating CD8-positive T cells and upregulated PD-1 expression. Upregulation of PD-1, combined with PD-L1 expression by tumor cells, likely inhibited the antitumor activity of single-agent T-VEC at distant sites. Concurrent PD-1 blockade overcame PD-L1induced resistance to oncolysis in the TME, however, and freed effector T cells to migrate to distant sites.

The findings provide a logical rationale for combining the 2 treatment modalities, and the combination is being studied further in the phase III KEYNOTE-034 trial (NCT02263508).

PD-1 Inhibition With Epigenetic Modulation

Drugs that inhibit DNA methylation or histone deacetylation (HDAC) sensitize cancer cells to ICIs in part by increasing expression of tumor-specific antigens and costimulatory molecules as well as upregulating TIL expression of PD-1/PD-L1.

The novel HDAC inhibitor entinostat (SNDX-275) also impairs the function of myeloid-derived suppressor cells, which cause immunosuppression in the TME.

At the 2019 AACR Annual Meeting, investigators presented preliminary findings from ENCORE-601, a clinical trial that paired entinostat with pembrolizumab.

The study included 53 patients with advanced melanoma who progressed during or after treatment with a PD-1 inhibitor; 70% had also used ipilimumab. The ORR with entinostat plus pembrolizumab was 19%, the median duration of response at data cutoff was 12.5 months, and the median PFS was 4.2 months. The most common grade 3/4 AEs were neutropenia, fatigue, and hyponatremia; only 5 patients experienced grade 3/4 irAEs.

Multiple clinical trials are evaluating the efficacy and safety of combining adoptive cell therapy (ACT) using TILs with a PD-1 inhibitor for patients with advanced melanoma.

One of the main focuses for TILs is in patients with melanoma who did not benefit from antiPD-1 therapy. Data were presented in November 2018 from a phase II trial of lifileucel (LN-144), an investigational cryopreserved autologous TIL therapy, in patients with metastatic melanoma that progressed despite PD-1 inhibitor therapy.

The multinational trial enrolled 47 patients, who were infused with manufactured TILs derived from a sample of their tumor. Before infusion, patients underwent nonmyeloablative lymphodepletion; after infusion, they received up to 6 doses of interleukin-2.

The ORR was 38% and included 1 CR; 38% of patients also achieved stable disease, which resulted in a DCR of 77%.

The median duration of response was 6.4 months (range, 1.3-14 months). Almost all the responders (16/17) had not responded to prior treatment with a PD-1 inhibitor. Hematologic toxicities grade &ge;3 were common, with high rates of thrombocytopenia (81%), neutropenia (53%), febrile neutropenia (53%), and anemia (47%). The study is ongoing and has already started recruiting patients for an expanded cohort that will include 80 to 100 participants.

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Investigators across the globe have expended considerable effort attempting to identify mechanisms of response and resistance to ICIs. The general conclusion is that each patient&rsquo;s response to immunotherapy is likely complex and highly individualized.

Christin L. Melton, ELS | Authors

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