Role of Anti-PD-1/PD-L1 Immunotherapy in Bladder Cancer

Surgical intervention, chemotherapeutic regimens, and radiation therapy have been the primary weapons in the frontline fight against cancer. However, immunotherapy, a fourth therapeutic modality, now holds the promise of a new age in the battle against even the most resistant cancers. Bladder cancer, in particular, has carried a poor prognosis in late-stage and metastatic disease. Current treatments have been inadequate with only minimal success. The discovery of checkpoint proteins, along with a better understanding of the role they play in tumor evasion mechanisms, has opened the door to mobilizing a significant immune response in patients with cancer. Using monoclonal antibodies (mAbs) to block the inhibitory signals of the programmed death-1 (PD-1)/ programmed death-ligand 1 (PD-L1) pathway has the potential to change traditional treatment pathways. This supplement will review the traditional treatments of bladder cancer, describe PD-1 and PD-L1 pathways, and present evidence from clinical trials evaluating the specific role of drugs targeting the PD-1 pathway in bladder cancer.

Bladder Cancer

Epidemiology

Bladder cancer is the fourth-most common cancer in the United States,¹ affecting men 3 times more than women.¹ In 2014, 74,690 new cases and 15,580 deaths are projected to occur in the United States.² This represents 4.5% of all new US cancer cases and 2.7% of all US cancer deaths.² Also, in 2011, approximately 571,518 patients were living with bladder cancer.² The median age of diagnosis is 65 years; in fact, diagnoses in patients under the age of 40 years are considered rare.¹

Types of Bladder Cancer

Bladder cancer is a broad term that encompasses a variety of cancer types that involve the epithelial lining of the urinary bladder. Bladder cancers can be classified into 1 of 3 tumor types: (1) nonmuscle invasive; (2) muscle invasive; and (3) metastatic tumors.¹ In the United States, 90% of urothelial tumors begin in the bladder, while only 8% originate in the renal pelvis and 2% in the ureter and urethra.¹ The most common subtype of bladder cancer in the United States is urothelial or transitional cell carcinoma. This subtype develops anywhere transitional urothelial epithelium exists, including the renal pelvis, ureter, bladder, the beginning two-thirds of the urethra, and anywhere in between. The remaining section of the urethra contains squamous epithelium; tumors located here are the cause of squamous cell tumors, which comprise just 3% of tumors in the United States.¹

Risk Factors

According to the American Cancer Society, smoking is the leading contributing factor to a diagnosis of bladder cancer. Overall, approximately 50% of bladder cancer cases can be attributed to smoking. Carcinogens from tobacco smoke are filtered by the kidneys, thus leading to bladder cell damage due to concentrations of these carcinogens in the urine. Therefore, people who smoke are 3 times more likely to develop bladder cancer compared with nonsmokers.

Exposure to certain industrial chemicals has also been linked to developing bladder cancer. Those occupations associated with an increased risk of bladder cancer include painters, machinists, printers, hairdressers, truck drivers, and any occupation that involves handling dye. The risk of developing bladder cancer increases significantly when industrial exposure is paired with smoking.

Other risk factors include race, age, sex, bladder birth defects, genetics, long-term chemotherapy with cyclophosphamide, pelvic radiation, and having taken pioglitazone for diabetes for more than 1 year.³

Pathophysiology

Non—muscle invasive disease (NMID) tumors comprise nearly 70% of newly detected cases of bladder cancer. Of these NMID tumors, 70% are confined to the mucosa, 25% are confined to the submucosa, and less than 5% are carcinoma in situ (CIS), which are flat, high-grade lesions. Characteristics of NMID tumors include being friable with high bleeding tendencies and a likely recurrence in the same area or a different area of the bladder.1 The rate of recurrence within 5 years for patients with tumors confined to the mucosa or submucosa is approximately 31% to 78%.¹

Diagnosis/Staging

Most often, patients with bladder cancer present with microscopic or gross hematuria. They may also present with frequent urination or a urinary tract infection as their primary complaint, albeit less frequently. Pain or an upper tract obstruction may also be a presenting symptom, but this is often indicative of a more advanced lesion.¹ In-office cystoscopy is typically performed in patients presenting with pain or upper urinary tract obstruction to determine whether or not a lesion is present.

Urine cytology may also be performed, which can detect abnormal cells in the urine. If the cystoscopy fails to identify a primary lesion and abnormal cells are detected with urine cytology, this warrants further investigation of areas of the urinary tract. If a lesion is observed during cystoscopy, a transurethral resection of the bladder tumor (TURBT) should be performed to confirm a diagnosis and evaluate the extent of the disease.¹ However, depending on the appearance of the tumor, a computerized tomography (CT) scan or magnetic resonance imaging may be warranted before TURBT.¹

Tumors that are determined to be high grade, invasive, or sessile (solid) need diagnostic studies prior to the TURBT. While there are many tests available to evaluate upper tracts, CT urography is considered the best modality, assuming the patient has no restrictions to receiving contrast.¹ Further diagnostic testing is driven based on findings during the initial evaluation. For example, a bone scan may be necessary if elevated levels of alkaline phosphatase are detected; chest imaging if the disease is considered invasive.¹ TURBT with a bimanual examination under anesthesia (EUA) is used in the diagnosis, staging, and treatment of visible tumors. For CIS, biopsies, including of the surrounding muscles, should be collected during the procedure, to determine whether the cancer is invasive. In male patients, additional biopsies should be considered, including a transurethral resection biopsy of the prostate.¹ Utilizing the tumor, node, metastasis (TNM) staging system shown in the Table,⁴ the cancer can be classified clinically or pathologically, which is noted with a “c” or “p.”¹ Two grading systems are currently employed to assign tumor grades. The 1973 World Health Organization (WHO) classification is the most widely utilized system; however, a new WHO system was introduced in 2004. Although the newer system may allow for more precise prognostic significance, it requires pathologists to determine classification and has yet to be validated by clinical trials.¹

Treatment of Bladder Cancer

Traditional treatment options for bladder cancer include surgery, radiation, chemotherapy, and biologic therapy. Treatment decisions are dependent on the type of bladder cancer, clinical stage and grade, and pathology.^1,5

Types of surgery include TURBT, radical cystectomy, partial cystectomy, and urinary diversion.⁵ As previously mentioned, TURBT is used in patients with observed lesions to confirm a diagnosis, assist in staging, and treat visible tumors.¹ TURBT can be performed with an EUA to resect any visible tumor and establish if muscle invasion has occurred.¹ Radical cystectomy includes removal of the bladder, lymph nodes, and nearby organs to which the cancer has spread.⁵ This procedure involves a cystoprostatectomy in males to remove the prostate and seminal vesicles.^1,5 Typically, but not always, a hysterectomy is also performed in women.^1,5 Once the bladder has been removed, the surgeon will form a urinary diversion to create a new way for urine to exit the body.

Types of urinary diversions include ileal conduits, a urinary reservoir, a continent cutaneous reservoir, or a neobladder. A urinary diversion using an ileal conduit consists of the surgeon removing a section of the bowel and repositioning it, thus allowing urine to drain from the ureters to a stoma and then out of the body into an external pouch.⁶ Another option for urinary diversion is the use of an internal reservoir formed from a section of bowel. An external pouch is not necessary because urine drains into an internal reservoir. A neobladder is intended to act similarly to a native bladder; however, it is associated with an increased risk of nocturnal incontinence or urinary retention. Unfortunately, urinary retention may result in the patient needing to insert a catheter through the urethra to ensure the neobladder is completely emptied.^1,6 A partial removal of the bladder is less common than a radical cystectomy and is surgically feasible in approximately 5% of cases.¹ Both radical and partial cystectomies involve the removal of pelvic lymph nodes.¹

Chemotherapy, drugs used to halt cancer cell growth by killing cells or inhibiting the cells from dividing, can be given systemically or intravesically, depending on the type and stage of bladder cancer. Often, a combination of drugs is used.⁵ Mitomycin C, thiotepa, doxorubicin, valrubicin, gemcitabine, and epirubicin may be used intravesically in bladder cancer. Of these, mitomycin C is most commonly used, although randomized trials have not demonstrated one agent to be superior to another.¹ Additionally, randomized trials have not demonstrated a reduction in progression or mortality with intravesical chemotherapy. A meta-analysis published in 1996 demonstrated a decreased tumor recurrence with intravesical chemotherapy but no decrease in disease progression or mortality.⁷ A more recent analysis, published in 2012, suggested that intravesical chemotherapy with gemcitabine may be more effective and less toxic than intravesical mitomycin C. Compared with intravesical immunotherapy with Bacillus Calmette-Guerin (BCG), the effects of intravesical gemcitabine were similar in intermediate-risk patients, less effective in high-risk patients, and superior in patients who were refractory to BCG therapy.⁸

Table. AJCC TNM Staging System for Bladder Cancer⁴

AJCC indicates American Joint Committee on Cancer.

Republished with permission of the American Joint Committee on Cancer from Edge S, Byrd DR, Compton CC, et al. AJCC Cancer Staging Manual. 7th ed. New York, NY: Springer; 2010. Permission conveyed through Copyright Clearance Center, Inc.

Both neoadjuvant and adjuvant systemic chemotherapy regimens are utilized in bladder cancer, yet evidence for using neoadjuvant therapy is stronger.¹ An intergroup, randomized phase III study assessed survival in patients treated with cystectomy alone versus 28-day cycles of methotrexate, vinblastine, doxorubicin, and cisplatin (MVAC) before cystectomy. Over 11 years, 317 patients were enrolled and randomized.⁹ Median survival was 46 months (95% CI, 25-60) in patients receiving the cystectomy alone compared with 77 months (95% CI, 55-104) in those receiving chemotherapy prior to cystectomy.⁹ Increased survival has been noted in studies with adjuvant therapy consisting of MVAC; methotrexate, vinblastine, epirubicin, and cisplatin (MVEC); and cyclophosphamide, doxorubicin, and cisplatin (CAP). Yet, due to the heterogeneity in methodology, whether or not this finding applies to all patients with urothelial tumors is questionable.¹

For patients with advanced or metastatic disease, 3 types of chemotherapy agents are generally utilized: cisplatin, taxanes, and gemcitabine. Gemcitabine plus cisplatin (GC) and dose-dense MVAC is the most common combination. Two large randomized phase III studies compared standard MVAC to dose-dense MVAC. Study results demonstrated that standard MVAC was inferior to the dose-dense regimen in terms of overall survival and toxicity among patients with locally advanced or metastatic disease. Additionally, standard MVAC was associated with higher mortality than GC due to drug toxicity, and offered no additional overall survival or progressionfree survival benefit; therefore, standard MVAC is no longer recommended.¹ The preferred treatment recommendation, according to the National Comprehensive Cancer Network (NCCN) guidelines for metastatic disease, is GC and dose-dense MVAC.¹ Based on results from studies showing that taxanes are effective as both first-line and palliative treatments, 2- and 3-drug combinations, with or without cisplatin, are being investigated for use as first-line treatment. It is important to keep in mind that the benefits of adding paclitaxel to regimens do not outweigh the risks associated with its use.¹

The third type of treatment available for patients with bladder cancer is biologic therapy (also referred to as biotherapy or immunotherapy). BCG, administered intravesically, is the only immunotherapy currently approved for bladder cancer by the FDA.⁵ Although treatment with BCG is largely more effective when compared with intravesical chemotherapy, the associated toxicity limits its use as first-line therapy in all patients.¹⁰ However, therapy with BCG is preferred over chemotherapy for adjuvant treatment of high-grade lesions.¹ Drugs targeting the PD-1/PD-L1 pathway are also a type of immunotherapy, but these agents are still under investigation for use as bladder cancer therapy. External or internal radiation may also be considered, and its use varies given the type and stage of bladder cancer.⁵

NCCN Guidelines

Non—Muscle-Invasive Disease

The standard of care for Ta, T1, and Tis tumors is TURBT followed by TUR. For high-grade T1 tumors and, in some cases, Ta tumors, this may be followed by biopsy of the prostate, and a possible second TUR of the prostate.¹ After a complete resection, prophylactic or adjuvant intravesical therapy may be warranted. Due to the somewhat high risk of recurrence for cTa (low-grade tumors), the guidelines recommend considering giving a single dose of immediate intravesical chemotherapy within 24 hours, possibly followed by a 6-week induction of intravesical chemotherapy. If a patient’s risk of recurrence is minimal, the immediate dose may be adequate. (Immunotherapy should not be used in patients who do not have high-grade tumors.1) However, in patients with high-grade tumors, NCCN guidelines support intravesical BCG over treatment with intravesical mitomycin C.¹

In contrast to the previously mentioned tumor types, cT1 tumors have a higher risk for recurrence and progression. TURBT is also recommended for this tumor type, and follow-up TUR is strongly advised—except in cases of high-risk tumors when cystectomy is recommended instead. After the second TUR, immunotherapy with BCG or cystectomy is recommended if residual disease is present.¹ If no residual disease is present, intravesical BCG or mitomycin C can be administered instead.¹ Given that primary CIS or Tis is a precursor of invasive disease, the usual course of therapy is TURBT followed by intravesical BCG administered weekly for 6 weeks. For patients unable to tolerate BCG therapy, mitomycin may be considered.¹

Patients with recurrent or persistent cTa, cT1, and Tis disease who had an initial TURBT should have another TURBT followed by intravesical adjuvant therapy. Patients who responded to intravesical therapy after initial treatment with positive results at their 3-month follow-up may be given a second induction course of BCG or mitomycin, but no more than 2 consecutive induction courses should be administered. If residual disease is still noted 3 months after the second induction course, the patient should undergo another TURBT. If recurrence involves Tis or cTa tumors post TURBT, intravesical therapy with a different agent may be considered as an alternative to cystectomy, although experts disagree on whether valrubicin is of value.¹

In patients with recurrence of high-grade cT1 disease, following TURBT and induction of BCG therapy, cystectomy is recommend as first-line therapy; for nonsurgical candidates, concurrent chemoradiation, use of a different intravesical agent, or a clinical trial may be considered.¹ After 1 or 2 courses of intravesical treatment, patients with no residual disease at follow-up may be candidates for BCG maintenance therapy (usually for 1-3 years). Although NCCN panelists do not believe that this should be routine practice, they do agree that maintenance therapy should be an available option.¹ If patients with no documented recurrence have cytologypositive cystoscopy with negative imaging, TUR needs to be performed with biopsies.¹ For biopsy-positive disease, intravesical BCG therapy should be considered, potentially followed by maintenance BCG therapy. Patients who fail to respond to BCG therapy or only have a partial response should be given alternative options, such as cystectomy, a different intravesical agent, or enrollment into a clinical trial.¹

Muscle-Invasive Disease

Most muscle-invasive tumors are high-grade urothelial carcinomas; therefore, in contrast to nonmuscle-invasive disease, muscle-invasive disease always requires TURBT with follow-up therapy, such as cystectomy, neoadjuvant or adjuvant therapy, bladder-preserving approaches, or chemotherapy. Available surgical treatments include radical cystectomy and partial cystectomy; however, the partial procedure is generally reserved for tumors on the dome of the bladder, for which less than about 5% of cases are actual candidates. Patients with lesions in the trigone or bladder neck should not have a partial cystectomy, as presence of a Tis tumor in other areas is a relative contraindication to partial cystectomy.

Select patients with T2 and T3a urothelial carcinomas may be candidates for bladder-preserving therapy. These treatment approaches include aggressive endoscopic TUR, TUR followed by chemotherapy, radiotherapy monotherapy, or a combination of chemotherapy with radiotherapy.¹ The International Consultation on Urologic Diseases-European Association of Urology evidence- based guidelines recommend the previously discussed bladder-preserving treatment modalities for patients who are medically unfit for surgery and therefore seeking an alternative to cystectomy.¹ This endorsement may be attributed to the underutilization of bladderpreserving treatment for those who are not candidates for surgery, such as the elderly and racial minorities.

Plus, it has been estimated that between 23% and 50% of patients over the age of 65 years with muscle-invasive bladder cancer receive no treatment or receive nonaggressive treatment.¹ However, if a lesion is solitary, less than 2 centimeters in size, or has minimally invaded the muscle and there is no associated in situ component, palpable mass, or hydronephrosis, TURBT may be a treatment option. If this option is chosen, a re-resection should occur within 4 weeks to guarantee that no residual disease is present.¹

Chemotherapy monotherapy after TUR is not recommended. More patients can avoid complete resection of the bladder when chemotherapy is instead combined with concurrent radiotherapy,¹ unless comorbidities prevent them from tolerating a cystectomy or chemotherapy. ¹ A phase III randomized trial of 360 patients with stage T2, T3, or T4a bladder cancer demonstrated that radiotherapy plus mitomycin C and fluorouracil (5-FU) resulted in a significantly higher locoregional diseasefree survival rate than radiotherapy alone (67% vs 54%, respectively; P = .01), with an absolute difference of approximately 12 percentage points (95% CI, 1.3-20) for 2-year recurrence-free rates.¹⁰

Evidence does not support the use of neoadjuvant chemotherapy before treatment with bladder-preserving chemotherapy given concurrently with radiation.¹ Radiotherapy with concurrent cisplatin-based chemotherapy as a radiosensitizer is the most commonly used and most extensively studied chemoradiation regimen for patients with muscle-invasive bladder disease. Radiation is given after TURBT, along with 2 doses of concurrent chemotherapy in weeks 1 and 4. If no disease is noted at follow-up, additional radiation with 1 dose of cisplatin is administered.¹ Cisplatin, cisplatin with 5-FU, 5-FU with mitomycin C, and cisplatin with paclitaxel are all practical options for bladder-preserving chemoradiation.¹

Recommended chemotherapy regimens for patients with advanced bladder disease depend on several factors, including comorbidities and risk classification based on the extent of disease. Patients with good performance status, no visceral involvement or bone disease, and normal alkaline phosphatase or lactic dehydrogenase levels generally have long-term survival rates with combination chemotherapy without other treatment modalities. Unfortunately, in terms of remission rates,¹ patients with poor performance status or visceral disease have demonstrated worse outcomes with multiagent combination chemotherapy. Palliative chemotherapy options depend on which agents the patient received earlier; evidence supports the use of docetaxel, paclitaxel, or gemcitabine alone. In some instances, chemotherapy can be combined with palliative radiation to treat metastases or pelvic recurrence; on the other hand, concurrent chemotherapy should not be used with high-dose radiation.¹

T2, T3, and T4 tumors should be treated with radical cystectomy and pelvic lymphadenectomy. However, stronger evidence exists for using neoadjuvant chemotherapy for stage T3 disease compared with stage T2 disease. As a result, neoadjuvant chemotherapy is recommended for treating cT3 tumors, and may be considered— but is not strongly recommended—for treating cT2 tumors. Based upon pathologic risk, adjuvant chemotherapy may be considered for patients who did not receive neoadjuvant treatment.¹ Patients with T4b disease that is node negative should be administered 2 to 3 courses of chemotherapy with or without radiotherapy.

But for certain patients with node-negative disease, cystectomy with or without chemotherapy may also be considered.¹ Unforfunately, patients with T3 or T4 lesions are not candidates for partial cystectomy. Instead, chemotherapy with or without radiation is a treatment option for patients with poor prognostic factors (eg, positive nodes). After the initial course of treatment, if cancer still exists, the patient should be treated using aggressive treatment pathways, such as those used for metastatic disease.¹ Then, after cystectomy, patients with metastatic disease or local recurrence can be managed with either palliative chemotherapy, radiation, or a combination of the treatments. If disseminated metastatic disease occurs, treatment options include palliative chemotherapy with an agent that has not been used in prior lines of therapy, radiotherapy (in radiotherapynaïve patients), chemoradiotherapy, or palliative TURBT.¹

Emerging Therapies: Anti-PD-1/PD-L1 Immunotherapy

Attempting to harness the body’s natural immune response to fight cancer is not a new concept. Over 100 years ago, the first evidence emerged that linked a boost in the immune system to spontaneous remission in patients with cancer. William Coley observed this response after giving patients with inoperable tumors a mixture of toxins created from inactivated infectious agents.^12,13 Since that time, many therapeutic targets have been identified to try and enhance the immune response.¹⁴

For instance, BCG therapy is one example of immunotherapy that has long been used to stimulate the host immune response; it contains a live attenuated mycobacterium that is administered intravesically.^15,16 In other cancer types, immunotherapies include amplification of the immune response using cytokines, such as interleukin- 2, in melanoma and renal cell carcinoma.^12,16,17 Vaccines have also shown some promise, but the results of these modalities have been modest at best.

As our understanding of the immune response has grown, it has become clear that there are adaptive mechanisms that allow cancers to evade and suppress the normal immune response to foreign antigens.^12,18,19 The immune system is a complex system that maintains a fine balance between activation in the presence of foreign antigens, such as an infection, and suppression under normal circumstances.^18,20 Regulatory checkpoints do not allow the inflammatory and cytotoxic process to spin out of control and harm normal “self” tissue. Instead, cancer cells are derivatives of normal cells containing “self”-recognized antigens that may not be seen as foreign to the immune system; over time, though, abnormal proteins appear on the surface of the cell and an immune response is generated.¹³ Tolerance to the immune response, in combination with the many characteristics that define cancer cells, ultimately leads to a malignant disease process and evasion from the host immune system.^12,19-22

Conventional cytotoxic therapies, such as chemotherapy and radiation treatment, have made great contributions in the fight against many cancers, as demonstrated by the significant gains in survival in breast cancer, non-small cell lung cancer, and many other cancers.13

However, in many cancers, such as bladder cancer, progress has been lacking.²³ Immunotherapy continues to gain momentum as an effective therapeutic approach for many types of cancer.¹³ It is directed at restoring the ability of a patient’s immune system to recognize the cancer as foreign and eliminate it.²⁴

Immunotherapy can be classified into 1 of 2 types: passive or active.¹³ Passive immunotherapy does not initiate the patient’s own immune system response, but rather directly has a cytotoxic effect on cancer cells by introducing exogenous immune system elements.¹³ The hallmark of this approach has been the development of mAbs directed against specific antigens unique to tumor cells, thus selectively targeting the tumors for destruction.^12,13 The mAbs have become valuable assets in the treatment strategies for many cancers. Specifically, rituximab and trastuzumab, which target the CD20 protein and the human epidermal growth factor receptor 2 (HER2), respectively, have shown very positive results.^25-27 Trastuzumab has been shown to increase time to progression, overall response, duration of response, and overall survival in patients with HER2-positive metastatic breast cancer. Rituximab has been widely used in B-cell malignancies such as non-Hodgkin lymphoma and chronic lymphocytic leukemia.^12,27 Immunotherapy that seeks to modify the endogenous immune response is referred to as active immunotherapy. ¹³ Sipuleucel-T, an autologous cellular immunotherapy approved by the FDA in 2010, induces an immune response targeted against an antigen expressed in most prostate cancers. Sipuleucel-T improved median overall survival by 4.1 months compared with placebo (25.8 months vs 21.7 months) in a randomized, doubleblind, placebo-controlled multicenter trial in patients with asymptomatic or minimally symptomatic metastatic castrate-resistant (hormone-refractory) prostate cancer.^16,28-31 Exhibiting a “vaccine-like” effect, this approach activates both the innate and adaptive sections of the immune response, providing the potential of a more sustained effect on tumors. Innate immunity is the body’s generic defense in the presence of foreign antigens, while an adaptive response recognizes and reacts to specific antigens. Adaptive immunity creates memory cells and confers a longer lasting immunity to a specific antigen.¹³ However, the effectiveness of using dendritic cell-based vaccinations did not achieve the same dramatic results that were observed in animal models.³⁰

With the discovery of immune checkpoints, there has been a renewal of interest in the active immunotherapy approach and a broad spectrum of research in the mechanisms that lead to immune evasion by cancer cells.¹³ Further analyzing the development of cancer and how it becomes resistant to treatment has led to recognition of a host of new therapeutic targets, including regulatory T cells, tumor-infiltrating lymphocytes, and interleukins.³² In reviewing an effective tumor response, there are several steps in the cycle of tumor cytotoxicity.¹³ First, tumor-specific antigens are captured by dendritic cells, which are a form of antigen-presenting cells.^16,27 Independent of that process, there must be coregulatory signals from pro-inflammatory cytokines that indicate whether the T cell should be turned on or off.^27,33 Antitumor activity will not occur without this secondary signal from co-signaling molecules, which serves to activate clonal expansion, cytokine secretion, and the action of the T cell.³³ In the presence of the proper signaling, cytotoxic T cells are presented with the captured antigen and thus will be primed to act against the specific cancer antigen.¹⁶ The primed and activated immune effector cells then travel to the tumor, and infiltrate and bind to the specific tumor antigen.^13,16,27 This results in cancer cell death and the release of additional antigens, thus repeating the cycle.¹³ Tumors may break any point in this cycle, which can lead to cancer progression.^13,34 Under normal circumstances, the secondary coregulatory signal that limits clonal expansion of T cells serves as a protective measure against the development of chronic inflammation and limits damage to normal tissue by ensuring that the immune response occurs at the right time under the right circumstances.¹³ Within the process, there are checkpoint pathways that manage these coregulatory signals leading to either the stimulation or inhibition of T cells (depending on the circumstances). ^27,33 By manipulating the checkpoint pathways that regulate T-cell response or function, cancer can evade the immune response.¹³ Researchers have identified 2 inhibitory pathways that utilize the cytotoxic T-lymphocyte antigen-4 (CTLA-4) and PD-1 protein receptors for signaling.^14,19,27,32 The effectiveness of targeting CTLA-4 was confirmed with the approval of ipilimumab by the FDA in March 2011.^12,16,27,13 At the time of its approval for the treatment of relapsed metastatic melanoma, the human CTLA-4 blocking antibody ipilimumab was the first significant advancement in over 10 years in the treatment of melanoma.^12,16,27,35

Activated T cells express the PD-1 receptor (also known as CD279) on their surface, which plays an active role throughout all stages of T-cell activity, as shown in the Figure.^19,20,22 PD-L1 (B7-H1; CD274) is found in multiple tissue types, including resting T cells, B cells, dendritic cells, macrophages, and parenchymal cells, and on pancreatic islet cells and the vascular endothelium. ^20,22 Programmed death-ligand 2 (PD-L2) (B7-DC; CD273) has a more limited distribution, as it is found on macrophages and dendritic cells.^20,22,32 The interaction between the PD-1 receptor and the PD-L1 ligand induces an inhibitory signal leading to a reduction in cytokine production, a decrease in T-cell proliferation, and suppression of cytolytic activity of CD4+ and CD8+ T cells. ^18-20,28,33 The PD-L1 is also known to bind to the B7-1 (CD80) receptor, which sits beside the PD-1 receptor on T cells.^14,18,33,36 This pathway is also inhibitory and causes a reduction in cytokine production and T-cell proliferation in addition to apoptosis of T cells.^14,18,36-38 During an active infection and subsequent inflammatory response, PD-1 serves to constrain the function of activated T cells in peripheral tissue to eliminate the potential for autoimmunity.^12,14 In the tumor microenvironment, overexpression of PD-L1 on cancer cells drives up regulation of inhibitory activity and the subsequent dysfunction of tumor-infiltrating T lymphocytes.²¹ The overall immunosuppressive effect on the T cells allows the tumor to thrive and evade the normal response. Indicative of poor clinical outcomes, the increased expression of PD-L1 has been documented in several malignant cell lines from the lung (in non-small cell lung cancer), esophagus, ovary, kidney, pancreas, skin (in melanoma), bone marrow (in multiple myeloma), and bladder.^19,20,22

To block the inhibitory signaling that allows the tumor to escape the body’s immune response, research has focused on developing mAbs directed against both the PD-1 receptor and its primary ligand PD-L1. Theoretically, by blocking the PD-1:PD-L1 interaction, T cells will be switched on, instead of off, and can assist in eliminating the tumor. Of course, this is an oversimplification of what we know as a very complex process that leads to the development of a malignancy, and there are certainly redundant pathways that must be explored. How ever, this line of research has vast potential to advance the treatment of cancer. In particular, metastatic bladder cancer carries a poor prognosis with its current standards of care. With MPDL3280A, a human anti-PD-L1 mAb, receiving breakthrough therapy designation from the FDA, the outlook for the treatment of bladder cancer, which has had no new treatment advances in almost 30 years, looks more promising.^23,39

Figure. Tumor Immunology and the PD-L1/PD-1 Pathway²²

Programmed death-1 (PD-1) is a T-cell molecule that binds to programmed death ligand-1 (PD-L1) or programmed death ligand-2 (PD-L2). PD-L1 is typically expressed on tumor cells and is induced by gamma interferon secreted by activated T cells. The activated T cells that could kill tumors are specifically disabled by those tumors that express PD-L1 and bind to PD-1 to create a phenotype known as T-cell exhaustion.

Reprinted from Chen DS, Irving BA, Hodi FS. Molecular pathways: next-generation immunotherapy—inhibiting programmed death-ligand 1 and programmed death-1. Clin Cancer Res. 2012;18(24):6580-6587, with permission from AACR.

Select Anti-PD-L1 Immunotherapies for Bladder Cancer

MPDL3280A (Genentech/Roche)

MPDL3280A, an anti-PD-1 immunotherapy with breakthrough designation from the FDA, inhibits the binding of PD-L1 to both PD-1 and B7-1. This action can restore antitumor T-cell activity and improve T-cell priming.³⁸ MPDL3280A does not interfere with the PD-L2:PD-1 interaction; therefore, immune homeostasis is maintained and autoimmunity is possibly prevented.³⁹ A multicentered, phase I study is currently evaluating the safety, tolerability, and pharmacokinetics of MPDL3280A, including the incidence of dose-limiting toxicities and the nature of those toxicities. Key eligibility criteria include measurable disease according to the Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 criteria and an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1.^40,41

An ongoing phase I expansion study is evaluating participants with renal cell carcinoma, melanoma, nonsmall- cell lung cancer, urothelial bladder cancer (UBC), and other tumor types. Preliminary data were presented at the European Society for Medical Oncology meeting in Madrid, Spain, in September 2014. Participants with UBC received MPDL3280A 15 mg/kg intravenously every 3 weeks for up to 16 cycles. Of the 70 participants with UBC, 33 were PD-L1-positive and 36 were PD-L1- negative; ¹ patient had unknown PD-L1 status. Of the efficacy evaluable population, 91% of participants received prior platinum-based therapy (mostly cisplatin) and 49% already had a cystectomy. The median age of participants was 65 years, and 73% were male.³⁹ Treatment with MPDL3280A was well tolerated, with no grade 5 treatmentrelated adverse events reported; only 5% of participants experienced a grade 3 or grade 4 adverse event. Therapy with MPDL3280A for UBC was not associated with renal toxicity, and no investigator-assessed immune-related toxicities were observed. The most common adverse events reported were fatigue (15%), decreased appetite (12%), and nausea (11%). Other adverse events included pruritus, pyrexia, asthenia, chills, dry skin, influenzalike illness, lethargy, and rash.³⁹ The median follow-up was 6 months for participants who were PD-L1-positive. Nineteen out of 22 responders had an ongoing response at the time of data cutoff. The objective response rate based on PD-L1 immunochemistry was 60% for IHC 3 (n = 10), 48% for IHC 2 (n = 23), 17% for IHC 1, and 8% for IHC 0.38 The median time to first response in PD-L1- positive patients (n = 17) was 43 days compared with 83 days for PD-L1-negative patients (n = 5). However, for the 22 participants evaluated, the median duration of response had not yet been reached.³⁹ Investigators observed that median progression-free survival appeared to be associated with PD-L1 expression (24 weeks in IHC 2/3 participants vs 8 weeks in IHC 0/1 participants, when comparing PD-L1-positive to PD-L1-negative).³⁹ A phase II, single-arm study is currently open for enrollment. This study consists of 2 cohorts: 1 of treatmentnaïve patients ineligible for platinum-containing therapy and 1 with patients who progressed during or after a prior platinum-based therapy. Participants in both cohorts will receive 1200 mg MPDL3280A intravenously every 3 weeks for 16 cycles, or 12 months. Patients in this study will be followed for up to 2 years.⁴²

Nivolumab (BMS-936558) (Bristol-Myers Squibb)

Nivolumab (Ono-4538/BMS-936558; Bristol-Myers Squibb; formerly MDX-1106), a fully human monoclonal antibody, was the first anti-PD-1 agent tested in clinical studies when it was evaluated in a phase I study with 6 patients with advanced, treatment-refractory solid tumors.¹³ A phase I/II open-label study comparing nivolumab monotherapy with nivolumab in combination with ipilimumab in 4 tumor types, including bladder cancer, is currently recruiting participants with measurable disease who are 18 years and older with an ECOG performance status of 0 or 1.^13,43 Patients excluded from this trial include those with active brain or leptomeningeal metastases, those who have an autoimmune disease; or those who have received systemic treatment with corticosteroids or other immunosuppressive agents within 10 days, or treatment with experimental antitumor vaccines, any T-cell costimulation or checkpoint pathway drugs, or anti-CTLA-4 antibodies.

The primary outcome of this study evaluating safety and efficacy is the objective response rate. A secondary objective will be measuring the rate of treatment-related adverse events leading to drug discontinuation during the first 12 weeks of treatment.⁴³ Participants in this parallel 2-arm study will receive: (1) nivolumab 3 mg/kg intravenously every 2 weeks or, (2) nivolumab 1 mg/kg plus ipilimumab 3 mg/kg intravenously every 3 weeks for 4 doses, followed by nivolumab 3 mg/kg intravenously every 2 weeks until documented disease progression, discontinuation due to toxicity, withdrawal of consent, or the end of the study.

Pembrolizumab (Keytruda) (Merck)

Pembrolizumab (Keytruda) was recently approved by the FDA for the treatment of patients with unresectable or metastatic melanoma and disease progression following previous treatment with certain other drugs. The drug was approved for this indication through the FDA’s accelerated approval program due to the tumor response rate and durability of response.⁴⁴

Pembrolizumab is a humanized immunoglobulin G4, anti-PD-1 monoclonal antibody with high affinity for the PD-1 receptor. This agent has no cytotoxic activity, but does have a low occurrence of antidrug antibodies, which has no impact on its pharmacokinetic properties. Pharmacokinetic studies demonstrate that pembrolizumab can be dosed every 2 or 3 weeks.⁴⁵

KEYNOTE-012, a phase Ib multi-cohort study of pembrolizumab in patients with PD-L1-positive advanced solid tumors, evaluated the safety, tolerability, and antitumor activity of pembrolizumab in participants with advanced triple-negative breast cancer, advanced head and neck cancer, advanced urothelial cancer, or gastric cancer.⁴⁶ The urothelial cohort evaluated participants with recurrent or metastatic cancer of the renal pelvis, ureter, bladder, or urethra with transitional or nontransitional histology. Participants had to have PD-L1-positive disease, which was defined as staining in the stroma or in 1% or more of tumor cells using a prototype immunohistochemistry assay and the 22C3 antibody clone. Candidates receiving systemic steroid therapy were excluded from the trial, as were those with an autoimmune disease or active brain metastases. Additionally, participants had to have an ECOG performance status score of 0 to 1. A total of 33 participants were enrolled in the urothelial study cohort and received treatment, including 3 participants with non-transitional cell or mixed histology.

The median age of participants was 70 years, and 24% of participants were treatment-naïve for advanced disease, 24% had tried 1 prior therapy, 18% had tried 2, 27% had tried 3, and 6% had tried 5 or more prior therapies. ⁴⁵ The majority of participants were male and had an ECOG performance status of 1 along with transitional cell histology.⁴⁴ Participants received pembrolizumab 10 mg/kg administered intravenously every 2 weeks and were assessed every 8 weeks per RECIST 1.1 criteria.⁴⁵ Participants classified as having a complete response were allowed to discontinue therapy. Those with a partial response or stable disease were treated for 24 months or until progression or intolerable toxicity occurred. For those participants with confirmed progressive disease, pembrolizumab was discontinued.⁴⁵ No treatment-related deaths or infusion-related reactions occurred in the study, and 1 participant discontinued due to a treatmentrelated adverse event. The most common adverse events were fatigue (18%), peripheral edema (12%), and nausea (9%). Grade 3 and grade 4 adverse events included thrombocytopenia, increased aspartate aminotransferase levels, dehydration, rhabdomyolysis, neuromyopathy, toxic encephalopathy, maculopapular rash, and pruritic rash; these were observed in 1 participant each.⁴⁵ Three participants had a complete response (10.3%; 95% CI, 2.2%-27.4%), and 4 had a partial response (13.8%; 95% CI, 3.9%-31.7%).

When evaluating the change from baseline in sum or longest diameter of target lesions, 64% of participants experienced a decrease in tumor burden, 3 of whom experienced a complete response.⁴⁵ The median time to response was 10 weeks, and ranged from 8 weeks to 17 weeks. A median response duration has yet to be determined; however, response duration ranged from 16 weeks to 40 or more weeks.⁴⁵ The median time of progression- free survival was 8.6 weeks (95% CI: 7.4-14.1), and 23.1% of participants demonstrated progressionfree survival at 6 months. The median overall survival was 9.3 months (95% CI, 3.6—not reached), with an overall survival rate of 58% at 6 months.⁴⁵ Outcomes from this study suggest that pembrolizumab should be studied further in patients with urothelial cancer.⁴⁵

Conclusion

Results from clinical trials demonstrate that immunotherapy is ready to take center stage in the fight against cancer. Targeting checkpoint proteins and activating the innate and adaptive immune responses has yielded impressive results. While there is still a great deal to learn about tumor evasion of the host response and what occurs in the tumor microenvironment, hope now exists. Further studies and trials are ongoing to determine the most effective therapeutic regimens, including the optimal sequencing of therapeutic agents and determining whether there is a synergistic effect utilizing combination therapy that blocks the PD-1 receptor in addition to the PD-L1 ligand. More importantly, immunotherapy that blocks the PD-1 and PD-L1 pathways has been associated with favorable adverse effect profiles, and these agents have been well tolerated in clinical trials.

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