Despite recent advancements, approximately 50% of patients with acute myeloid leukemia (AML) do not respond to induction therapy (primary induction failure, PIF) or relapse after <6 months (early relapse, ER). We have recently shown an association between an immune-infiltrated tumor microenvironment (TME) and resistance to cytarabine-based chemotherapy but responsiveness to flotetuzumab, a bispecific DART® antibody-based molecule to CD3ε and CD123. This study reports the results of a multicenter, open-label, phase 1/2 study of flotetuzumab in adults with relapsed/refractory AML. Eighty-eight AML patients were enrolled, 42 in dose-finding and 46 at the recommended phase 2 dose (RP2D) of 500ng/kg/day. Consistent with flotetuzumab's mode of action, the most frequent adverse events were infusion-related reactions (IRR)/cytokine release syndrome (CRS), the majority as grade 1-2. Stepwise dosing during week 1, pre-treatment dexamethasone, prompt use of tocilizumab and temporary dose reductions/interruptions successfully prevented severe IRR/CRS, resulting in acceptable tolerability. Clinical benefit accrued to PIF/ER AML patients, who showed an immune-infiltrated TME. Among 30 PIF/ER patients treated at the RP2D, the CR/CRh rate was 26.7%, with an overall response rate (CR/CRh/CRi) of 30.0%. In PIF/ER patients who achieved CR/CRh, median OS was 10.2 months (range 1.87-27.27), with 6- and 12-month survival rates of 75% (95%CI, 0.450-1.05) and 50% (95%CI, 0.154-0.846). Bone marrow transcriptomic analysis showed that a parsimonious 10-gene signature predicted complete responses to flotetuzumab (AUROC=0.904 versus 0.672 for the ELN risk classifier). Flotetuzumab represents an innovative experimental approach associated with acceptable safety and encouraging evidence of activity in PIF/ER AML patients. Trial registration number: NCT02152956.

The role of prostate-specific antigen (PSA) or kallkirein-related peptidase 3 (KLK3) as a biomarker for prostate cancer is well known; however, the precise physiologic role of its serine protease activity in prostate cancer remains a mystery. PSA is produced at high levels by both androgen-dependent and -independent prostate cancers. Studies have documented high levels of active PSA in the milieu surrounding osseous and soft tissue metastases. This evidence, coupled with growing experimental evidence, suggests that PSA plays an important role in the pathobiology of prostate cancer. These observations support the development of PSA-selective inhibitors as useful tools for the targeted treatment and imaging of prostate cancer. Here we review the research that has been conducted to date on developing selective inhibitors for PSA. The different approaches used to determine PSA substrate specificity and for creating inhibitors is discussed. In addition, the unique active site characteristics of PSA and how these motifs aided our research in developing PSA targeted agents is highlighted.

Prostate-Specific Antigen (PSA) is a serine protease produced at high levels by normal and malignant prostate epithelial cells that is used extensively as a biomarker in the clinical management of prostate cancer. To better understand PSA’s role in prostate cancer progression we prepared a library of peptidyl boronic acid based inhibitors. To enhance selectivity for PSA vs. other serine proteases, we modified the P1 site of the inhibitors to incorporate a bromopropylglycine group. This allowed the inhibitors to participate in halogen bond formation with the serine found at the bottom of the specificity pocket. The best of these Ahx-FSQn(boro)Bpg had PSA Ki of 72 nM and chymotrypsin Ki of 580 nM. In vivo studies using PSA-producing xenografts demonstrated that candidate inhibitors had minimal effect on growth but significantly altered serum levels of PSA. Biodistribution of 125I labeled peptides showed low levels of uptake into tumors compared to other normal tissues.

Prostate-specific Ag (PSA) is a serine protease that is expressed exclusively by normal and malignant prostate epithelial cells. The continued high-level expression of PSA by the majority of men with both high- and low-grade prostate cancer throughout the course of disease progression, even in the androgen-ablated state, suggests that PSA has a role in the pathogenesis of disease. Current experimental and clinical evidence suggests that chronic inflammation, regardless of the cause, may predispose men to prostate cancer. The responsibility of the immune system in immune surveillance and eventually tumor progression is well appreciated but not completely understood. In this study, we used a mass spectrometry–based evaluation of prostatic fluid obtained from diseased prostates after removal by radical prostatectomy to identify potential immunoregulatory proteins. This analysis revealed the presence of Igs and the complement system proteins C3, factor B, and clusterin. Verification of these findings by Western blot confirmed the high-level expression of C3 in the prostatic fluid and the presence of a previously uncharacterized C-terminal C3 cleavage product. Biochemical analysis of this C3 cleavage fragment revealed a putative PSA cleavage site after tyrosine-1348. Purified PSA was able to cleave iC3b and the related complement protein C5. These results suggest a previously uncharacterized function of PSA as an immunoregulatory protease that could help to create an environment hospitable to malignancy through proteolysis of the complement system.