Development of a Diagnostic Programmed Cell Death 1-Ligand 1 Immunohistochemistry Assay for Nivolumab Therapy in Melanoma.

Nivolumab is a monoclonal antibody that blocks the interaction between programmed cell death 1 (PD1) and programmed cell death 1-ligand 1 (PD-L1), resulting in enhanced antitumor activity by the immune system. Nivolumab is currently approved by the US Food and Drug Administration (FDA) for melanoma, non-small cell lung cancer (NSCLC), renal cell carcinoma, classical Hodgkin lymphoma, squamous cell carcinoma of the head and neck, and urothelial carcinoma. PD-L1 IHC 28-8 pharmDx is FDA-approved as a complementary diagnostic for immunohistochemical (IHC) detection of PD-L1 in non-squamous NSCLC and melanoma. We report validation of PD-L1 IHC 28-8 pharmDx for PD-L1 detection on formalin-fixed, paraffin-embedded human melanoma specimens using Autostainer Link 48. A prevalence assessment of 104 melanoma specimens indicated that PD-L1 was detected across the full expression level range (0% to 100% of tumor cells). Assay robustness and precision studies were conducted at Agilent Technologies, with additional reproducibility studies performed at 3 external laboratories. Precision studies evaluated at ≥1% and ≥5% expression levels revealed a range of average negative agreement from 89.5%, 95% CI (83.2, 93.6) to 100%, 95% CI (97.3, 100), and average positive agreement from 85.5%, 95% CI (77.6, 90.9) to 100%, 95% CI (97.9, 100). For external reproducibility, precise results were obtained. These results demonstrate PD-L1 IHC 28-8 pharmDx is a precise, robust, and reproducible assay for determining PD-L1 expression in melanoma. This is the first PD-L1 IHC test to receive FDA approval as a complementary diagnostic in melanoma patients whereby positive PD-L1 expression is correlated with the magnitude of nivolumab treatment effect.

EP1: a novel rabbit monoclonal antibody for detection of oestrogen receptor α.

AIMS: Assessment of hormone receptor expression is part of routine examination of every breast cancer. In this study, we report the characterisation of a novel rabbit monoclonal antibody, clone EP1, directed against oestrogen receptor (ER) α. Additionally, its immunohistochemical performance characteristics in archival tissues are evaluated in normal tissues and two distinct cohorts of breast cancer patients. METHODS: Comparative analyses between EP1 and the anti-ERα component of the ER/PR pharmDx kit (cocktail of mouse monoclonal antibody clones 1D5 and ER-2-123) and between EP1 and another commercially available rabbit monoclonal antibody, clone SP1, are described. RESULTS: Clone EP1 specifically detects nuclear ER in all tissues examined; cytoplasmic staining was not observed. The analysis shows a high degree of concordance (~95%) between EP1 and both the ERα component of the Dako ER/PR pharmDx kit and Ventana clone SP1. However, the use of EP1 antibody together with Dako EnVision FLEX detection system resulted in a stronger staining intensity as compared with SP1 antibody using the Ventana ultraView DAB detection system resulting in better 'ease of use.' CONCLUSIONS: The use of EPI can result in better interpretation of the results of the ER analysis.

IMP3, NESP55, TTF-1 and CDX2 serve as an immunohistochemical panel in the distinction among small-cell carcinoma, gastrointestinal carcinoid, and pancreatic endocrine tumor metastasized to the liver.

Histopathologic distinction among small-cell carcinoma (SCC), pancreatic endocrine tumor (PET), and gastrointestinal carcinoids metastasized to the liver in needle core biopsies can be extremely challenging because of limited material, crush artifact, and lack of detailed clinical history. In this study, a total of 61 surgically resected or biopsied specimens, including 27 SCCs (lung, 17; colon, 1; gallbladder, 2; stomach, 1; and unknown primary, 6), 18 gastrointestinal carcinoid tumors (GICTs) (stomach, 2; small intestine, 14; colon, 2), and 16 PETs were immunohistochemically examined for the expression of IMP3, TTF-1, CDX2, and NESP55 to evaluate their diagnostic value. The results showed that 24 (89%) of 27 SCCs exhibited strong cytoplasmic staining for IMP3 in 60% to 100% of the tumor cells. Eighteen (67%) SCCs were strongly and diffusely positive for TTF-1. In the remaining 9 TTF-1-negative SCCs (including 4 extrapulmonary cases), 7 showed strong and diffuse IMP3 expression. All SCCs were negative for CDX2 except for 1 case of colonic origin that showed strong CDX2 immunoreactivity. All 16 metastatic PETs were positively stained for IMP3 with 12 cases (75%) showing a diffuse and moderate-to-strong staining pattern while they were negative for TTF-1. Six PETs exhibited moderate-to-strong positivity for CDX2 with nuclear staining in 5% to 40% of tumor cells, and 5 showed a varying degree of positivity for NESP55. Three (17%) of 18 metastatic GICTs showed moderate IMP3 staining in 50% to 90% of the tumor cells, whereas CDX2 was expressed in 17 (94%) cases with moderate-to-strong staining in 50% to 100% of tumor cells. No NESP55 immunoreactivity was detected in metastatic SCCs and GICTs. In conclusion, a panel of these 4 markers is useful in segregating among SCC, PET, and GICT to help determine the primary site of hepatic metastasis.

S100P, von Hippel-Lindau gene product, and IMP3 serve as a useful immunohistochemical panel in the diagnosis of adenocarcinoma on endoscopic bile duct biopsy.

Histopathologic distinction between benign and malignant bile duct epithelial lesions on endoscopic biopsies can be extremely challenging because of limited material, crush artifact, and compounding inflammatory and/or reactive changes particularly after stent placement. In this study, a total of 72 endoscopic bile duct biopsies, including 40 adenocarcinomas and 32 benign cases, were immunohistochemically examined for the expression of S100P, von Hippel-Lindau gene product (pVHL), and IMP3 to evaluate their diagnostic value. The results showed that 36 adenocarcinomas (90%) exhibited strong nuclear and cytoplasmic staining for S100P, of which 30 (83.3%) showed diffuse immunoreactivity. Intermediate to strong cytoplasmic staining for IMP3 was demonstrated in 31 tumors (77.5%) (15 diffuse, 16 focal). Completely negative staining for pVHL was observed in 37 adenocarcinomas. In the remaining 3 tumors, focal (1) or diffuse (2) membranous and cytoplasmic pVHL immunoreactivity was detected. Twenty-eight tumors (70%) showed a S100P+/IMP3+/pVHL- staining pattern, 6 (15%) with a S100P+/IMP3-/pVHL- pattern, and 2 (5%) with a S100P-/IMP3+/pVHL- pattern. All 32 benign biopsies were completely negative for IMP3 with the exception of 2 cases with focal dysplasia where focal immunoreactivity was observed. Thirty benign biopsies (93.8%) were positive for pVHL with a diffuse staining pattern observed in 28 cases (93.3%). Eight benign biopsies (25%) showed focal S100P positivity. Twenty-two benign biopsies (68.8%) displayed a S100P-/IMP3-/pVHL+ staining pattern. In conclusion, an immunohistochemical panel consisting of S100P, pVHL, and IMP3 can be helpful in distinguishing adenocarcinoma from reactive epithelial changes on challenging bile duct biopsies. The findings of focal S100P and/or IMP3 expression with reciprocal loss of pVHL immunoreactivity in a few benign biopsies suggest a use of these markers in the detection of early epithelial dysplasia that may be beyond histologic recognition.

IMP3 expression is correlated with histologic grade of lung adenocarcinoma.

Insulin-like growth factor II mRNA binding protein 3 is an oncofetal protein that is expressed in multiple malignancies. This study aimed to determine the correlation of insulin-like growth factor II mRNA binding protein 3 expression with histologic grade of lung adenocarcinoma. Eighty-nine cases, including 11 atypical adenomatous hyperplasias, 10 pure bronchioloalveolar carcinomas, 36 well-differentiated adenocarcinomas and 41 moderately or poorly differentiated adenocarcinomas, were immunohistochemically studied using a monoclonal antibody against insulin-like growth factor II mRNA binding protein 3. Twenty-nine (70.7%) of 41 moderately to poorly differentiated adenocarcinomas were positive for insulin-like growth factor II mRNA binding protein 3, with 26 (89.7%) tumors demonstrating either a strong staining or staining in greater than 30% of tumor cells. Four (40.0%) of 10 bronchioloalveolar carcinomas and 13 (36.1%) of 36 well-differentiated adenocarcinomas exhibited insulin-like growth factor II mRNA binding protein 3 positivity with a variable degree and percentage of tumor cells staining. When bronchioloalveolar carcinomas were present in a pure form or as a component of adenocarcinomas, positive insulin-like growth factor II mRNA binding protein 3 staining was always patchy, with less than 20% of tumor cells stained. Overall, the frequency of positive insulin-like growth factor II mRNA binding protein 3 staining was lower in bronchioloalveolar carcinomas and well-differentiated adenocarcinomas compared to moderately/poorly differentiated adenocarcinomas (P < .01). No insulin-like growth factor II mRNA binding protein 3 signal was detected in any case of atypical adenomatous hyperplasia. These findings show that insulin-like growth factor II mRNA binding protein 3 is strongly and diffusely expressed in a large proportion of moderately/poorly differentiated lung adenocarcinomas, in particular in the solid component of mixed subtype adenocarcinomas, less frequently expressed in well-differentiated adenocarcinomas and bronchioloalveolar carcinomas, and negative in atypical adenomatous hyperplasias. The higher frequency of expression in moderately/poorly differentiated adenocarcinomas suggests that insulin-like growth factor II mRNA binding protein 3 expression may be associated with an aggressive biological behavior.

Diagnostic utility of IMP3 in segregating metastatic melanoma from benign nevi in lymph nodes.

Depending on the Breslow depth of the primary melanoma, sentinel lymph node biopsy is considered as standard of care for the staging of cutaneous melanoma, and is one of the most important prognostic factors. The histologic analysis of these specimens becomes difficult to interpret when benign intranodal nevic cells mimic metastases. Insulin-like growth factor-II messenger RNA (mRNA)-binding protein-3 (IMP3), also known as K homology domain-containing protein overexpressed in cancer or L523S, is a member of the insulin-like growth factor-II mRNA-binding protein family and has been shown to have diagnostic utility in distinguishing cutaneous melanoma from benign nevi. In this study, 43 sentinel lymph node biopsy specimens, including 13 with benign intranodal nevi and 30 with metastatic melanoma (two cases containing both benign nevi and metastatic melanoma), from 41 patients were immunohistochemically analyzed with a monoclonal antibody against IMP3. None of the benign intranodal nevi expressed IMP3, whereas 21 out of 30 (70%) of the lymph nodes containing metastatic melanoma did. It seems that IMP3 is helpful in distinguishing benign intranodal nevi from metastatic melanoma in sentinel lymph node biopsy specimens, and could be a valuable diagnostic adjunct in sentinel lymph node biopsy assessment in which questions arise as to the malignancy of the melanocytes present.

IGF2BP3 (IMP3) expression is a marker of unfavorable prognosis in ovarian carcinoma of clear cell subtype.

Clear cell carcinoma is an uncommon subtype of ovarian carcinoma, accounting for 10% of cases. Clear cell carcinoma typically presents with stage I or II disease, and in this setting prognostic markers could aid in management decisions, in particular the decision to treat with adjuvant chemotherapy. We tested whether expression of insulin-like growth factor 2 mRNA-binding protein 3 (IGF2BP3, also known as IMP3) can serve as a new biomarker to predict outcome for patients with clear cell carcinoma and other subtypes of ovarian carcinoma. The expression of IGF2BP3 was evaluated by immunohistochemistry in 475 ovarian carcinomas of different subtypes and correlated with disease-specific survival. IGF2BP3 antibody specificity was validated by correlation of IGF2BP3 protein with mRNA expression level in a series of 35 ovarian carcinomas (r=0.849, P<0.0001). IGF2BP3 protein expression was an independent marker of reduced disease-specific survival (risk ratio 2.9, 95% confidence interval 1.4-5.8) in the clear cell subtype (N=128), but not in high-grade serous (N=198) or endometrioid (N=121) carcinomas. The prognostic significance of IGF2BP3 expression for reduced disease-specific survival (risk ratio 2.6, 95% confidence interval 1.3-5.0) was confirmed in an independent series of cases (N=150) from three different centers in North America. We conclude that IGF2BP3 is the first biomarker of prognostic significance in ovarian clear cell carcinoma that has been validated in an independent case series.

Mammaglobin, a valuable diagnostic marker for metastatic breast carcinoma.

Identification of metastasis and occult micrometastases of breast cancer demands sensitive and specific diagnostic markers. In this study, we assessed the utility of a mouse monoclonal antibody to human mammaglobin for one such purpose. Immunohistochemical stains were performed on paraffin-embedded sections from a total of 284 cases, which consisted of primary breast invasive carcinomas (41 cases) with matched metastases to ipsilateral axillary lymph nodes, metastatic breast carcinoma to liver (1 case) and kidney (1 case), non-breast neoplasms (161 cases), and normal human tissues (39 cases). The results showed 31 of the 41 cases of primary breast cancer with axillary lymph node metastases were positive for mammaglobin (76%). In the meantime, we documented expression of mammaglobin in occasional cases of endometrial carcinoma (17%). Our data further validated that mammaglobin is a valuable diagnostic marker for metastatic carcinoma of breast origin, although endometrial carcinoma should be considered as a major differential diagnosis.

Merkel cell carcinoma expresses K homology domain-containing protein overexpressed in cancer similar to other high-grade neuroendocrine carcinomas.

Merkel cell carcinoma is an uncommon and aggressive primary cutaneous neuroendocrine carcinoma with a high rate of recurrence and metastasis. Optimal management is controversial; consequently, it is imperative to identify the signaling pathways involved in the pathogenesis of Merkel cell carcinoma so that effective therapeutic targeting agents can be developed. We previously reported that K homology domain-containing protein overexpressed in cancer is expressed in high-grade neuroendocrine carcinomas of the lung and extrapulmonary small cell carcinomas. The K homology domain-containing protein overexpressed in cancer (KOC), also known as L523S and IMP-3, is an insulin-like growth factor II messenger RNA-binding protein that promotes tumor cell proliferation by enhancing insulin-like growth factor II protein expression. Expression of KOC in Merkel cell carcinoma has not been investigated. We studied 20 Merkel cell carcinomas by immunohistochemistry using a monoclonal antibody against L523S/KOC. Of 20 Merkel cell carcinomas, 18 (90%) overexpressed KOC, with 11 (55%) overexpressing KOC in greater than 90% of tumor cells, 3 (15%) overexpressing KOC in 50% to 90% of tumor cells, 3 (15%) overexpressing KOC in 10% to 50% of tumor cells, and 1 (5%) overexpressing KOC in less than 10% of tumor cells. The immunostaining intensity was variable, with moderate to strong staining in 14 cases and weak staining in the remaining 4. Extent of expression of K homology domain-containing protein overexpressed in cancer predicted metastasis (P = .04) and was weakly correlated with increased tumor size (P = .08). In conclusion, Merkel cell carcinoma expresses K homology domain-containing protein overexpressed in cancer with an expression pattern similar to high-grade neuroendocrine carcinomas of the lung and extrapulmonary small cell carcinomas. We propose K homology domain-containing protein overexpressed in cancer as a potential target molecule for the treatment of high-grade neuroendocrine carcinomas, irrespective of anatomical location, and a potential marker to predict metastatic disease.

Expression of RNA-binding protein IMP3 (KOC) in benign urothelium and urothelial tumors.

RNA-binding protein IMP3 is a KH-domain-containing protein and a member of the insulin-like growth factor messenger RNA-binding protein family. It is identical to K-homology protein overexpressed in cancer that was identified through screening for genes differentially expressed between benign pancreatic tissue and pancreatic cancer. Several studies have shown that IMP3 is associated with aggressive and advanced tumors in various organs. We studied the expression of IMP3 in benign urothelium and urothelial tumors by immunohistochemistry. The expression pattern of IMP3 was further compared with that of p53 and p16. Our study shows that IMP3 is generally not expressed in benign urothelium or low-grade urothelial tumors including urothelial dysplasia, papillary urothelial neoplasm of low malignant potential, and low-grade papillary urothelial carcinoma. The expression of IMP3 is significantly increased in high-grade urothelial tumors including high-grade papillary urothelial carcinoma, urothelial carcinoma in situ, and invasive urothelial carcinoma. Expression of IMP3 in urothelial tumors parallels the accumulation of nuclear p53, although there is not always a one to one correlation. In contrast, expression of p16 in the different groups of urothelial tumors is more variable. Urothelial carcinomas with invasion of muscularis propria appear to express IMP3 more frequently than lower-stage tumors. These findings suggest that IMP3 may be involved in the progression of urothelial tumors from low grade to high grade in both papillary and flat lesions. Immunohistochemical detection of the combined expression of IMP3 and p53 is useful in the diagnosis of high-grade urothelial tumors, particularly in small, superficial materials.

IMP-3 is a novel progression marker in malignant melanoma.

Insulin-like growth factor-II messenger RNA (mRNA)-binding protein-3 (IMP-3), also known as K homology domain-containing protein overexpressed in cancer (KOC) and L523S, is a member of the insulin-like growth factor-II mRNA-binding protein family and is expressed during embryogenesis and in some malignancies. IMP-3 expression in melanocytic neoplasms has not been investigated. Fifty-six melanocytic neoplasms from 48 subjects were immunohistochemically studied using a monoclonal antibody against L523S/IMP-3. IMP-3 expression in melanoma was significantly higher than in Spitz nevi (P<0.05), and the staining intensity in the Spitz nevi was weak. IMP-3 expression in metastatic melanoma was significantly higher than in primary cutaneous melanoma with a Breslow depth

Utility of anti-L523S antibody in the diagnosis of benign and malignant serous effusions.

BACKGROUND: Immunohistochemistry is helpful in distinguishing metastatic carcinoma from atypical mesothelial cells; however, it is not useful in differentiating atypical mesothelial cells from malignant mesothelial cells. K homolog domain containing protein overexpressed in cancer (KOC), a member of the insulin-like growth factor mRNA-binding protein (IMP) family, also known as L523S and IMP3, is expressed during embryogenesis and in various malignancies. Using a mouse monoclonal antibody (L523S) against KOC, KOC expression was investigated in malignant tumors and reactive mesothelial cells in serous effusions. METHODS: Seventy-six cases with paraffin-embedded pleural, pericardial, and peritoneal serous effusion cell blocks including 60 malignant serous effusions (11 malignant pleural mesotheliomas and 49 metastatic carcinomas) and benign pleural effusions (14 cases with reactive mesothelial cells and 2 cases with atypical cells with uncertain significance) were selected for immunohistochemical analysis with L523S, calretinin, and CK5/6. RESULTS: Immunohistochemical studies showed that positive staining for KOC of variable degrees of intensity was observed in 47 of 60 cases in malignant serous effusions including 10 of 11 mesotheliomas and 36 of 49 metastatic carcinomas. The associated reactive mesothelial cells were negative for KOC but positive for calretinin and CK5/6. All 11 malignant mesotheliomas exhibited positivity for calretinin, and 9 of 11 cases had CK5/6 staining. In addition, 16 cases that were originally diagnosed either as pleural effusions with reactive mesothelial cells (14) or atypical cells with uncertain significance (2) were also tested for KOC expression. Interestingly, 3 of 16 cases exhibited various degrees of positivity for KOC, 2 of which were diagnosed as lung adenocarcinoma with a recurrence after tumor resection and 1 as malignant pleural mesothelioma. CONCLUSIONS: Anti-L523S antibody is a useful marker for the detection of malignant cells in serous effusions and it can have significant utility in differentiating reactive mesothelial cells from malignant mesothelioma and metastatic carcinoma in combination with calretinin and CK5/6 staining.

Extrapulmonary small cell carcinomas express K homology domain containing protein overexpressed in cancer, but carcinoid tumors do not.

A number of malignancies, including high-grade neuroendocrine carcinomas of the lung, have been reported to express K homology domain containing protein overexpressed in cancer (KOC), a member of the insulin-like growth factor messenger RNA-binding protein (IMP) family also known as L523S and IMP3. KOC acts to promote tumor cell proliferation by enhancing insulin-like growth factor-II protein expression. This study aimed to examine KOC expression pattern in extrapulmonary neuroendocrine tumors. Seventy-five extrapulmonary neuroendocrine tumors that were surgically resected or had undergone biopsy, including 53 small cell carcinomas (uterine cervix, 21; bladder, 10; colorectum, 13; prostate, 7; stomach, 1; and esophagus, 1) and 22 carcinoid tumors (colorectum, 10; appendix, 5; ileum, 4; duodenum, 2; and stomach, 1), were immunohistochemically studied using a monoclonal antibody against KOC. Our results demonstrated that 47 small cell carcinomas (89%) showed moderate to strong positive staining for KOC, with 25 cases (53%) showing positivity in more than 90% of tumor cells and 22 cases (47%) in 40% to 80% of tumor cells. Three cases showed weak staining in 5% to 10% of the tumor cells. The remaining 3 cases (uterine cervix, 2; bladder, 1) showed completely negative immunoreactivity. No KOC immunostaining was detected in 22 carcinoid tumors. These findings indicate that KOC may play an important role in the regulation of biologic behavior of extrapulmonary small cell carcinomas. In addition, immunohistochemical detection of KOC expression may serve as a useful diagnostic tool in the distinction between small cell carcinoma and carcinoid tumor, particularly when the diagnostic material is a small biopsy with crushing artifact.

High-grade neuroendocrine carcinomas of the lung express K homology domain containing protein overexpressed in cancer but carcinoid tumors do not.

K homology domain containing protein overexpressed in cancer (KOC) is a member of the insulin-like growth factor (IGF) messenger RNA-binding protein family and is expressed during embryogenesis and in certain malignancies. KOC, known as L523S and IGF messenger RNA-binding protein 3, was shown to be frequently expressed in high-grade neuroendocrine carcinomas of the lung in our immunohistochemical studies using a monoclonal antibody against human KOC. Specifically, all 10 small cell lung carcinomas (SCLCs) exhibited strong cytoplasmic staining, 9 with diffuse positivity and 1 with focal positivity. Among 14 large cell neuroendocrine carcinomas (LCNECs), 9 exhibited strong and diffuse cytoplasmic staining, and 5 cases showed focal immunoreactivity. In contrast, no KOC was detected in 21 typical and atypical carcinoids, except for one atypical carcinoid with oncocytic cells showing weak cytoplasmic staining. Although SCLCs exhibited a strong and diffuse staining pattern more frequently (90%) than LCNECs (64%), the difference did not reach statistical significance (P = .3408). Interestingly, our immunohistochemical studies demonstrated that IGF-II, reportedly regulated by KOC, was comparably expressed in SCLC, LCNEC, and typical and atypical carcinoids, irrespective of KOC expression status of the tumors. These results support the formulation that KOC may play an important role in the regulation of biologic behavior of high-grade neuroendocrine carcinomas. In addition, detection of KOC expression may be diagnostically useful in distinguishing high-grade neuroendocrine carcinomas from carcinoid tumors. Our findings of equivalent IGF-II expression in KOC-positive SCLC and LCNEC and KOC-negative carcinoid tumors suggest different regulatory mechanisms involved in the control of IGF-II expression in these tumors.

Epidermal growth factor receptor expression and measurement in solid tumors.

The efficacies of targeted therapies for cancer treatment are dependent on the expression of the targeted molecule in the tumors of the treated patient population. Immunohistochemistry is an attractive assay format for determining protein expression in biopsies of solid tumors, and is widely used in pathology laboratories. An optimized immunohistochemistry assay for the epidermal growth factor receptor (EGFR) has been developed for the detection of EGFR in colorectal, head and neck, and other tumors that express EGFR, and the assay is being used in colorectal cancer trials of IMC-C225 to select patients for treatment. The specificity of the assay and the detection technology used will be described, along with other assay methodologies and applications for EGFR testing.

Comparison of different antibodies for detection of progesterone receptor in breast cancer.

Monoclonal antibodies directed against human estrogen receptor (ER) and progesterone receptor (PR) have been used extensively for biochemical and immunohistochemical detection of receptors independent of hormone-binding assays. These antibodies have been valuable both for experimental work and for detection of receptors in clinical breast cancer specimens. The purpose of this study was to characterize the sensitivity and specificity of different antibodies for detection of PR by immunohistochemistry (IHC) of formalin-fixed paraffin breast carcinoma sections. The panel of twelve antibodies included two new ones (PgR636 and PgR1294) produced prospectively to be resistant to formalin fixation and paraffin embedding. Fifty-nine breast carcinomas, having known PR levels by biochemical ligand-binding assay, were used to prepare multitumor paraffin-embedded tissue blocks for characterization of the PR antibodies. Of all the antibodies tested, both PgR636 and PgR1294 stained the highest percentage of breast carcinomas known to be positive by the biochemical assay (95-98%) and they exhibited the highest concordance with the biochemical assay (88-90%). The PgR636 and PgR1294 antibodies, along with one other, PR 88, also gave the highest intensity of nuclear staining, while PgR636 and PgR1294 stained the highest mean percentage of tumor cell nuclei. Antigen retrieval was not necessary for PR immunostaining by PgR636 and PgR1294 in most tumors and other tissues examined, but did slightly increase the staining intensity. The majority of the other antibodies tested were highly dependent on antigen retrieval; only PR 88 and KD 68 antibodies approached the performance of PgR636 and PgR1294 without antigen retrieval. These results indicate that PgR636 and PgR1294 are optimal antibodies for IHC detection of PR in routine paraffin tissue blocks.