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The International Consortium for Innovation and Quality (IQ) Physiologically Based Pharmacokinetic (PBPK) Modeling Induction Working Group (IWG) conducted a survey across participating companies around general strategies for PBPK modeling of induction, including experience with its utility to address various questions, regulatory interactions, and regulatory acceptance. The results highlight areas where PBPK modeling is used with high confidence and identifies opportunities where confidence is lower and further evaluation is needed. To enhance the survey results, the PBPK-IWG also collected case studies and analyzed recent literature examples where PBPK models were applied to predict CYP3A induction-mediated drug-drug interactions. PBPK modeling of induction has evolved and progressed significantly, proving to have great potential to accelerate drug discovery and development. With the aim of enabling optimal use for new molecular entities that are either substrates and/or inducers of CYP3A, the PBPK-IWG proposes initial workflows for PBPK application, discusses future trends, and identifies gaps that need to be addressed.

Edoxaban 60 mg is approved for stroke prevention in patients with atrial fibrillation (AF) not fulfilling any dose-reduction criteria. As edoxaban is partially renally cleared (≈50%), this study compared pharmacokinetics (PK) and pharmacodynamics of edoxaban 60 mg once daily with edoxaban 75 mg once daily in patients with AF with high renal clearance (creatinine clearance 100 mL/min) over 12 months. Primary PK and pharmacodynamics end points were plasma edoxaban exposure and anti-factor Xa (FXa) concentration. A population PK model estimated edoxaban exposure at steady state. Efficacy and safety outcomes included composites of stroke, transient ischemic attack, systemic embolism, and major and clinically relevant nonmajor bleeding. Of 607 patients, 303 and 304 were randomized to edoxaban 60 and 75 mg, respectively. Edoxaban 75 mg provided ≈25% higher exposure than 60 mg. This increase was accurately depicted in the population PK model; anti-factor Xa concentration correlated with edoxaban exposure. Rates of composite and individual outcomes were similarly low between doses. In conclusion, the 25% increase in edoxaban dose (60-75 mg) resulted in ≈25% exposure increase in the 75-mg group. Higher exposure was not associated with reduced stroke risk in patients with AF with high renal clearance.

Ephrin type-A 2 (EphA2) is a transmembrane receptor expressed in epithelial cancers. We report on a phase I dose escalation and biodistribution study of DS-8895a, an anti-EphA2 antibody, in patients with advanced EphA2 positive cancers. DS-8895a was administered at 1, 3, 10 or 20 mg/kg every 2 weeks to determine safety, pharmacokinetics and anti-tumor efficacy. All patients underwent

B7-H3 is a transmembrane protein widely expressed in a variety of cancers and has been shown to play a role in anti-tumor immunity. This study aims to develop a molecular imaging probe to identify B7-H3 expression in tumors and to develop 89Zr-DS-5573a as a theranostic that could aid patient selection in clinical Phase I studies. Methods: The anti-B7-H3 humanised monoclonal antibody DS-5573a was labeled with zirconium-89 (89Zr-), and assessed for radiochemical purity, immunoreactivity (Lindmo analysis), antigen binding affinity (Scatchard analysis), and serum stability in vitro. In vivo biodistribution and imaging studies were performed with positron emission tomography and magnetic resonance imaging (PET/MRI) studies to identify and quantitate 89Zr-DS-5573a tumor uptake in a B7-H3-positive breast cancer model (MDA-MB-231) and a B7-H3-negative murine colon cancer model (CT26). Imaging and biodistribution studies were also performed in MDA-MB-231 tumor-bearing SCID mice in the absence and presence of therapeutic DS-5573a antibody dose (3 mg/kg DS-5573a). Results: 89Zr-DS-5573a showed high and specific binding to B7-H3-expressing MDA-MB-231 cells (immunoreactivity on day 0, 75.0 ± 2.9%), and low binding to B7-H3-negative CT26 cells (immunoreactivity on day 0, 10.85 ± 0.11%) in vitro. 89Zr-DS-5573a demonstrated good serum stability in vitro with 57.2 ± 2.0% of immunoreactivity remaining on day 7. In vivo biodistribution studies showed high uptake of 89Zr-DS-5573a in B7-H3-expressing MDA-MB-231 tumor-bearing mice, achieving 32.32 ± 6.55 %ID/g on day 7 post injection in BALB/c nu/nu mice and 25.76 ± 1.79 %ID/g in SCID mice, with minimal evidence of non-specific uptake in normal tissues, and excellent tumor localization on PET/MRI. In a combined imaging/therapy study, receptor saturation was demonstrated in tumors responding to therapy. Conclusion: 89Zr-DS-5573a demonstrates specific and prolonged targeting of B7-H3-expressing tumors in vivo. Saturation of binding sites was demonstrated in tumors responding to DS-5573a therapy. These results indicate that 89Zr-DS-5573a has potential to target B7-H3-expressing tumors in cancer patients. Furthermore 89Zr-DS-5573a has the potential to provide important insights into T cell biology through its specific binding to B7-H3.

Competitive radiolabeled antibody imaging can determine the unlabeled intact antibody dose that fully blocks target binding but may be confounded by heterogeneous tumor penetration. We evaluated the hypothesis that smaller radiolabeled constructs can be used to more accurately evaluate tumor expressed receptors. The Krogh cylinder distributed model, including bivalent binding and variable intervessel distances, simulated distribution of smaller constructs in the presence of increasing doses of labeled antibody forms. Smaller constructs

1. Esaxerenone (CS-3150) is a novel non-steroidal mineralocorticoid receptor antagonist. The pharmacokinetics, tissue distribution, excretion, and metabolism of esaxerenone were evaluated in rats and monkeys. 2. Following intravenous dosing of esaxerenone at 0.1-3 mg/kg, the total body clearance and the volume of distribution were 3.53-6.69 mL/min/kg and 1.47-2.49 L/kg, respectively, in rats, and 2.79-3.69 mL/min/kg and 1.34-1.54 L/kg, respectively, in monkeys. The absolute oral bioavailability was 61.0-127% in rats and 63.7-73.8% in monkeys. 3. After oral administration of [

Background: DS-8273a, an anti-human death receptor 5 (DR5) agonistic antibody, has cytotoxic activity against human cancer cells and induces apoptosis after specific binding to DR5. DS-8273a is currently being used in clinical Phase I trials. This study evaluated the molecular imaging of DR5 expression in vivo in mouse tumor models using SPECT/CT and PET/MRI, as a tool for drug development and trial design. Methods: DS-8273a was radiolabeled with indium-111 and zirconium-89. Radiochemical purity, immunoreactivity, antigen binding affinity and serum stability were assessed in vitro. In vivo biodistribution and pharmacokinetic studies were performed, including SPECT/CT and PET/MR imaging. A dose-escalation study using a PET/MR imaging quantitative analysis was also performed to determine DR5 receptor saturability in a mouse model. Results: 111In-CHX-A″-DTPA-DS-8273a and 89Zr-Df-Bz-NCS-DS-8273a showed high immunoreactivity (100%), high serum stability, and bound to DR5 expressing cells with high affinity (Ka, 1.02-1.22 × 1010 M-1). The number of antibodies bound per cell was 32,000. In vivo biodistribution studies showed high and specific uptake of 111In-CHX-A″-DTPA-DS-8273a and 89Zr-Df-Bz-NCS-DS-8273a in DR5 expressing COLO205 xenografts, with no specific uptake in normal tissues or in DR5-negative CT26 xenografts. DR5 receptor saturation was observed in vivo by biodistribution studies and quantitative PET/MRI analysis. Conclusion: 89Zr-Df-Bz-NCS-DS-8273a is a potential novel PET imaging reagent for human bioimaging trials, and can be used for effective dose assessment and patient response evaluation in clinical trials.

Purpose CS-1008 (tigatuzumab) is a humanized, monoclonal immunoglobulin G1 (IgG1) agonistic antibody to human death receptor 5. The purpose of this study was to investigate the impact of CS-1008 dose on the biodistribution, quantitative tumor uptake, and antitumor response in patients with metastatic colorectal cancer (mCRC). Patients and Methods Patients with mCRC who had received at least one course of chemotherapy were assigned to one of five dosage cohorts and infused with a weekly dose of CS-1008. Day 1 and day 36 doses were trace-labeled with indium-111 (111In), followed by whole-body planar and regional single-photon emission computed tomography (SPECT) imaging at several time points over the course of 10 days. Results Nineteen patients were enrolled. 111In-CS-1008 uptake in tumor was observed in only 12 patients (63%). 111In-CS-1008 uptake and pharmacokinetics were not affected by dose or repeated drug administration. 111In-CS-1008 biodistribution showed gradual blood-pool clearance and no abnormal uptake in normal tissue. No anti–CS-1008 antibody development was detected. One patient achieved partial response (3.7 months duration), eight patients had stable disease, and 10 patients had progressive disease. Clinical benefit rate (stable disease + partial response) in patients with 111In-CS-1008 uptake in tumor was 58% versus 28% in patients with no uptake. An analysis of individual lesions showed that lesions with antibody uptake were one third as likely to progress as those without antibody uptake (P = .07). Death-receptor–5 expression in archived tumor samples did not correlate with 111In-CS-1008 uptake (P = .5) or tumor response (P = .6). Conclusion Death-receptor–5 imaging with 111In-CS-1008 reveals interpatient and intrapatient heterogeneity of uptake in tumor, is not dose dependent, and is predictive of clinical benefit in the treatment of patients who have mCRC.

Objectives The aim of this study was to noninvasively detect the anti-inflammatory properties of the novel liver X receptor agonist R211945. Background R211945 induces reversal cholesterol transport and modulates inflammation in atherosclerotic plaques. We aimed to characterize with 18F-fluorodeoxyglucose (FDG)–positron emission tomography (PET)/computed tomography (CT) and dynamic contrast-enhanced cardiac magnetic resonance (DCE-CMR) inflammation and neovascularization, respectively, in atherosclerotic plaques with R211945 treatment compared with atorvastatin treatment and a control. Methods Twenty-one atherosclerotic New Zealand white rabbits were divided into 3 groups (control, R211945 [3 mg/kg orally], and atorvastatin [3 mg/kg orally] groups). All groups underwent 18F-FDG–PET/CT and DCE-CMR at baseline and at 1 and 3 months after treatment initiation. Concomitantly, serum metabolic parameters and histology were assessed. For statistical analysis, continuous DCE-CMR and PET/CT outcomes were modeled as linear functions of time by using a linear mixed model, whereas the histological data, animal characteristics data, and nonlinear regression imaging data were analyzed with a 2-tailed Student t test. Results 18F-FDG–PET/CT detected a decrease in mean and maximum standard uptake values (SUV) over time in the R211945 group (both p = 0.001), indicating inflammation regression. The atorvastatin group displayed no significant change (p = 0.371 and p = 0.600, respectively), indicating no progression or regression. The control group demonstrated an increase in SUV (p = 0.01 and p = 0.04, respectively), indicating progression. There was a significant interaction between time and group for mean and maximum SUV (p = 0.0003 and p = 0.0016, respectively) . DCE-CMR detected a trend toward difference (p = 0.06) in the area under the curve in the atorvastatin group, suggesting a decrease in neovascularization. There was no significant interaction between time and group (p = 0.6350 and p = 0.8011, respectively). Macrophage and apolipoprotein B immunoreactivity decreased in the R211945 and atorvastatin groups (p Conclusions Noninvasive imaging with 18F-FDG–PET/CT and DCE-CMR and histological analysis demonstrated significant anti-inflammatory effects of the LXR agonist R211945 compared with atorvastatin. The results suggest a possible role for LXR agonists in the treatment of atherosclerosis.

Intravital Microscopy (IVM) is a powerful tool for imaging of dynamic events in living subjects and benefits from flexibility of various tissue preparation techniques. For example, a “tissue flap” (TF) approach initially affords high spatial resolution and physiological imaging with minimal tissue preparation, but serial TF imaging greatly increases the effects of pathological inflammation, resulting in postoperative adhesions and tissue injury. We took a materials science approach by implanting a commercially available, thin film, biopolymer tissue adhesion barrier (TAB) beneath the TF during serial imaging of the normal and developing breast in transgenic fluorescent mice, and with a fluorescent orthotopic mouse lymphoma model. We applied the TAB post-operatively beneath the TF to isolate the TF from the underlying peritoneum. When re-imaging the TF every 3–4 d, with a new TAB placed each time, we observed reduced hemorrhage, fibrous connective tissue and soft tissue damage. The presence of the TAB enab...

Subtype A2 of the erythropoietin-producing hepatocellular tyrosine kinase (EphA2) cell surface receptor is expressed in a range of epithelial cancers. This study evaluated the molecular imaging of EphA2 expression in vivo in mouse tumor models using SPECT/MR and PET/MR and a humanized anti-EphA2 antibody, DS-8895a. Methods: DS-8895a was labeled with 111In, 125I, and 89Zr and assessed for radiochemical purity, immunoreactivity (Lindmo analysis), antigen-binding affinity (Scatchard analysis), and serum stability in vitro. In vivo biodistribution, imaging, and pharmacokinetic studies were performed with SPECT/MR and PET/MR. A dose-escalation study was also performed to determine EphA2 receptor saturability through tissue and imaging quantitative analysis. Results: All conjugates demonstrated good serum stability and specific binding to EphA2-expressing cells in vitro. In vivo biodistribution studies showed high uptake of 111In-CHX-A″-DTPA-DS-8895a and 89Zr-Df-Bz-NCS-DS-8895a in EphA2-expressing xenograft models, with no specific uptake in normal tissues. In comparison, retention of 125I-DS-8895a in tumors was lower because of internalization of the radioconjugate and dehalogenation. These results were confirmed by SPECT/MR and PET/MR. EphA2 receptor saturation was observed at the 30 mg/kg dose. Conclusion: Molecular imaging of tumor uptake of DS-8895a allows noninvasive measurement of EphA2 expression in tumors in vivo and determination of receptor saturation. 89Zr-Df-Bz-NCS-DS-8895a is suited for human bioimaging trials on the basis of superior imaging characteristics and will inform DS-8895a dose assessment and patient response evaluation in clinical trials.

We combine mathematical modeling with experiments in living mice to quantify the relative roles of intrinsic cellular vs. tissue-scale physiological contributors to chemotherapy drug resistance, which are difficult to understand solely through experimentation. Experiments in cell culture and in mice with drug-sensitive (Eµ-myc/Arf-/-) and drug-resistant (Eµ-myc/p53-/-) lymphoma cell lines were conducted to calibrate and validate a mechanistic mathematical model. Inputs to inform the model include tumor drug transport characteristics, such as blood volume fraction, average geometric mean blood vessel radius, drug diffusion penetration distance, and drug response in cell culture. Model results show that the drug response in mice, represented by the fraction of dead tumor volume, can be reliably predicted from these inputs. Hence, a proof-of-principle for predictive quantification of lymphoma drug therapy was established based on both cellular and tissue-scale physiological contributions. We further demonstrate that, if the in vitro cytotoxic response of a specific cancer cell line under chemotherapy is known, the model is then able to predict the treatment efficacy in vivo. Lastly, tissue blood volume fraction was determined to be the most sensitive model parameter and a primary contributor to drug resistance.

The aim of this study was to noninvasively detect the anti-inflammatory properties of the novel liver X receptor agonist R211945.R211945 induces reversal cholesterol transport and modulates inflammation in atherosclerotic plaques. We aimed to characterize with (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) and dynamic contrast-enhanced cardiac magnetic resonance (DCE-CMR) inflammation and neovascularization, respectively, in atherosclerotic plaques with R211945 treatment compared with atorvastatin treatment and a control.Twenty-one atherosclerotic New Zealand white rabbits were divided into 3 groups (control, R211945 [3 mg/kg orally], and atorvastatin [3 mg/kg orally] groups). All groups underwent (18)F-FDG-PET/CT and DCE-CMR at baseline and at 1 and 3 months after treatment initiation. Concomitantly, serum metabolic parameters and histology were assessed. For statistical analysis, continuous DCE-CMR and PET/CT outcomes were modeled as linear functions of time by using a linear mixed model, whereas the histological data, animal characteristics data, and nonlinear regression imaging data were analyzed with a 2-tailed Student t test.(18)F-FDG-PET/CT detected a decrease in mean and maximum standard uptake values (SUV) over time in the R211945 group (both p = 0.001), indicating inflammation regression. The atorvastatin group displayed no significant change (p = 0.371 and p = 0.600, respectively), indicating no progression or regression. The control group demonstrated an increase in SUV (p = 0.01 and p = 0.04, respectively), indicating progression. There was a significant interaction between time and group for mean and maximum SUV (p = 0.0003 and p = 0.0016, respectively) . DCE-CMR detected a trend toward difference (p = 0.06) in the area under the curve in the atorvastatin group, suggesting a decrease in neovascularization. There was no significant interaction between time and group (p = 0.6350 and p = 0.8011, respectively). Macrophage and apolipoprotein B immunoreactivity decreased in the R211945 and atorvastatin groups (p0.0001 and p = 0.0004, respectively), and R211945 decreased oxidized phospholipid immunoreactivity (p = 0.02).Noninvasive imaging with (18)F-FDG-PET/CT and DCE-CMR and histological analysis demonstrated significant anti-inflammatory effects of the LXR agonist R211945 compared with atorvastatin. The results suggest a possible role for LXR agonists in the treatment of atherosclerosis.

Purpose: Pactimibe is a novel ACAT inhibitor. The pharmacokinetics of pactimibe and its pharmacologically inactive plasma metabolite, R-125528, of which the main clearance pathway is CYP2D6, was affected by coadministration of quinidine. The aim of this study was to investigate the influence of CYP2D6 polymorphism on pharmacokinetics of pactimibe and R-125528. In addition, exposure was examined after multiple doses of pactimibe sulfate in CYP2D6 poor metabolizer (PMs). Methods: 24 healthy male Caucasian volunteers, genotyped as extensive, intermediate, and poor metabolizers, were received single dose of 25 mg pactimibe. In a multiple-dose study, six CYP2D6 PMs received 100 mg pactimibe for 21 days and exposure of pactimibe and R-125528 was examined. Results: In contrast to the mild 1.7-fold increase in AUC 0-inf of pactimibe, a marked 3.1-fold increase in AUC 0-tz of R-125528 was observed in CYP2D6 PMs. After multiple doses of 100 mg pactimibe to CYP2D6 PMs, the accumulation ratio of R-125528 reached 8.8-fold, however, the exposure of R-125528 in CYP2D6 PMs was covered by the exposure in additional metabolite safety testing. Conclusions: Although CYP2D6 polymorphism greatly affected the pharmacokinetics of R-125528 rather than pactimibe, the exposure in CYP2D6 PMs after a multiple dose of 100 mg pactimibe sulfate was covered by additional non-clinical metabolite safety testing. The finding is clinically informative with respect to the safety testing of drug metabolite present at disproportionately high levels in a special population with specific genetic back ground.

Typical CYP2D6 substrates generally contain a basic nitrogen atom that interacts with Asp(301) and/or Glu(216) and an aromatic moiety adjacent to the site of metabolism. Recently, we found novel acidic substrates for CYP2D6, pactimibe, and its indole metabolite, R-125528, that are not protonated but are negatively charged at physiological pH. The K(m) value of R-125528 in CYP2D6-expressing microsomes was determined to be 1.74 microM, which was comparable with those of typical basic CYP2D6 substrates (1-10 microM). Pactimibe has lower affinity than R-125528; however, the K(m) value was comparable with that of metoprolol. Interestingly, their sites of metabolism, the omega-1 position of the N-octyl indoline/indole group, were relatively distant from the aromatic moiety. A pactimibe analog with an N-decyl chain was similarly labile against CYP2D6; however, analogs with N-hexyl or N-dodecyl chains were stable to CYP2D6. An induced fit docking of the ligands with an X-ray crystal structure of substrate-free CYP2D6 (Protein Data Bank code 2F9Q) indicated the involvement of an electrostatic interaction between the carboxyl group and Arg(221) and hydrophobic interaction between the aromatic moiety and Phe(483). The docking model correctly positioned the site of metabolism above the heme. The effect of the N-alkyl chain length of pactimibe analogs on their CYP2D6 metabolic stability was plausibly explained by the docking model. In conclusion, we report herein a novel CYP2D6 binding mode for the acidic substrates pactimibe and R-125528. Further investigation, such as a site-directed mutation, will be necessary to directly demonstrate the involvement of Arg(221) in CYP2D6 binding.

Pactimibe sulfate is a novel acyl coenzyme A:cholesterol acyltransferase inhibitor developed for the treatment of hypercholesterolemia and atherosclerotic diseases. Pactimibe has two equally dominant clearance pathways forming R-125528 by CYP3A4 and M-1 by CYP2D6 in vitro. R-125528 is a plasma metabolite and is cleared solely by CYP2D6 despite its acidity. To evaluate contributions of the cytochrome P450 enzymes on the pharmacokinetics of pactimibe and R-125528 in humans, drug-drug interaction studies using ketoconazole and quinidine were conducted. Eighteen healthy male subjects were given a single dose of pactimibe sulfate without and with 400 mg of ketoconazole (q.d.). With the concomitant treatment, the area under the plasma concentration-time curve (AUC(0-inf)) of pactimibe modestly increased 1.7-fold and AUC(0-tz) of R-125528 decreased by 55%. In addition, 17 healthy male subjects were given a single dose of pactimibe sulfate without and with 600 mg of quinidine (b.i.d.). With the concomitant treatment, the AUC(0-inf) for pactimibe modestly increased 1.7-fold. On the other hand, the AUC(0-tz) of R-125528 was markedly elevated 5.0-fold, although the AUC(0-inf) could not be adequately defined because the terminal elimination phase of R-125528 was not obtained in the study period up to 72 h. As the f(m CYP3A4) and f(m CYP2D6) values of pactimibe estimated from in vitro studies were 0.40 and 0.33, respectively, AUC increase ratios of pactimibe were estimated to be 1.7 with ketoconazole and 1.5 with quinidine. These values were well in accordance with the values observed in this study. Moreover, the f(m CYP2D6) of R-125528 estimated to be almost 1 would well explain the accumulation of R-125528 observed with the quinidine treatment.

Pactimibe sulfate is a novel acyl coenzyme A:cholesterol acyltransferase inhibitor. We conducted metabolic studies of pactimibe and its plasma metabolite, R-125528. Pactimibe had multiple metabolic pathways including indolin oxidation to form R-125528, omega-1 oxidation, N-dealkylation, and glucuronidation. Among them, the indolin oxidation and the omega-1 oxidation were dominant and were mainly catalyzed by CYP3A4 and CYP2D6, respectively. The intrinsic clearance (CL(int)) values for these pathways in human hepatic microsomes were 0.63 and 0.76 microl/min/mg-protein, respectively. On the other hand, the metabolic reaction for R-125528 was restricted. It was demonstrated that omega-1 oxidation was the only pathway that could eliminate R-125528 from the systemic circulation. To our surprise, only CYP2D6-expressing microsomes could catalyze the reaction, and omega-1 oxidation was strongly correlated with the CYP2D6 marker reaction, dextromethorphan O-demethylation (r(2) = 0.90), in human hepatic microsomes. Although R-125528 is an atypical substrate for CYP2D6 because of its acidity, the K(m) value was 1.8 microM for the reaction in human hepatic microsomes and the CL(int) value was as high as 75.0 microl/min/mg-protein. These results suggested that the systemic clearance of R-125528 was highly dependent on CYP2D6 activity and that several studies with CYP2D6 including drug-drug interaction and polymorphism sensitivity should be performed during development from the viewpoint of metabolite safety assessment. The finding that R-125528, an acidic compound devoid of basic nitrogen, was a good substrate for CYP2D6 raised a question about previously reported CYP2D6 models based on a critical electrostatic interaction with Asp(301) and/or Glu(216).

Background: Non-uniform tumor penetration by antibodies may contribute to therapeutic failure. Competition experiments by radiolabeled antibody imaging can guide selection of the antibody dose that fully blocks target binding and thus corresponds to complete therapeutic coverage. However, because antibodies penetrate tumors slowly and non-uniformly, such a competition experiment may reflect only the most accessible sites. A novel approach, termed enhanced competition, has been proposed in which the radiolabeled moiety is smaller and more readily diffusible. In this study, simulations were conducted in order to examine the enhanced competition experiment. Materials and Methods: The Krogh cylinder distributed model was implemented to simulate the distribution of antibody, antibody fragment, affibody, or peptide in the tumor as a function of time and distance from the capillary wall. Simulations explored traditional and enhanced competition experiments, varying parameters including the nature of the small construct, time between administration of unlabeled and radiolabeled compounds, mass dose, affinity, antigen density, and internalization rate. Results were plotted as nanomolar concentration versus time and distance from the capillary wall. In addition, total tumor %ID/g is presented as a function of time and as a function of unlabeled antibody dose. Results: Simulation results showed that small constructs access sites in tumors at distances far from capillaries that are not readily accessible to intact antibodies. Under conditions of high affinity (1-10 nM), high antigen density (150,000 sites/cell), and compound internalization (t1/2 = 13 h), a mass of > 40 μg/kg of peptide or affibody construct required a higher dose of antibody to displace the radioactive signal compared to traditional competition with radiolabeled antibody. However, the difference in imaging signal between sub-saturating and fully displacing antibody doses is expected to be difficult to detect. Conclusions: The phenomenon predicted by the enhanced competition paradigm using small constructs is validated by modeling. Simulated imaging competition experiments indicate that affinity and mass dose levels are critical factors in this approach. High affinity small constructs and enhanced precision of imaging techniques will be required to fully evaluate target occupancy and saturation in vivo. Citation Format: Kelly D. Orcutt, Gregory P. Adams, Anna M. Wu, Matthew Silva, Jack Hoppin, Catey Harwell, Manabu Matsumura, Masakatsu Kotsuma, Daniel Freeman, Archie Tse, Jonathan Greenberg, Andrew Scott, Robert A. Beckman. Receptor occupancy and tumor penetration by antibodies, peptides, and antibody fragments: Molecular simulation of imaging assessment. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4300. doi:10.1158/1538-7445.AM2014-4300