Efficacy of panitumumab (pmab) vs. cetuximab (cmab) in patients (pts) with wild-type (WT) KRAS exon 2 metastatic colorectal cancer (mCRC) treated with prior bevacizumab (bev): Results from ASPECCT

519

Background: The phase 3 ASPECCT trial of pts with chemorefractory WT KRAS exon 2 mCRC demonstrated that pmab was noninferior to cmab for overall survival (OS). A previous subgroup analysis of hazard ratios (HRs) suggested that pts who had received prior bev (any line, at any point before study start) in the pmab arm may have had better outcomes vs pts in the cmab arm (Price, 2014). Methods: Pts were randomized 1:1 to receive pmab or cmab. The subset of pts who had received prior bev were analyzed based on the final analysis of ASPECCT. Results: 999 pts were randomized and treated: 499 pmab and 500 cmab. The prior bev subset included 126 pts in the pmab arm (25%) and 132 pts in the cmab arm (26%). Pts in the pmab arm had longer median OS and progression-free survival (PFS) and higher objective response rates (ORR) compared with pts in the cmab arm. Results are shown (table). After adjustment for baseline covariates including ECOG performance status, number of metastatic sites, and baseline LDH, OS HR was 0.65 (95%CI=0.49-0.85) with pmab vs cmab in pts who had received prior bev. Pts in the pmab and cmab arms who did not receive prior bev had similar OS, PFS, and ORR. Post-progression antitumor therapy was similar between the pmab (47%) and cmab arms (52%) in pts who received prior bev. Conclusions: In ASPECCT, pts with WT KRASexon 2 mCRC who received prior bev-containing regimens may have derived greater benefit with pmab versus cmab monotherapy. Clinical trial information: NCT01001377. [Table: see text]

Outcomes by hypomagnesemia (hypomag) in the randomized phase III ASPECCT trial of patients (pts) with chemofractory wild-type (WT) KRAS exon 2 metastatic colorectal cancer (mCRC)

507

Background: ASPECCT demonstrated that panitumumab (pmab) was noninferior to cetuximab (cmab) for overall survival (OS). Ad hoc analyses from ASPECCT suggested that hypomag was associated with better outcomes for pmab and cmab (Price 2015). However, results from the phase 3 NCIC CTG/AGITG CO.17 trial indicated hypomag at day 28 was associated with worse outcomes for cmab (Vickers 2013). Methods: Patients (pts) with chemorefractory WT KRASexon 2 mCRC were randomized 1:1 to receive pmab or cmab. Ad hoc analyses by hypomag were performed from the final analysis of ASPECCT at week 5, consistent with the NCIC CTG/AGITG CO.17 trial analysis (Vickers, 2013). Results: 999 pts were treated: 496 received pmab and 503 received cmab. Any grade hypomag was 29.0% and grade ≥3 was 7.3% in the pmab arm vs 19.3% and 2.8% in the cmab arm, respectively. In the pmab arm, 1.2% of pts discontinued treatment and 5% of pts had dose modifications due to hypomag vs 0.4% and 3% in the cmab arm, respectively. Efficacy results by hypomag are shown (Table). Conclusions: In ASPECCT, rates of hypomag were higher in the pmab vs the cmab arm. Pts who developed any grade hypomag with pmab or cmab had longer median OS compared with those pts who did not. Consistent with previous analyses, development of hypomag at week 5 was associated with worse median OS for cmab. Clinical trial information: NCT01001377. [Table: see text]

Phase III study (MONET1) of motesanib plus carboplatin/paclitaxel (C/P) in patients with advanced nonsquamous non-small cell lung cancer (NSCLC): Asian subgroup analysis

7549

Background: MONET1 evaluated overall survival (OS) in patients (pts) with nonsquamous NSCLC receiving motesanib (an oral VEGFR 1, 2 and 3, PDGFR and Kit inhibitor) plus C/P compared with pts receiving placebo plus C/P. Analysis of the total population (N=1090) showed that motesanib + C/P did not significantly improve OS vs C/P alone (primary endpoint). Here we present results of a subgroup analysis of Asian pts. Methods: Asian pts (Japan, S. Korea, Philippines, Hong Kong, Taiwan, Singapore) with stage IIIB/IV or recurrent nonsquamous NSCLC and no prior systemic therapy for advanced NSCLC were analysed. Pts were randomized to up to six 3-wk cycles of C (AUC 6 mg/mL·min) and P (200 mg/m2) with either motesanib 125 mg QD (Arm A) or placebo QD (Arm B) orally continuously. Results: 227 Asian pts (incl. 106 Japanese pts) with nonsquamous NSCLC were randomized (Arm A/B, n=110/117); 198 had adenocarcinoma (n=97/101). Median age was 60 y (range 30–78); 80% had stage IV disease. At the time of analysis, 139 pts had died (118 pts with adenocarcinoma). Pts received a median of 164 days of motesanib vs 125 days of placebo (vs 106 and 126 days in non-Asian pts). Median follow-up was 63 wks. Efficacy results are shown in the table. Motesanib/placebo-related AEs were seen in 94/74% of pts respectively; Gr ≥3 related AEs in 48/22%. Most common emergent AEs were (Arm A/B) alopecia (78/76%), diarrhea (63/33%), and nausea (55/43%); gallbladder disorders (Gr 1–2) were seen in 9/2% of pts. Gr ≥3 AEs more frequent in Arm A vs B included neutropenia (36/22%) and hypertension (13/3%). Emergent Gr 5 events were seen in (Arm A/B) 5/4% vs 16/11% in non-Asian pts. Conclusions: In contrast to non-Asian pts, in the subgroup of Asian pts with advanced nonsquamous NSCLC, motesanib plus C/P treatment was associated with increased OS, PFS, and objective response rates (ORR) compared with C/P alone, with no excess of treatment-related mortality. [Table: see text]

Conatumumab (CON) plus FOLFIRI (F) or ganitumab (GAN) plus F for second-line treatment of mutant (MT) KRAS metastatic colorectal cancer (mCRC)

534

Background: EGFR inhibitors are not an option for patients (pts) with MT KRAS mCRC; therefore, there is an unmet medical need for novel agents to treat this pt population. CON (agonist) and GAN (antagonist) are investigational, fully human monoclonal antibodies to death receptor 5 and type 1 insulin-like growth factor receptor, respectively. In colorectal xenografts, CON enhanced the activity of 5FU, and both agents enhanced the activity of irinotecan. A 3-arm, placebo (pbo)-controlled, double-blind, double dummy, randomized phase 2 study evaluated both agents in MT KRAS mCRC.

Methods: Planned sample size = 150. Eligibility: MT KRAS mCRC; disease progression on 1 prior oxaliplatin and fluoropyrimidine therapy. Randomization was 1:1:1 to Arm 1: CON 10 mg/kg + GAN pbo + F Q2W; Arm 2: CON pbo + GAN 12 mg/kg + F Q2W; Arm 3: CON pbo + GAN pbo + F Q2W. Randomization was stratified by PS (0 vs 1) and prior anti-VEGF therapy (yes vs no). Pts received CON, GAN, or pbo IV day (D)1 and 15; irinotecan 180 mg/m2 D1; leucovorin 400 mg/m2 over 2 h D1; 5FU bolus 400 mg/m2 over 2-4 min D1 then 5FU 2400 mg/m2 over 46-48 h. Primary endpoint: PFS by investigator assessment.

Results: 155 pts (Arms 1/2/3: 51/52/52 pts) were randomized between 3/09 and 2/11; 50/50/52 received ≥ 1 dose of study drug. Median age, 59/58/59 years; women, 47/54/56%; colon cancer, 75/65/83%; PS 1, 37/42/40%; prior anti-VEGF therapy, 43/40/42%. See table for efficacy. % pts with grade 3-5 adverse events (Arms 1/2/3) included: neutropenia 30/25/18%; diarrhea 18/2/10%; intestinal obstruction 4/6/8%. There was 1 treatment-related death of unknown cause on Arm 1.

Conclusions: CON + F and GAN + F were tolerable in this pt population. CON + F, but not GAN + F, improved PFS. ORR was increased vs pbo. OS data are immature. Further study of CON may be warranted in mCRC.

[Table: see text]

Predictors of skeletal complications in men with hormone-refractory metastatic prostate cancer

OBJECTIVES

Skeletal complications are a major cause of morbidity in men with hormone-refractory metastatic prostate cancer. These analyses were designed to identify the variables associated with a greater risk of skeletal complications.

METHODS

The 643 subjects in this report were participants in a randomized placebo-controlled trial to evaluate the effects of zoledronic acid on the incidence of skeletal-related events. All subjects had bone metastases and disease progression despite medical or surgical castration. The relationships between the baseline covariates and the time to the first skeletal-related event were assessed by Cox proportional hazard analyses. The serum bone-specific alkaline phosphatase (BAP) and urinary N-telopeptide level was assessed as a representative specific marker of osteoblastic and osteoclastic activity, respectively. The other covariates included in the model were age, cancer duration, Eastern Cooperative Oncology Group performance status, analgesic use, and prostate-specific antigen, hemoglobin, and lactate dehydrogenase levels.

RESULTS

Elevated BAP levels were consistently associated with a greater risk of adverse skeletal outcomes. Elevated BAP was significantly associated with a shorter time to the first skeletal-related event on multivariate analyses of the entire study population (relative risk 1.84, 95% confidence interval 1.40 to 2.43; P <0.001) and in subset analyses of the placebo and zoledronic acid groups. Elevated BAP levels were also consistently associated with adverse skeletal outcomes on multivariate analyses of the time to radiotherapy and pathologic fracture, the most common types of skeletal-related events in the study population. No other baseline variable was consistently associated with the risk of adverse skeletal outcomes.

CONCLUSIONS

The results of our study have shown that elevated serum BAP levels are associated with a greater risk of adverse skeletal outcomes in men with hormone-refractory prostate cancer and bone metastases.

Markers of Bone Metabolism and Survival in Men with Hormone-Refractory Metastatic Prostate Cancer

PURPOSE

To evaluate the relative prognostic value for specific markers of osteoblast and osteoclast activity while controlling for previously reported prognostic variables among men with hormone-refractory metastatic prostate cancer.

EXPERIMENTAL DESIGN

The 643 subjects in this report were participants in multicenter randomized controlled trial of zoledronic acid in men with metastatic prostate cancer. All subjects had bone metastases and disease progression despite medical or surgical castration. Relationships between baseline covariates and overall survival were examined by Cox proportional hazard model. Serum bone-specific alkaline phosphatase (BAP) and urinary N-telopeptide were assessed as representative specific markers of osteoblast and osteoclast activity, respectively. Other covariates in the model were age, log prostate-specific antigen, hemoglobin, lactate dehydrogenase, albumin, analgesic use, and Eastern Cooperative Oncology Group performance status.

RESULTS

Serum BAP was significantly correlated with urinary N-telopeptide (correlation coefficient = 0.674; 95% confidence interval, 0.628-0.715; P < 0.0001). In univariate analyses, higher levels of serum BAP and urinary N-telopeptide levels were significantly associated with shorter overall survival. After controlling for the other variables, including N-telopeptide, in multivariate models, higher serum BAP levels were consistently associated with shorter survival. In contrast, urinary N-telopeptide levels were not significantly associated with survival in multivariate analyses. Variables retained in the reduced multivariate model were age, log prostate-specific antigen, hemoglobin, lactate dehydrogenase, analgesic use, and BAP.

CONCLUSIONS

Serum BAP significantly correlates with urinary N-telopeptide in men with androgen-independent prostate cancer and bone metastases. In multivariate models, higher levels of serum BAP but not urinary N-telopeptide are associated with shorter overall survival.

Predictive Value of Bone Resorption and Formation Markers in Cancer Patients With Bone Metastases Receiving the Bisphosphonate Zoledronic Acid

Purpose

Three large, randomized trials of patients with bone metastases recently demonstrated that zoledronic acid reduces the risk of skeletal-related events. These trials provide an opportunity for investigating the correlation between bone metabolism and clinical outcome during bisphosphonate therapy.

Patients and Methods

Urinary measurements of N-telopeptide (Ntx) and serum bone alkaline phosphatase (BAP) were obtained in 1,824 bisphosphonate-treated patients—1,462 with zoledronic acid (breast, 490; prostate, 411; myeloma, 210; non–small-cell lung, 183; other, 168) and 362 with pamidronate (breast, 254; myeloma, 108). This exploratory cohort analysis grouped patients by baseline and most recent levels of Ntx as low (< 50 nmol/mmol creatinine), moderate (50 to 99 nmol/mmol creatinine), or high (≥ 100 nmol/mmol creatinine), and BAP as low (< 146 U/L) or high (≥ 146 U/L). The relative risks for negative clinical outcomes were estimated for each group using multiple-event and Cox regression models with time-varying covariates.

Results

Patients with high and moderate Ntx levels had 2-fold increases in their risk of skeletal complications and disease progression compared with patients with low Ntx levels (P < .001 for all). High Ntx levels in each solid tumor category were associated with a 4- to 6-fold increased risk of death on study, and moderate Ntx levels a 2- to 4-fold increased risk compared with low Ntx levels (P < .001 for all). Bone alkaline phosphatase also showed some correlation with risk of negative clinical outcomes.

Conclusion

The bone resorption marker Ntx provides valuable prognostic information in patients with bone metastases receiving bisphosphonates.

Nuclear localization conferred by the pocket domain of the retinoblastoma gene product

The tumor suppressor Rb is a nuclear phosphoprotein that controls cell growth and differentiation by modulating the activity of certain transcription factors. Transport of Rb to the nucleus is affected by both a bipartite nuclear localization signal (NLS) in the C-terminus of the protein and a central domain, termed A/B or pocket, through which Rb interacts with transcription factors and viral oncoproteins. Mutations in either the A or B subdomains of the pocket render a NLS-deficient Rb completely cytoplasmic. Fusing the A/B domain of Rb to the Escherichia coli beta-galactosidase, to create betagal-A/B, confers nuclear localization upon this bacterial protein. Moreover, co-expression with the adenovirus oncoprotein, E1A, further augments nuclear localization of betagal-A/B. These findings provide direct evidence that the pocket domain of Rb is not only required but also sufficient to induce nuclear transport by a 'piggyback' mechanism. Thus, nuclear localization of Rb is dictated by two independent and autonomous domains: (i) the bipartite NLS and (ii) the pocket domain. We suggest that via these domains, Rb chaperons and co-compartmentalizes with its associated factors and preempts their activity prior to nuclear transport.

Stimulation of MAP kinase and S6 kinase by vanadium and selenium in rat adipocytes

To explore the mechanism underlying the insulin-mimetic actions of vanadium and selenium we examined their effects on the mitogen activated protein/myelin basic protein kinases (MAPK) and ribosomal S6 protein kinases, which are among the best characterized of the kinases that comprise the phosphorylation cascade in insulin signal transduction. We observed a transient activation of MAPK and S6 kinases by insulin in rat adipocytes, while both sodium selenate and vanadyl sulphate produced prolonged activation of the kinases. Vanadyl sulphate stimulated the activity of MAPK and S6 kinase by as much as 6 fold and 15 fold, respectively. Pretreatment of the cells with genistein did not affect the activation of MAPK by insulin, but partially blocked the effects of sodium selenate and vanadyl sulphate. Genistein did not change the activation of S6 kinase by insulin, but blocked the activation in vanadyl sulphate- and sodium selenate-treated-cells, suggesting that a genistein sensitive tyrosine kinase may be involved in the activation by these two compounds. Rapamycin, a specific inhibitor of the p70s6k isoform of S6 kinase, partially reduced the activation of S6 kinase activity by sodium selenate, indicating a role for this kinase in the overall activity of the S6 kinase in sodium selenate-treated cells. A similar trend was noted in vanadyl sulphate-treated cells. Thus, this study supports the involvement of MAPK and S6 kinases in the insulin-mimetic actions of vanadium and selenium.

Loss of Rb and Myc activation co-operate to suppress cyclin D1 and contribute to transformation

Cyclin D1 can bind and phosphorylate the product (pRb) of the retinoblastoma gene (RB-1) and recent evidence suggests pRb, in turn, may regulate cyclin D1 protein expression. In transformed cell lines, loss of pRb activity strongly correlates with a decrease in cyclin D1 protein expression, and conversely, introduction of pRb can induce cyclin D1 promoter activity. We show here that pRb does not regulate cyclin D1 directly as basal and serum-stimulated levels of cyclin D1 protein and kinase activity are similar in wildtype and pRb-deficient primary mouse embryonic fibroblasts (MEFs). These observations suggest that the suppression of cyclin D1 in pRb-minus tumour cell lines requires both loss of pRb and at least one additional genetic event. We have determined that constitutive, ectopic Myc expression in pRb-deficient, but not wildtype, MEFs suppresses cyclin D1 protein expression and kinase activity. Regulation is evident at either the level of RNA or protein expression. Phenotypically, pRb-deficient MEFs consistently exhibited a delayed growth response in comparison to wildtype MEFs. This growth delay is abrogated in pRb-deficient MEFs which are expressing ectopic Myc protein, coincident with the loss of cyclin D1 protein expression. Moreover, these cells exhibit an increased proliferative capacity, and they no longer show contact inhibition. Our results support a cross-regulatory mechanism between Myc, pRb and cyclin D1 and suggest a novel role for cyclin D1 in tumorigenesis.

Skeletal muscle mitogen-activated protein kinases and ribosomal S6 kinases. Suppression in chronic diabetic rats and reversal by vanadium

The mitogen-activated protein (MAP) kinases and ribosomal S6 protein kinases in the skeletal muscle of insulin-resistant long-term (2 and 6 months' duration) diabetic rats were investigated to understand further the changes in insulin intracellular signaling pathways that accompany diabetes. The effects of insulin-mimetic vanadium compounds on the activity of these kinases were also examined. In the insulin-resistant 2-month diabetic rats, the basal activities of MAP kinases were relatively unchanged, while the basal activities of S6 kinases were significantly increased. Intravenous injection of insulin moderately activated both the 42-kDa MAP kinase (p42mapk) and a 44-kDa MAP kinase (p44erk1) in the 2-month control rats but not in the 2-month diabetic rats. Insulin treatment markedly stimulated the activity of a novel 31-kDa S6 kinase and the previously described 90-kDa ribosomal S6 kinase encoded by one of the rsk genes (p90rsk) in the 2-month control rats, while the effect was substantially reduced in the diabetic rats. In the 6-month diabetic rats, the basal phosphotransferase activities of both MAP kinases were depressed threefold or greater. This correlated with reductions in the amount of immunoreactive p42mapk and p44erk1 proteins in extracts from the diabetic rats. The basal activity of the 31-kDa S6 kinase activity was also reduced fourfold in the 6-month diabetic rats. Treatment of the 2-month diabetic rats with vanadyl sulfate resulted in euglycemia, prevented the increase in the basal activity of S6 kinase, and improved the activation of S6 kinase by insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of a novel ribosomal S6 kinase from skeletal muscle of insulin-treated rats

The predominant 40 S ribosomal protein S6 kinase in skeletal muscle extracts from insulin-treated rats was purified over 10,000-fold to near homogeneity with approximately 4.5% recovery of starting activity. This S6 kinase was resolved from the catalytic subunit of cAMP-dependent protein kinase only by the seventh and final column chromatography step. The purified S6 kinase migrated as a tight doublet of approximately 31 kDa on an SDS-polyacrylamide gel, and it was eluted from gel filtration columns with a similar apparent M(r), which indicated that the enzyme exists as a monomer. This S6 kinase was immunologically distinct from the other known insulin-activated S6 kinases, i.e. p70S6K and p90rsk. It was inhibited by [ethylenebis(oxyethylenenitrilo)]tetraacetic acid and beta-glycerophosphate at concentrations routinely used to stabilize p70S6K and p90rsk. In addition to S6, phosvitin was also a substrate, whereas myelin basic protein, casein, protamine, and histones were poorly phosphorylated if at all by the purified S6 kinase. The purified enzyme was inactivated upon incubation with serine/threonine-specific protein phosphatase 2A, which indicated that it may be an intermediary component in a cascade of insulin-activated protein kinases.

Distribution of MAP kinase, S6 kinase, and casein kinase II in rat tissues: activation by insulin in spleen

We examined the distribution of mitogen-activated protein (MAP) kinase, S6 kinase, and casein kinase II (CK-II) in the muscle, spleen, brain, and testes of Wistar rats. It was observed that spleen extracts contained the highest activity of all the kinases. Anion-exchange chromatography of spleen extracts by a MonoQ column resolved a single peak of myelin basic protein phosphotransferase activity that eluted after the usual position of the previously described p42 and p44 MAP kinases. Immunoblotting of the peak fractions with anti-MAP kinase antibody did not detect any immunoreactive bands that coincided with the activity peak, suggesting that the activity may represent a potentially novel MAP kinase. The MonoQ fractionation also resolved a single peak of phosvitin phosphotransferase activity which coincided with the intensity of two immunoreactive bands of 39 and 43 kilodaltons that were detected with antibodies against CK-II. The chromatographic behaviour and immunoblotting data indicate that the phosvitin kinase peak represented CK-II and suggested that the rat spleen CK-II had a molecular structure of alpha alpha ' beta 2. Furthermore, using an intact rat model, we showed that the potentially novel spleen MAP kinase and CK-II were markedly activated following intravenous injection of insulin. The significance of these findings remains to be determined.

Characterization of insulin-stimulated seryl/threonyl protein kinases in rat skeletal muscle

Post-insulin receptor signal transduction is mediated by a cascade of seryl/threonyl protein kinases which includes a family of mitogen-activated protein (MAP) kinases, ribosomal protein S6 kinases, and casein kinase-2. Previous studies have characterized these kinases primarily in cultured or isolated cells. We have demonstrated that intravenous injection of insulin into fasted rats significantly stimulated the activities of MAP kinases and S6 kinases in skeletal muscle, independently of the blood glucose levels in these animals. Anion exchange chromatography on Mono Q afforded the resolution of at least five peaks of insulin-stimulated myelin basic protein kinase activity. By immunological criteria, these myelin basic protein kinases included the p42mapk and p44erk1 as well as other potentially novel 44-kDa MAP kinases. Insulin-activated ribosomal S6 kinases were resolved into two major peaks by Mono Q chromatography, the latter of which contained a 100-kDa isoform of p90rsk as revealed by immunoblotting with an anti-rsk-peptide antibody. A 32-kDa S6 kinase in the earlier peak may represent a novel protein kinase in this tissue. Skeletal muscle casein kinase-2 was not significantly stimulated following insulin injection into rats under our experimental conditions. These results indicate that the intact rat can serve as a useful model system to investigate the mechanisms of insulin signal transduction.

Alterations of G protein function in cardiac tissues from streptozotocin-induced chronic diabetic rats

1. This study was performed to investigate G protein function in cardiac tissues from chronic diabetic rats by using pertussis toxin (PTX) and cholera toxin (CTX) as probes for G(i) and Gs proteins, respectively. 2. In the 10-week control group, i.v. injection of PTX significantly elevated the basal heart rate without having any effect on the chronotropic response of right atria to increasing concentrations of isoproterenol (ISO). In the 10-week diabetic rats, PTX treatment had no effect on the basal heart rate or on the response of right atria to ISO. In the 6-month groups, PTX did not exert any effects on basal or ISO-stimulated heart rate in either control or diabetic rat. 3. The inhibitory effect of carbachol (CCH) on cardiac tension in ISO-stimulated left atria was completely abolished by i.v. injection of PTX in the 10-week groups (both control and diabetic rats). The same treatment, however, only slightly reduced the effect of CCH on left atria contraction in rats from 6-month groups. 4. In both control and diabetic rats in the 10-week groups, incubation with CTX caused a significant increase in heart rate in right atria, and in developed cardiac tension in left atria preparations. The magnitude of the increase was the same in both control and diabetic rats. 5. Studies carried out using ADP-ribosylation technique indicated that the amount of G(i) protein was not changed in the ventricular muscle of the 10-week diabetic rat. Labelling of Gs protein could not be detected in either control or diabetic rat heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of correlation between activation of cyclic AMP-dependent protein kinase and inhibition of contraction of rat vas deferens by cyclic AMP analogs

The effects of N6,O2-dibutyrl-adenosine-3',5'-cyclic monophosphate (db-cAMP) and 8-bromo-adenosine-3',5'-cyclic monophosphate (8-Br-cAMP) on tension and cAMP-dependent protein kinase (PKA) activities in rat vas deferens were investigated. A soluble enzyme fraction obtained from the vas deferens was found to contain both type I and type II isozymes of PKA, whereas a particulate fraction contained only the type II isozyme. Exposure of the vas deferens to db-cAMP (1-100 microM) for 30 min caused a concentration-dependent inhibition of phenylephrine-induced contractions, with an EC50 of less than 10 microM. 8-Br-cAMP had no significant effect on contractions over a similar concentration range. Both of the analogs were able to activate PKA significantly at a concentration of 10 microM, and the magnitude of the PKA activation was greater with 8-Br-cAMP than with db-cAMP. Charcoal was added to the homogenization buffer in these experiments to prevent the artifactual activation of PKA by cAMP analogs trapped in the extracellular space. The ability of db-cAMP, but not 8-Br-cAMP, to inhibit the contraction of vas deferens could not be explained on the basis of differential activation of soluble or particulate PKA or of specific isozymes of the enzyme. It is, therefore, concluded that activation of PKA is not responsible for the relaxant effects of cAMP analogs in some smooth muscle.