Spontaneous development of myasthenia gravis and myositis following treatment with pembrolizumab: a case report.

BACKGROUND: Immune checkpoint inhibitors are a relatively new advancement in the world of cancer therapy. As such, their adverse effects have yet to be fully understood, with only recent literature documenting autoimmune phenomena secondary to their utilization. Specific immune checkpoint inhibitors have recently been linked with the development of myasthenia gravis, which is classically known to manifest spontaneously in patients. Given the relative rarity of this presentation, the risk of misdiagnosis and subsequent mortality and morbidity is concerning. CASE PRESENTATION: We discuss the case of a 73-year-old male who presented with clinical symptoms of myasthenia gravis and myositis shortly after beginning treatment with Pembrolizumab. The diagnosis of myasthenia gravis was initially missed at an outside hospital, which delayed initiation of proper treatment. CONCLUSION: While the incidence of "de-novo" diseases secondary to immune checkpoint inhibitors might be increasing, guidelines regarding best treatment options do not yet exist, leaving many providers at a loss when faced with making clinical decisions surrounding patients with De novo myasthenia gravis. Thus, our goal is to underscore the importance of early recognition of this disease, and emphasize the need for a standard of care as immune checkpoint inhibitors usage becomes more prevalent.

Long-Lasting Therapeutic Response following Treatment with Pembrolizumab in Patients with Non-Small Cell Lung Cancer: A Real-World Experience.

Immune checkpoint inhibitors (ICIs), pembrolizumab in particular, have been shown to be vastly more efficacious than traditional cytotoxic or platinum-based chemotherapies in the treatment of non-small cell lung cancer (NSCLC). While there are plenty of data showing their efficacy and safety profiles, very little exists about the long-term effects of pembrolizumab. We compiled all patients with NSCLC who were treated with pembrolizumab at our institution and had progression-free survival (PFS) of at least 2 years during or after the treatment period. Within this group, we examined the long-term rates of PFS and overall survival (OS), side effect profiles, treatment, and overall disease course up to 60 months after starting treatment. This study included 36 patients with median (range) follow up times from treatment initiation in months as follows: 36 (28-65) overall; 39.5 (28-65) for adenocarcinoma; and 36 (30-58) for squamous cell carcinoma. The median (range) of OS and PFS (months) was comparable for adenocarcinoma, 36 (23-55); and squamous cell carcinoma, 35.5 (28-65). Overall, pembrolizumab shows remarkable long-term safety and efficacy in NSCLC patients. In patients who show an initially strong response and can make it to 24 months of PFS, disease progression after this period seems increasingly unlikely.

Pembrolizumab-induced severe rejection and graft intolerance syndrome resulting in renal allograft nephrectomy.

INTRODUCTION: Pembrolizumab is a selective anti-programmed cell death protein-1 (PD-1) humanized monoclonal antibody that inhibits PD-1 activity by binding to the PD-1 receptor that is found on activated T-cells. The goal of the treatment is to allow the immune system to target and destroy cancer cells by preventing cancer cells from binding to PD-1 receptors, leading to decreased tumor growth. The activation of T-cells by pembrolizumab not only leads to the destruction of malignant cells but also attacks the donor alloantigens that are present in a renal transplant, resulting in graft rejection. CASE REPORT: We present a case of a 46-year-old African American female with history of renal transplant who was treated with pembrolizumab for stage IV B endometrial adenocarcinoma and experienced renal transplant rejection and severe graft intolerance syndrome.Management and outcome: Due to ongoing graft intolerance, a transplant nephrectomy was performed. Allograft pathology was consistent with non-viable kidney with tubulitis, interstitial fibrosis and necrosis consistent with transplant rejection without any evidence of malignancy. DISCUSSION: As emphasized in our case, there is a very high risk of graft rejection in patients who need to be placed on immunomodulators such as pembrolizumab, so the risk versus benefit needs to be assessed and discussed. Our case is unique because pembrolizumab not only caused graft rejection but also severe graft intolerance syndrome which led to transplant nephrectomy. Further guidelines are needed in renal transplant patients requiring PD-1 inhibitors to establish the ideal treatment plan of immunosuppression management and anti-cancer treatments.

Physicochemical stability of pembrolizumab admixture solution in normal saline intravenous infusion bag.

INTRODUCTION: Pembrolizumab is an anti-PD-1 monoclonal antibody, approved and under development for numerous indications in oncology. It is available as either lyophilized powder for reconstitution or ready-to-use solution. Both are required to be diluted in saline or dextrose solution prior to intravenous infusion. After dilution, the recommendation per summary of product characteristics is 24 h at 2-8℃ and 6 h at room temperature. The purpose of this study was to investigate the physicochemical stability of pembrolizumab diluted solution (1 mg/mL) at both refrigerated and room temperature conditions for an extended period. METHODS: Under aseptic conditions, pembrolizumab was diluted in 250 mL of saline injection in polyolefin bags to obtain the final protein concentration of 1 mg/mL. Thus, prepared bags were then stored at either 5℃ ± 3℃, refrigerator exposing the product to ambient light or room temperature (20℃ ± 3℃) on the benchtop. RESULTS: Using several analytical methods, it was demonstrated that pembrolizumab solution for infusion, diluted in normal saline can be stored in polyolefin infusion bags for at least 1 week at 5℃ or RT with no evidence of chemical or physical instability. No aggregation was observed. CONCLUSION: Thus, the practical use of aseptically prepared diluted pembrolizumab in saline can be safely extended to optimize the workload of centralized preparation units and to minimize costs. However, it is the responsibility of the end-user to maintain overall quality of prepared admixture solution that is administered to patient, by following aseptic compounding process as recommended in the packaging insert.

Posterior reversible encephalopathy syndrome following pembrolizumab therapy for relapsed Hodgkin's lymphoma.

Posterior reversible encephalopathy syndrome (PRES) is characterized by a group of central nervous system related symptoms. Diagnosis is usually made by computed tomography or magnetic resonance imaging. Common causes can be arterial hypertension, sepsis, autoimmune disorders, and medications. We report PRES in a relapsed Hodgkin's Lymphoma patient after a dose of pembrolizumab.

Immunotherapy Induced Adrenal Insufficiency: An Underdiagnosed Cause of Persistent Hypotension in Cancer.

Endocrinopathies following immunotherapy have infrequently been documented in the literature. Adrenal insufficiency is a rare consequence of pembrolizumab immunotherapy, with incidence reported to be between 0.98 and 1.3%. We present the case of a 34-year-old female with triple negative breast cancer on chemotherapy who presented with generalized weakness with tachycardia, tachypnea and hypotension unresponsive to fluids. Despite initial improvement with intravenous hydrocortisone and midodrine, the patient continued to be symptomatically hypotensive following discharge and required re-admission. AM cortisol level was found to be < 0.5 ug/dl and ACTH was <1.5 pg/dL, consistent with secondary adrenal insufficiency. CT abdomen and pelvis was unremarkable for adrenal pathology. Patient had been initiated on pembrolizumab (Keytruda) 4 months prior to presentation as part of neoadjuvant chemotherapy. The patient was provided supportive treatment with discharge on fludrocortisone, prednisone, and midodrine. This case reports an unusual consequence of immune checkpoint inhibitors, in which early diagnostic testing, identification, and management is critical.

Comparison of Clinical Trial Results of the Recently Approved Immunotherapeutic Drugs for Advanced Biliary Tract Cancers.

The recently approved immunotherapeutic drugs are Keytruda (pembrolizumab) and Imfinzi (durvalumab) for advanced biliary tract cancers that inhibit PD-1 receptor and PD-L1 ligand, respectively. In this perspective, the results of the two clinical trials, i.e., TOPAZ-1 (NCT03875235) and KEYNOTE-966 (NCT04003636), are critically appraised, compared, and discussed to assess the benefits of these two drugs in the context of the treatment of advanced biliary tract cancers with a focus on PD-L1 status and MIS (microsatellite instability) status and therapy responsiveness in the subgroups. Analyzing the PD-L2 status in biliary tract cancer patients can aid in assessing the prognostic value of PD-L2 expression in determining the clinical response and this may aid in appropriate patient stratification.

Secondary Adrenal Insufficiency as an Immune-Related Adverse Event of Pembrolizumab Therapy.

Pembrolizumab is an immune checkpoint inhibitor that blocks the PD1 receptor and is currently used in the treatment of a vast variety of malignancies. Despite adrenal insufficiency already being documented as a rare but potential side effect, its diagnosis is usually delayed due to its vague presentation, increasing the risk of deleterious outcomes. We present a case of secondary adrenal insufficiency while on pembrolizumab, in which the diagnosis was delayed due to a nonspecific clinical picture. This case highlights the importance of maintaining a high index of suspicion for endocrine dysfunction in patients treated with immune checkpoint inhibitors, as early recognition and appropriate intervention are paramount to preventing serious complications in these patients.

Assessment of chemical stability of monoclonal antibody and antibody drug conjugate administered by pressurized intraperitoneal aerosol chemotherapy.

Pressurized intraperitoneal aerosol chemotherapy (PIPAC) is a new therapeutic approach for patients with peritoneal cancer. So far, most published studies investigated the administration of established cytostatic agents through PIPAC. This study aimed to evaluate the effect of PIPAC on two breakthrough anti-cancer agents, specifically anti-PD1 pembrolizumab, and anti-HER2 antibody-drug conjugate (ADC) - trastuzumab-deruxtecan. We conducted systematic analyses on samples of pembrolizumab and trastuzumab-deruxtecan at clinically relevant concentrations before and after PIPAC administration using an experimental setup of a hermetic container system, mimicking the abdominal cavity and using identical features as in clinical use. We utilized a range of chromatographic and spectroscopic techniques to explore potential alterations in the primary, secondary, and tertiary structures of the drugs, focusing on post-translational modifications resulting from the aerosolization. Our findings indicate that PIPAC did not compromise the integrity of tested biopharmaceuticals. The size variants of both drugs, assessed by size exclusion chromatography (SEC), remained unchanged. Reversed-phase liquid chromatography (RPLC) and hydrophobic interaction chromatography (HIC) revealed no significant differences in hydrophobicity variants, the average drug-to-antibody ratio (DAR), or DAR distribution before and after PIPAC treatment. Circular dichroism (CD) spectroscopy confirmed that the secondary and tertiary structures were preserved. While pembrolizumab showed no change in charge variants post-PIPAC, trastuzumab-deruxtecan exhibited a non-negligible change in the quantity of charge variants on the monoclonal antibody itself, while the payload remained unchanged. This shift could possibly be related to the metallic composition of the CapnoPen® device (made of nickel and chromium) used in PIPAC and for these experiments. Together, our results suggest that PIPAC does not alter the structure of pembrolizumab and trastuzumab-deruxtecan, paving the way for future clinical trials.

Autoimmune diabetes from pembrolizumab: A case report and review of literature.

BACKGROUND: Immunotherapy, specifically the use of checkpoint inhibitors such as pembrolizumab, has become an important tool in personalized cancer therapy. These inhibitors target proteins on T-cells that regulate the immune response against tumor cells. Pembrolizumab, which targets the programmed cell death 1 receptor on T-cells, has been approved for the treatment of metastatic melanoma and non-small cell lung cancer. However, it can also lead to immune-related side effects, including pneumonitis, colitis, thyroid abnormalities, and rare cases of type 1 diabetes. CASE SUMMARY: The case presented involves an adult patient in 30s with breast cancer who developed hyperglycemia after receiving pembrolizumab treatment. The patient was diagnosed with diabetic ketoacidosis and further investigations were performed to evaluate for new-onset type 1 diabetes. The patient had a history of hypothyroidism and a family history of breast cancer. Treatment for diabetic ketoacidosis was initiated, and the patient was discharged for close follow-up with an endocrinologist. CONCLUSION: This literature review highlights the occurrence of diabetic ketoacidosis and new-onset type 1 diabetes in patients receiving pembrolizumab treatment for different types of cancer. Overall, the article emphasizes the therapeutic benefits of immunotherapy in cancer treatment, particularly pembrolizumab, while also highlighting the potential side effect of immune-related diabetes that can occur in a small percentage of patients. Here we present a case where pembrolizumab lead to development of diabetes after a few cycles highlighting one of the rare yet a serious toxicity of the drug.

A Case of Pembrolizumab-Induced Myasthenia Gravis.

With the advent of new cancer treatments, immunotherapy has emerged as an increasingly promising strategy. Undoubtedly, it has pushed oncology into a new era and is providing patients with unprecedented results. As with many treatments, however, adverse effects lead to setbacks in progress. Pembrolizumab is an immunomodulating medication that functions by binding to programmed cell death protein 1 (PD-1) receptors of T cells thereby upregulating the immune system to more effectively detect and target cancer. Myasthenia gravis (MG) is a reported side effect of this medication. Our patient is an 87-year-old male with urothelial cell bladder cancer who developed MG following the administration of pembrolizumab and was treated with plasma exchange therapy. We aim to examine the existing literature concerning treatments for MG, with a particular focus on myasthenia gravis induced by pembrolizumab. We will discuss the occurrence rates and results of such instances, along with their implications for the future of these potential therapies.

A Rare Case of Pembrolizumab-Associated Graves' Disease.

Immune checkpoint inhibitors (ICPis), such as pembrolizumab (Keytruda®), are associated with the development of several immune-related adverse events (irAEs). Thyroid dysfunction is a common endocrine irAE associated with pembrolizumab; however, Graves' disease induced by pembrolizumab is extremely rare. Few cases of this condition have been reported in the literature. Here, we report the case of a 50-year-old patient who presented with thyrotoxicosis that was attributed to Graves' disease secondary to pembrolizumab therapy.

Triple negative breast cancer: approved treatment options and their mechanisms of action.

PURPOSE: Breast cancer, the most prevalent cancer worldwide, consists of 4 main subtypes, namely, Luminal A, Luminal B, HER2-positive, and Triple-negative breast cancer (TNBC). Triple-negative breast tumors, which do not express estrogen, progesterone, and HER2 receptors, account for approximately 15-20% of breast cancer cases. The lack of traditional receptor targets contributes to the heterogenous, aggressive, and refractory nature of these tumors, resulting in limited therapeutic strategies. METHODS: Chemotherapeutics such as taxanes and anthracyclines have been the traditional go to treatment regimens for TNBC patients. Paclitaxel, docetaxel, doxorubicin, and epirubicin have been longstanding, Food and Drug Administration (FDA)-approved therapies against TNBC. Additionally, the FDA approved PARP inhibitors such as olaparib and atezolizumab to be used in combination with chemotherapies, primarily to improve their efficiency and reduce adverse patient outcomes. The immunotherapeutic Keytruda was the latest addition to the FDA-approved list of drugs used to treat TNBC. RESULTS: The following review aims to elucidate current FDA-approved therapeutics and their mechanisms of action, shedding a light on the various strategies currently used to circumvent the treatment-resistant nature of TNBC cases. CONCLUSION: The recent approval and use of therapies such as Trodelvy, olaparib and Keytruda has its roots in the development of an understanding of signaling pathways that drive tumour growth. In the future, the emergence of novel drug delivery methods may help increase the efficiency of these therapies whiel also reducing adverse side effects.

Pembrolizumab-Induced Immune-Mediated Glossitis.

Pembrolizumab (Keytruda), an anti-PD-1 antibody used in the treatment of several different malignancies has been identified to cause adverse effects pertaining to multiple body systems which include respiratory, gastrointestinal, dermatologic, and endocrine manifestations known as immune-related adverse events (IRAEs). Skin manifestations have been most described in current literature highlighting the most common adverse effects of this agent. However, adverse outcomes involving the oral mucosa have been rarely identified in the PD-1 and PD-L1 inhibitor classes of immunotherapeutic agents. We present a case of a 71-year-old male who was treated with a chemotherapeutic regimen including pembrolizumab for newly diagnosed squamous cell carcinoma of the lung, who later developed ulcerations on his tongue that were consistent with glossitis. Upon determining that this adverse effect may be immune-related, the patient was treated with oral prednisone 40 mg with a 10 mg taper each subsequent week, which resulted in significant improvement in the patient's symptoms following one month of treatment.

What is the importance of difference in LCM strategy in drug development? - Learnings from Keytruda and Opdivo.

Immune checkpoint inhibitors have changed how cancer is treated. As of February 2022, six immune checkpoint inhibitors had been approved, with Keytruda and Opdivo accounting for the majority of global sales. Here, the impact of the differences in commercial success between Keytruda and Opdivo is reviewed by analyzing inter-organizational deals since their launch in 2014. Both showed a correlation between the cumulative number of indications and product sales trends, the latter crossing from 2017 to 2018. Differences in sales were due to different approaches to life cycle management by inter-organizational deals and drug development strategies, the key to commercial success.

QbD-guided pharmaceutical development of Pembrolizumab biosimilar candidate PSG-024 propelled to industry meeting primary requirements of comparability to Keytruda®.

Pharmaceutical development of biosimilars is primarily focused on meeting the regulatory requirements for analytical comparability of the product's critical quality attributes (CQAs), concerning safety and efficacy, to those of the originator drug of interest. To this end, the early adoption of a systematic science-based approach, as guided by quality-by-design (QbD) principles, is crucial due to the blind starting point where the same insights of an originator developer into the challenges of a given biopharmaceutical and its manufacturing process are lacking. In this study, we devised a pharmaceutical QbD-guided approach to undertake the biosimilar development of Pembrolizumab (Keytruda®), the ace of therapeutic monoclonal antibodies (mAbs) in terms of approved indications and market sales, and its manufacturing process development. Quality target product profile (QTPP) for Pembrolizumab biosimilar product was assembled using publicly available information on Keytruda®. Upon preliminary analyses of four different lots of Keytruda®, the product CQAs and their acceptable ranges of specification were determined via risk assessment based on the relevant pharmaceutical development quality guidelines, particularly those of the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH). The development and clone selection of Chinese Hamster Ovary (CHO) DG44 cell line was performed using DHFR expression vectors and Methotrexate (MTX) selective pressure. The CHO clone stably expressing relatively higher mAb titer (∼1200 mg/l) in small-scale shake-flask cultures, with the highest similarity of the CQAs charge variants contents (CVCs), N-glycan profile, and biological potency to those of Keytruda® reference standard was selected as the lead clone and the produced Pembrolizumab candidate was named PSG-024. The upstream process (USP) and downstream process (DSP) developments for production were started with the process evaluation screening experiments for the identification of critical process parameters (CPPs) founded upon the prior knowledge on different process stages, input process parameters (iPPs), output process parameters (oPPs), and their impacts on product CQAs. Thereby, screening experiments of USP fed-batch cell culture in 5-liter bioreactor resulted in improvement of PSG-024 expression titer to 2060 ± 70 mg/l and selection of the iPPs feed amount (A), glucose setpoint (B), culture temperature (C), and agitation rate (D) for the optimization design of experiments (DoEs) mainly focused on the CQA acidic CVC and the oPPs mAb expression yield. The USP optimization DoEs using response surface methodology (RSM) yielded valid prediction models and optimal conditions of A = 35%, B = 4.5 g/l, C = 37 °C, and D = 160-220 rpm, which resulted in the final PSG-024 expression titer of 3170 ± 40 mg/l without an excessive rise in acidic CVC. The DSP screening experiments led to achieving the mAb recovery rates of 94% ± 3% and 71.5% ± 3.5% for affinity (capture) and cation-exchange (polishing) chromatography stages, respectively. The capture eluate buffer and viral inactivation conditions were optimized to prevent mAb eluate turbidity and protein aggregation. Moreover, the polishing stage optimization DoEs via one-factor-at-a-time method focused on wash and elution steps for control of the acidic CVC CQA and achieving >80% mAb recovery rate. By shifting to Step elution from the primary salt gradient method and considering an additional intermediate wash step, the maximum mAb recovery of 87% ± 1.5% was achievable while maintaining the CQA acidic CVC within the acceptable range. The consistency of final analytical comparability of PSG-024 demonstrated the effectiveness of the adopted pharmaceutical QbD approach for Pembrolizumab biosimilar development, paving the way for the technology transfer to the client to proceed further development.

Immune checkpoints inhibitors in the management of high-risk non-muscle-invasive bladder cancer. A scoping review.

PURPOSE: Provide the current state of trials investigating the effectiveness and safety of checkpoint inhibitors in patients with non-muscle invasive bladder cancer. METHODS: We conducted this scoping review following the recommendations of the Joanna Briggs Institute. We searched for MEDLINE, EMBASE, CENTRAL databases, and clinical trials in search engines such as clinicaltrials.gov and clinicaltrialsregister.eu. We included clinical trials in patients over 18 years of age diagnosed with high-risk non-muscle-invasive bladder cancer who suffer treatment failure with Bacillus Calmette-Guérin (BCG). Even those who have not received prior therapy. Those clinical trials also evaluated intravenous- or intravesical-administered immune checkpoint inhibitors and those associated with BCG. RESULTS: Thirteen clinical trials are currently being developed with immune checkpoint inhibitors as a therapeutic alternative in patients diagnosed with non-muscle-invasive urothelial carcinoma of the bladder. Five of these have not received prior therapy with bacillus Calmette-Guérin. The remaining eight studies in patients who received BCG immunotherapy with a poor response are classified as persistent, refractory, or non-responders to BCG therapy. Also, in patients who do not accept surgical management with radical cystectomy. Preliminary results from studies such as SWOG S1605 (NCT02844816) show encouraging antitumor activity and long-lasting response patients with carcinoma in situ with or without papillary disease in terms of disease-free survival and rate free of adverse events. Recently, Keynote-057 (NCT02625961) evidenced that after 36 months of follow-up, Pembrolizumab as monotherapy was tolerable and showed promising antitumor activity in patients with BCG-unresponsive non-muscle-invasive bladder cancer. CONCLUSION: The checkpoint inhibitor response may offer a therapeutic alternative for patients diagnosed with high-risk non-muscle-invasive bladder cancer. However, the complete response rate documented in this scoping review is limited to patients with carcinoma in situ, with mild adverse effects, without reporting severity or death from the intervention.

Pembrolizumab related Guillain barre syndrome, a rare presentation in a patient with a history of lupus and bladder cancer.

Immune checkpoint inhibitor-related neurotoxicity causing Guillain Barre Syndrome is relatively uncommon. We discussed an 80-year-old patient with known systemic lupus erythematosus who presented with lower extremity weakness, areflexia and then progressed to respiratory muscle and upper extremity weakness after receiving immunotherapy with checkpoint inhibitors for metastatic bladder cancer. With the increasing use of immunotherapy for the management of cancer, awareness of neurological autoimmune side effects is essential. Immune checkpoint inhibitor-mediated GBS can be severe and fatal if not diagnosed promptly. The hospitalists, neurologists, and oncologists should be aware of neurotoxicity related to immune checkpoint inhibitor therapy requiring a multidisciplinary approach to patient care. Prompt initiation of immunosuppressive therapy is required for the management of immune checkpoint inhibitor-related neurotoxicity.

Apatinib combined with Keytruda treatment induces apoptosis of gastric carcinoma cells through CES4/miR-616-5p/DUSP2 axis.

Gastric carcinoma (GC) is a highly malignant and heterogeneous tumour. Long non-coding RNA CES4 is down-regulated in GC. However, whether CES4 can participate in GC remains unclear; we have carried out research on this topic. GC cells (HGC-27 and MKN-7) were treated with anti-tumour drugs: apatinib combined with Keytruda. Cell viability and apoptosis were detected by CCK-8 assay and flow cytometry. Gene and protein expression were examined by quantitative real-time PCR and western blot. Luciferase reporter assay was performed to verify the relationship among CES4, miR-616-5p and dual-specificity phosphatase-2 (DUSP2). CES4 was highly expressed in the apatinib combined with Keytruda-treated HGC-27 and MKN-7 cells. Apatinib combined with Keytruda treatment repressed cell viability and promoted apoptosis of HGC-27 and MKN-7 cells, which was abrogated by CES4 knockdown. Furthermore, CES4 promoted DUSP2 expression by sponging miR-616-5p in HGC-27 and MKN-7 cells. CES4 knockdown promoted cell viability and inhibited apoptosis of drug-treated HGC-27 and MKN-7 cells by regulating miR-616-5p/DUSP2 axis. In conclusion, these data demonstrate that apatinib combined with Keytruda treatment induces apoptosis of GC cells through CES4/miR-616-5p/DUSP2 axis. Thus, this work provides the experimental basis for the combination of apatinib and Keytruda as a treatment for GC.

Real World Experience With Pembrolizumab in Recurrent or Advanced Prostate Cancer.

INTRODUCTION: Phase II trials have shown activity with pembrolizumab against prostate cancer. However, the clinical factors predictive of a response to pembrolizumab in men with prostate cancer are unknown. PATIENTS AND METHODS: A total of 54 consecutive men with progressive, recurrent, or advanced prostate cancer were treated with 1 to 12 cycles of pembrolizumab 200 mg every 3 weeks with or without stereotactic body radiotherapy (SBRT). RESULTS: For the 31 men evaluable for response, the median age, prostate-specific antigen (PSA) level, and Gleason score were 75 years, 30 ng/mL, and 8 (4 + 4), respectively, which were similar to those for the 23 nonevaluable patients. The treatments received before pembrolizumab were enzalutamide in 26, abiraterone in 18, and sipuleucel-T in 23. All but 4 men had had castrate-resistant disease. Of the 54 men, 31 had completed ≥ 4 cycles of pembrolizumab and were evaluable for the response. Ten men had undergone SBRT to an isolated metastasis shortly before or during pembrolizumab treatment, with the goal of inducing an abscopal effect. The clinical characteristics of the 17 men with a response or stable disease were compared with those of the 14 men with progressive disease. Grade ≥ 2 toxicity occurred in 16 men (30%). PSA stabilization or a response occurred in slightly more than one half (55%) of the men treated with ≥ 4 cycles of pembrolizumab. Five patients had a notable PSA decline of > 50%, which were sustained as long as they had continued receiving pembrolizumab. A PSA response or stabilization was more common for men who had begun taking pembrolizumab with a lower PSA level, fewer bone metastases, and fewer mutations and without previous chemotherapy. A statistically nonsignificant trend toward stabilization or a response was observed in men who had undergone concomitant SBRT. CONCLUSION: Pembrolizumab showed modest anticancer activity against metastatic castrate-resistant prostate cancer. A PSA response or stabilization occurred more frequently in men with less-advanced disease.