Leucine-rich-repeat-containing protein 15 (LRRC15) is selectively expressed on cancer-associated fibroblasts (CAFs) and constitutes a promising biomarker for imaging the tumor microenvironment. Using a combinatorial library approach, assisted by machine learning, we developed disulfide-constrained peptides (DCPs), notably ML-YSD-07 and ML-PD-03, that demonstrate subnanomolar affinities for murine LRRC15 (muLRRC15) and specifically localize onto muLRRC15-expressing fibroblasts. PET imaging with 18F-radiolabeled ML-YSD-07 exhibits specific tumor accumulation in a murine pancreatic cancer model highly enriched with LRRC15-expressing CAFs. Crystal structures of apo-muLRRC15 and of ML-YSD-07-bound muLRRC15 show that the DCPs evolved to adopt a distinct binding conformation that efficiently interacts with a flat epitope on muLRRC15. Collectively, this work identifies potent, molecularly engineered LRRC15-binding peptides and further highlights LRRC15 as a valuable CAF biomarker for cancer imaging applications.

We are developing a 14.3 kDa anti-CD8 VHH tracer, [ ^18 F]-2C8v144 (binding tracer, B), along with a nonbinding control tracer, [ ^18 F]-2C8v145 (C), in order to track CD8+ (“cytotoxic”) T cells in malignant tumors during immunotherapy by Positron Emission Tomography (PET). Arterial blood concentrations of C and B were monitored in three rhesus monkeys upon i.v. injection of mass doses/kg varying approximately 5–fold for both tracers. Plasma concentrations, calculated from individual hematocrit measurements assuming no cellular uptake, were analyzed by mixed–effects compartmental modeling, by assuming the parameters of non-specific distribution and elimination (estimated based on the PET-based arterial blood concentrations) to be the same for B as for C. C exhibited linear three–compartment kinetics with a mean residence time of 26 min. Nonlinear kinetics of B suggesting saturable, reversible binding outside of circulating blood were described by an average association rate constant, k_on.4 , and an equilibrium dissociation constant, K_D . Binding in blood to circulating CD8+ cells was described by a different association rate constant, k_on.b , and the same K_D . The total body content of CD8 receptors was estimated at 1.7 nmol/kg body weight, with 2.7 k_on.4 , 0.0097/(min · nM), was > 50–fold lower than k_on determined in vitro by Surface Plasmon Resonance (SPR, 0.50/(min · nM), yet the estimated K_D , 0.23 nM, was similar to the SPR estimate (0.13 nM) suggesting that the model informs about affinity. The model also yields predicted total body receptor occupancies and plasma concentrations of unbound B, i.e. the arterial input function needed for analyzing tracer kinetics in malignant tumors.

The incorporation of privileged saturated fragments into pharmaceutically relevant scaffolds has seen increased prevalence in drug discovery. Electrochemistry, a single-electron enabling platform, has demonstrated remarkable synthetic potential, particularly in alkyl functionalizations. However, these two concepts have not yet been fully considered in 18F-radiochemistry workflows. This work presents the development of a practical late-stage electrochemical platform that utilizes mild conditions, low voltages, and applications toward the installation of rigid alkyl bioisosteres onto relevant drug scaffolds. The utility of this platform is further demonstrated in the electrochemical synthesis of a PARP1 radiotracer and its application in subsequent biodistribution studies.

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Tau PET imaging using the tau specific PET tracer [18F]GTP1 has been and is part of therapeutic trials in Alzheimer's disease to monitor the accumulation of tau aggregates in the brain. Herein, we examined the metabolic processes of GTP1 and assessed the influence of smoking on its metabolism through in vitro assays. The tracer metabolic profile was assessed by incubating GTP1 with human liver microsomes (HLM) and human hepatocytes. Since smoking strongly stimulates the CYP1A2 enzyme activity, we incubated GTP1 with recombinant CYP1A2 to evaluate the role of the enzyme in tracer metabolism. It was found that GTP1 could form up to eleven oxidative metabolites with higher polarity than the parent. Only a small amount (2.6 % at 60 min) of a defluorinated metabolite was detected in HLM and human hepatocytes incubations highlighting the stability of GTP1 with respect to enzymatic defluorination. Moreover, the major GTP1 metabolites were not the product of CYP1A2 activity suggesting that smoking may not impact in vivo tracer metabolism and subsequently GTP1 brain kinetics.

Supplementary Figure 1 Legend from Use of a Peptide Derived from Foot-and-Mouth Disease Virus for the Noninvasive Imaging of Human Cancer: Generation and Evaluation of 4-[18F]Fluorobenzoyl A20FMDV2 for In vivo Imaging of Integrin αvβ6 Expression with Positron Emission Tomography

Supplementary Figure 1: Serum levels of AMA-MMAE in mice. Supplementary Figure 2: Correlation plot of tumor growth inhibition versus specific tumor uptake. Supplementary Table 1: Overview results. Supplementary Table 2: Tumor growth inhibition study of mice bearing OVCAR3-X2.1 cells comparing the efficacy of 5mg/kg and 20mg/kg doses

Supplementary Methods, Table, Figures 1-4 from Targeted In vivo Imaging of Integrin αvβ6 with an Improved Radiotracer and Its Relevance in a Pancreatic Tumor Model

AbstractExpression of the epithelial-specific integrin αvβ6 is low or undetectable in most adult tissues but may be increased during wound healing and inflammation and is up-regulated dramatically by many different carcinomas, making αvβ6 a promising target for the in vivo detection of cancer using noninvasive imaging. In addition, αvβ6 is recognized as promoting invasion and correlates with aggressive behavior of human cancers and thus agents that recognize αvβ6 specifically in vivo will be an essential tool for the future management of αvβ6-positive cancers. Recently, we identified the peptide NAVPNLRGDLQVLAQKVART (A20FMDV2), derived from foot-and-mouth disease virus, as a potent inhibitor of αvβ6. Using flow cytometry and ELISA, we show that this peptide is highly selective, inhibiting αvβ6-ligand binding with a IC50 of 3 nmol/L, an activity 1,000-fold more selective for αvβ6 than for other RGD-directed integrins (αvβ3, αvβ5, and α5β1). A20FMDV2 was radiolabeled on solid-phase using 4-[18F]fluorobenzoic acid, injected into mice bearing both αvβ6-negative and αvβ6-positive (DX3puro/DX3puroβ6 cell lines) xenografts and imaged using a small animal positron emission tomography (PET) scanner. Rapid uptake (<30 min) and selective retention (>5 h) of radioactivity in the αvβ6-positive versus the αvβ6-negative tumor, together with fast renal elimination of nonspecifically bound activity, resulted in specific imaging of the αvβ6-positive neoplasm. These data suggest that PET imaging of αvβ6-positive tumors is feasible and will provide an important new tool for early detection and improved management of many types of cancers. [Cancer Res 2007;67(16):7833–40]

ZED8 is a novel monovalent antibody labeled with zirconium-89 for the molecular imaging of CD8. This work describes nonclinical studies performed in part to provide rationale for and to inform expectations in the early clinical development of ZED8, such as in the studies outlined in clinical trial registry NCT04029181 [1]. Surface plasmon resonance, X-ray crystallography, and flow cytometry were used to characterize the ZED8-CD8 binding interaction, its specificity, and its impact on T cell function. Immuno-PET with ZED8 was assessed in huCD8+ tumor-bearing mice and in non-human primates. Plasma antibody levels were measured by ELISA to determine pharmacokinetic parameters, and OLINDA 1.0 was used to estimate radiation dosimetry from image-derived biodistribution data. ZED8 selectively binds to human CD8α at a binding site approximately 9 Å from that of MHCI making mutual interference unlikely. The equilibrium dissociation constant (KD) is 5 nM. ZED8 binds to cynomolgus CD8 with reduced affinity (66 nM) but it has no measurable affinity for rat or mouse CD8. In a series of lymphoma xenografts, ZED8 imaging was able to identify different CD8 levels concordant with flow cytometry. In cynomolgus monkeys with tool compound 89Zr-aCD8v17, lymph nodes were conspicuous by imaging 24 h post-injection, and the pharmacokinetics suggested a flat-fixed first-in-human dose of 4 mg per subject. The whole-body effective dose for an adult human was estimated to be 0.48 mSv/MBq, comparable to existing 89Zr immuno-PET reagents. 89Zr immuno-PET with ZED8 appears to be a promising biomarker of tissue CD8 levels suitable for clinical evaluation in cancer patients eligible for immunotherapy.

Cancer immunotherapies (CITs) have revolutionized the treatment of certain cancers, but many patients fail to respond or relapse from current therapies, prompting the need for new CIT agents. CD8+ T cells play a central role in the activity of many CITs, and thus, the rapid imaging of CD8+ cells could provide a critical biomarker for new CIT agents. However, existing 89Zr-labeled CD8 PET imaging reagents exhibit a long circulatory half-life and high radiation burden that limit potential applications such as same-day and longitudinal imaging. To this end, we discovered and developed a 13-kDa single-domain antibody (VHH5v2) against human CD8 to enable high-quality, same-day imaging with a reduced radiation burden. To enable sensitive and rapid imaging, we employed a site-specific conjugation strategy to introduce an 18F radiolabel to the VHH. The anti-CD8 VHH, VHH5v2, demonstrated binding to a membrane distal epitope of human CD8 with a binding affinity (KD) of 500 pM. Subsequent imaging experiments in several xenografts that express varying levels of CD8 demonstrated rapid tumor uptake and fast clearance from the blood. High-quality images were obtained within 1 h post-injection and could quantitatively differentiate the tumor models based on CD8 expression level. Our work reveals the potential of this anti-human CD8 VHH [18F]F-VHH5v2 to enable rapid and specific imaging of CD8+ cells in the clinic.

The "free drug hypothesis " assumes that, in the absence of transporters, the steady state free plasma concentrations equal to that at the site of action that elicit pharmacologic effects. While it is important to utilize the free drug hypothesis, exceptions exist that the free plasma exposures, either at C-max, C-trough, and C-average, or at other time points, cannot represent the corresponding free tissue concentrations. This "drug concentration asymmetry " in both total and free form can influence drug disposition and pharmacological effects. In this review, we first discuss options to assess total and free drug concentrations in tissues. Then various drug design strategies to achieve concentration asymmetry are presented. Last, the utilities of tissue concentrations in understanding exposure-effect relationships and translational projections to humans are discussed for several therapeutic areas and modalities. A thorough understanding in plasma and tissue exposures correlation with pharmacologic effects can provide insightful guidance to aid drug discovery.

Purpose Almost all radiolabellings of antibodies with 89 Zr currently employ the hexadentate chelator desferrioxamine (DFO). However, DFO can lead to unwanted uptake of 89 Zr in bones due to instability of the resulting metal complex. DFO*-NCS and the squaramide ester of DFO, DFOSq, are novel analogues that gave more stable 89 Zr complexes than DFO in pilot experiments. Here, we directly compare these linker-chelator systems to identify optimal immuno-PET reagents. Methods Cetuximab, trastuzumab and B12 (non-binding control antibody) were labelled with 89 Zr via DFO*-NCS, DFOSq, DFO-NCS or DFO*Sq. Stability in vitro was compared at 37 °C in serum (7 days), in formulation solution (24 h ± chelator challenges) and in vivo with N87 and A431 tumour-bearing mice. Finally, to demonstrate the practical benefit of more stable complexation for the accurate detection of bone metastases, [ 89 Zr]Zr-DFO*-NCS and [ 89 Zr]Zr-DFO-NCS-labelled trastuzumab and B12 were evaluated in a bone metastasis mouse model where BT-474 breast cancer cells were injected intratibially. Results [ 89 Zr]Zr-DFO*-NCS-trastuzumab and [ 89 Zr]Zr-DFO*Sq-trastuzumab showed excellent stability in vitro, superior to their [ 89 Zr]Zr-DFO counterparts under all conditions. While tumour uptake was similar for all conjugates, bone uptake was lower for DFO* conjugates. Lower bone uptake for DFO* conjugates was confirmed using a second xenograft model: A431 combined with cetuximab. Finally, in the intratibial BT-474 bone metastasis model, the DFO* conjugates provided superior detection of tumour-specific signal over the DFO conjugates. Conclusion DFO*-mAb conjugates provide lower bone uptake than their DFO analogues; thus, DFO* is a superior candidate for preclinical and clinical 89 Zr-immuno-PET.

Immuno-PET is a molecular imaging technique utilizing positron emission tomography (PET) to measure the biodistribution of an antibody species labeled with a radioactive isotope. When applied as a clinical imaging technique, an immuno-PET imaging agent must be manufactured with quality standards appropriate for regulatory approval. This paper describes methods relevant to the chemistry, manufacturing, and controls component of an immuno-PET regulatory filing, such as an investigational new drug application. Namely, the production, quality control, and characterization of the immuno-PET clinical imaging agent, ZED8, an 89Zr-labeled CD8-specific monovalent antibody as well as its desferrioxamine-conjugated precursor, CED8, is described and evaluated. PET imaging data in a human CD8-expressing tumor murine model is presented as a proof of concept that the imaging agent exhibits target specificity and comparable biodistribution across a range of desferrioxamine conjugate loads.

The use of 89Zr-antibody PET imaging to measure antibody biodistribution and tissue pharmacokinetics is well established, but current PET systems lack the sensitivity needed to study 89Zr-labeled antibodies beyond 2-3 isotope half-lives (7-10 d), after which a poor signal-to-noise ratio is problematic. However, studies across many weeks are desirable to better match antibody circulation half-life in human and nonhuman primates. These studies investigated the technical feasibility of using the primate mini-EXPLORER PET scanner, making use of its high sensitivity and 45-cm axial field of view, for total-body imaging of 89Zr-labeled antibodies in rhesus monkeys up to 30 d after injection. Methods: A humanized monoclonal IgG antibody against the herpes simplex viral protein glycoprotein D (gD) was radiolabeled with 89Zr via 1 of 4 chelator-linker combinations (benzyl isothiocyanate-DFO [DFO-Bz-NCS], where DFO is desferrioxamine B; DFO-squaramide; DFO*-Bz-NCS, where DFO* is desferrioxamine*; and DFO*-squaramide). The pharmacokinetics associated with these 4 chelator-linker combinations were compared in 12 healthy young male rhesus monkeys (∼1-2 y old, ∼3 ± 1 kg). Each animal was initially injected intravenously with unlabeled antibody in a peripheral vessel in the right arm (10 mg/kg, providing therapeutic-level antibody concentrations), immediately followed by approximately 40 MBq of one of the 89Zr-labeled antibodies injected intravenously in a peripheral vessel in the left arm. All animals were imaged 6 times over a period of 30 d, with an initial 60-min dynamic scan on day 0 (day of injection) followed by static scans of 30-45 min on approximately days 3, 7, 14, 21, and 30, with all acquired using a single bed position and images reconstructed using time-of-flight list-mode ordered-subsets expectation maximization. Activity concentrations in various organs were extracted from the PET images using manually defined regions of interest. Results: Excellent image quality was obtained, capturing the initial distribution phase in the whole-body scan; later time points showed residual 89Zr mainly in the liver. Even at 30 d after injection, representing approximately 9 half-lives of 89Zr and with a total residual activity of only 20-40 kBq in the animal, the image quality was sufficient to readily identify activity in the liver, kidneys, and upper and lower limb joints. Significant differences were noted in late time point liver uptake, bone uptake, and whole-body clearance between chelator-linker types, whereas little variation (±10%) was observed within each type. Conclusion: These studies demonstrate the ability to image 89Zr-radiolabeled antibodies up to 30 d after injection while maintaining satisfactory image quality, as provided by the primate mini-EXPLORER with high sensitivity and long axial field of view. Quantification demonstrated potentially important differences in the behavior of the 4 chelators. This finding supports further investigation.

Cancer immunotherapies have demonstrated durable responses in a range of different cancers. However, only a subset of patients responds to these therapies. We set out to test if non-invasive imaging of tumor perfusion and vascular inflammation may be able to explain differences in T-cell infiltration in pre-clinical tumor models, relevant for treatment outcomes. Tumor perfusion and vascular cell adhesion molecule (VCAM-1) density were quantified using magnetic resonance imaging (MRI) and correlated with infiltration of adoptively transferred and endogenous T-cells. MRI biomarkers were evaluated for their ability to detect tumor rejection 3 days after T-cell transfer. Baseline levels of these markers were used to assess their ability to predict PD-L1 treatment response. We found correlations between MRI-derived VCAM-1 density and infiltration of endogenous or adoptively transferred T-cells in some preclinical tumor models. Blocking T-cell binding to endothelial cell adhesion molecules (VCAM-1/ICAM) prevented T-cell mediated tumor rejection. Tumor rejection could be detected 3 days after adoptive T-cell transfer prior to tumor volume changes by monitoring the extracellular extravascular volume fraction. Imaging tumor perfusion and VCAM-1 density before treatment initiation was able to predict the response of MC38 tumors to PD-L1 blockade. These results indicate that MRI based assessment of tumor perfusion and VCAM-1 density can inform about the permissibility of the tumor vasculature for T-cell infiltration which may explain some of the observed variance in treatment response for cancer immunotherapies.