Tumor-targeted CD28 bispecific antibodies enhance the antitumor efficacy of PD-1 immunotherapy.

Monoclonal antibodies that block the programmed cell death 1 (PD-1) checkpoint have revolutionized cancer immunotherapy. However, many major tumor types remain unresponsive to anti-PD-1 therapy, and even among responsive tumor types, most of the patients do not develop durable antitumor immunity. It has been shown that bispecific antibodies activate T cells by cross-linking the TCR/CD3 complex with a tumor-specific antigen (TSA). The class of TSAxCD3 bispecific antibodies have generated exciting results in early clinical trials. We have recently described another class of "costimulatory bispecifics" that cross-link a TSA to CD28 (TSAxCD28) and cooperate with TSAxCD3 bispecifics. Here, we demonstrate that these TSAxCD28 bispecifics (one specific for prostate cancer and the other for epithelial tumors) can also synergize with the broader anti-PD-1 approach and endow responsiveness-as well as long-term immune memory-against tumors that otherwise do not respond to anti-PD-1 alone. Unlike CD28 superagonists, which broadly activate T cells and induce cytokine storm, TSAxCD28 bispecifics display little or no toxicity when used alone or in combination with a PD-1 blocker in genetically humanized immunocompetent mouse models or in primates and thus may provide a well-tolerated and "off the shelf" combination approach with PD-1 immunotherapy that can markedly enhance antitumor efficacy.

The efficacy and pharmacokinetics of brincidofovir for the treatment of lethal rabbitpox virus infection: a model of smallpox disease.

Brincidofovir (BCV) has broad-spectrum in vitro activity against dsDNA viruses, including smallpox, and is being developed as a treatment for smallpox as well as infections caused by other dsDNA viruses. BCV has previously been shown to be active in multiple animal models of smallpox. Here we present the results of a randomized, blinded, placebo-controlled study of the efficacy and pharmacokinetics of a novel, "humanized" regimen of BCV for treatment of New Zealand White rabbits infected with a highly lethal inoculum of rabbitpox virus, a well characterized model of smallpox. Compared with placebo, a dose-dependent increase in survival was observed in all BCV-treatment groups. Concentrations of cidofovir diphosphate (CDV-PP), the active antiviral, in rabbit peripheral blood mononuclear cells (PBMCs) were determined for comparison to those produced in humans at the dose proposed for treatment of smallpox. CDV-PP exposure in PBMCs from rabbits given BCV scaled to human exposures at the dose proposed for treatment of smallpox, which is also currently under evaluation for other indications. The results of this study demonstrate the activity of BCV in the rabbitpox model of smallpox and the feasibility of scaling doses efficacious in the model to a proposed human dose and regimen for treatment of smallpox.

Physiologically based pharmacokinetic modeling of deltamethrin: development of a rat and human diffusion-limited model.

Mirfazaelian et al. developed a physiologically based pharmacokinetic (PBPK) model for the pyrethroid pesticide deltamethrin in the rat. This model describes gastrointestinal (GI) tract absorption as a saturable process mediated by phase III efflux transporters which pump deltamethrin out of the intestinal enterocytes into the GI tract lumen, resulting in minimal net absorption at low concentrations and increasing absorption at higher concentrations. In the present study, the dose dependency in absorption of deltamethrin was examined in male Long Evans rats using po exposures predicted by the Mirfazaelian model to yield different po bioavailability values. No difference in the bioavailability from single po doses of 0.3 and 3.0 mg/kg deltamethrin was observed. Based on this finding, the Mirfazaelian PBPK model was modified to exclude a saturable absorption process. Other changes to the Mirfazaelian model included describing all tissue compartments with diffusion-limited kinetics and a single blood compartment. These changes improved model predictions of deltamethrin tissue concentration data from the present study and the literature. The rat model was then scaled to humans. The model predicted a twofold greater peak deltamethrin brain concentration and threefold greater area under the curve (AUC(0-48 h)) for humans following an po exposure of 1 mg/kg. Based on this model, humans would have greater distribution of deltamethrin to the brain for the same administered po dose compared to rats. The relative sensitivity to deltamethrin between rats and humans depends on both pharmacokinetic and pharmacodynamic differences. Species differences in the pharmacodynamic responses to deltamethrin between rats and humans remain uncharacterized.

In vitro metabolism of pyrethroid pesticides by rat and human hepatic microsomes and cytochrome p450 isoforms.

Species differences in the intrinsic clearance (CL(int)) and the enzymes involved in the metabolism of pyrethroid pesticides were examined in rat and human hepatic microsomes. The pyrethroids bifenthrin, S-bioallethrin, bioresmethrin, beta-cyfluthrin, cypermethrin, cis-permethrin, and trans-permethrin were incubated in rat and human hepatic microsomes in the presence or absence of NADPH. Metabolism was measured using a parent depletion approach. The CL(int) of the pyrethroids was 5- to 15-fold greater in rat relative to human microsomes except for trans-permethrin, which was approximately 45% greater in human microsomes. The metabolism of bifenthrin, S-bioallethrin, and cis-permethrin in rat and human hepatic microsomes was solely the result of oxidative processes. The metabolism of bioresmethrin and cypermethrin in human hepatic microsomes was solely the result of hydrolytic processes. Bioresmethrin and cypermethrin in rat hepatic microsomes and beta-cyfluthrin and trans-permethrin in microsomes from both species were metabolized by both oxidative and hydrolytic pathways. The metabolism of trans-permethrin was reduced when incubated with its diastereomer, cis-permethrin, in both rat and human hepatic microsomes. Rat cytochrome P450 (P450) isoforms that showed activity toward several pyrethroids included CYP1A1, CYP1A2, CYP2C6, CYP2C11, CYP3A1, and CYP3A2. Human P450 isoforms that showed activity toward multiple pyrethroids were CYP2C8, CYP2C9, CYP2C19, and CYP3A4. Species-specific differences in metabolism may result in variable detoxification of pyrethroids, which may in turn result in divergent neurotoxic outcomes. These species differences and isomer interactions in metabolism of pyrethroids should be considered when assessing the potential adverse health effects of pyrethroid pesticides.

Identification of rat and human cytochrome p450 isoforms and a rat serum esterase that metabolize the pyrethroid insecticides deltamethrin and esfenvalerate.

The metabolism of (alphaS)-cyano-3-phenoxybenzyl (1R, 3R)-cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropane carboxylate (deltamethrin) and (alphaS)-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)-3-methylbutyrate (esfenvalerate) by rat and human liver microsomes differs with respect to the biotransformation pathway (oxidation versus hydrolysis) responsible for their clearance. This study aims to further explore the species differences in the metabolism of these chemicals. Using a parent depletion approach, rat and human cytochromes P450 (P450s) were screened for their ability to eliminate deltamethrin or esfenvalerate during in vitro incubations. Rat P450 isoforms CYP1A1, CYP2C6, CYP2C11, and CYP3A2 and human P450 isoforms CYP2C8, CYP2C19, and CYP3A5 were capable of metabolizing either pyrethroid. Human CYP2C9 metabolized esfenvalerate but not deltamethrin. Rat and human P450s that metabolize esfenvalerate and deltamethrin do so with similar kinetics. In addition to the liver, a potential site of metabolic elimination of pyrethroids is the blood via serum carboxylesterase (CE) hydrolysis. The serum of rats, but not humans, contains significant quantities of CE. Deltamethrin and esfenvalerate were metabolized effectively by rat serum and a purified rat serum CE. In contrast, neither pyrethroid was metabolized by human serum or purified human serum esterases (acetylcholinesterase and butyrylcholinesterase). These studies suggest that the difference in rates of oxidative metabolism of pyrethroids by rat and human hepatic microsomes is dependent on the expression levels of individual P450 isoforms rather than their specific activity. Furthermore, these studies show that the metabolic elimination of deltamethrin and esfenvalerate in blood may be important to their disposition in rats but not in humans.

Species differences in the in vitro metabolism of deltamethrin and esfenvalerate: differential oxidative and hydrolytic metabolism by humans and rats.

Pyrethroids are neurotoxic pesticides whose pharmacokinetic behavior plays a role in their potency. This study examined the elimination of esfenvalerate and deltamethrin from rat and human liver microsomes. A parent depletion approach in the presence and absence of NADPH was used to assess species differences in biotransformation pathways, rates of elimination, and intrinsic hepatic clearance. Esfenvalerate was eliminated primarily via NADPH-dependent oxidative metabolism in both rat and human liver microsomes. The intrinsic hepatic clearance (CL(INT)) of esfenvalerate was estimated to be 3-fold greater in rodents than in humans on a per kilogram body weight basis. Deltamethrin was also eliminated primarily via NADPH-dependent oxidative metabolism in rat liver microsomes; however, in human liver microsomes, deltamethrin was eliminated almost entirely via NADPH-independent hydrolytic metabolism. The CL(INT) for deltamethrin was estimated to be 2-fold more rapid in humans than in rats on a per kilogram body weight basis. Metabolism by purified rat and human carboxylesterases (CEs) were used to further examine the species differences in hydrolysis of deltamethrin and esfenvalerate. Results of CE metabolism revealed that human carboxylesterase 1 (hCE-1) was markedly more active toward deltamethrin than the class 1 rat CEs hydrolase A and B and the class 2 human CE (hCE-2); however, hydrolase A metabolized esfenvalerate 2-fold faster than hCE-1, whereas hydrolase B and hCE-1 hydrolyzed esfenvalerate at equal rates. These studies demonstrate a significant species difference in the in vitro pathways of biotransformation of deltamethrin in rat and human liver microsomes, which is due in part to differences in the intrinsic activities of rat and human carboxylestersases.