Prospective Design of Anti-Transferrin Receptor Bispecific Antibodies for Optimal Delivery into the Human Brain

Anti‐transferrin receptor (TfR)‐based bispecific antibodies have shown promise for boosting antibody uptake in the brain. Nevertheless, there are limited data on the molecular properties, including affinity required for successful development of TfR‐based therapeutics. A complex nonmonotonic relationship exists between affinity of the anti‐TfR arm and brain uptake at therapeutically relevant doses. However, the quantitative nature of this relationship and its translatability to humans is heretofore unexplored. Therefore, we developed a mechanistic pharmacokinetic‐pharmacodynamic (PK‐PD) model for bispecific anti‐TfR/BACE1 antibodies that accounts for antibody‐TfR interactions at the blood‐brain barrier (BBB) as well as the pharmacodynamic (PD) effect of anti‐BACE1 arm. The calibrated model correctly predicted the optimal anti‐TfR affinity required to maximize brain exposure of therapeutic antibodies in the cynomolgus monkey and was scaled to predict the optimal affinity of anti‐TfR bispecifics in humans. Thus, this model provides a framework for testing critical translational predictions for anti‐TfR bispecific antibodies, including choice of candidate molecule for clinical development.

Abnormal social behaviors in mice lacking Fgf17

The fibroblast growth factor family of secreted signaling molecules is essential for patterning in the central nervous system. Fibroblast growth factor 17 (Fgf17) has been shown to contribute to regionalization of the rodent frontal cortex. To determine how Fgf17 signaling modulates behavior, both during development and in adulthood, we studied mice lacking one or two copies of the Fgf17 gene. Fgf17-deficient mice showed no abnormalities in overall physical growth, activity level, exploration, anxiety-like behaviors, motor co-ordination, motor learning, acoustic startle, prepulse inhibition, feeding, fear conditioning, aggression and olfactory exploration. However, they displayed striking deficits in several behaviors involving specific social interactions. Fgf17-deficient pups vocalized less than wild-type controls when separated from their mother and siblings. Elimination of Fgf17 also decreased the interaction of adult males with a novel ovariectomized female in a social recognition test and reduced the amount of time opposite-sex pairs spent engaged in prolonged, affiliative interactions during exploration of a novel environment. After social exploration of a novel environment, Fgf17-deficient mice showed less activation of the immediate-early gene Fos in the frontal cortex than wild-type controls. Our findings show that Fgf17 is required for several complex social behaviors and suggest that disturbances in Fgf17 signaling may contribute to neuropsychiatric diseases that affect such behaviors.

Enkephalin contributes to the locomotor stimulating effects of 3,4-methylenedioxy-N-methylamphetamine

3,4-methylenedioxy-N-methylamphetamine (MDMA, 'Ecstasy') is a potent inhibitor of serotonin uptake, which induces both an increase in locomotion and a decrease in exploratory activity in rodents. Serotonin 5-HT1B receptors, located on the terminals of striatal efferent neurons, have been suggested to mediate these motor effects of MDMA. Striatal neurons projecting to the globus pallidus contain met-enkephalin, whilst those projecting to the substantia nigra contain substance P. We therefore analysed the levels of both peptides using radioimmunocytochemistry after MDMA administration (10 mg/kg, 3 h) in wild-type and 5-HT1B receptor knockout mice. Our results demonstrate that MDMA induces a decrease in pallidal met-enkephalin immunolabelling in wild-type, but not in 5-HT1B receptor knockout mice. Similar results were obtained following treatment with the 5-HT1A/1B agonist RU24969 (5 mg/kg, 3 h), suggesting that activation of 5-HT1B receptors leads to a reduction in met-enkephalin levels in the globus pallidus. In contrast, MDMA had no effect on the nigral substance P levels. We have previously shown that both MDMA and RU24969 fail to stimulate locomotor activity in 5-HT1B receptor knockout mice. Our present data indicate that the opioid antagonist naloxone suppressed the locomotor effects of MDMA. This study is the first to demonstrate that Enk contributes to MDMA-induced increases in locomotor activity. Such an effect may be related to the 5-HT control of pallidal met-enkephalin levels via the 5-HT1B receptors.

Engineering receptors activated solely by synthetic ligands (RASSLs)

The functional and molecular diversity of G-protein-coupled receptors presents a significant challenge to understanding the connection between a single receptor signaling pathway and a specific physiological or pathological response. To gain control over the timing and specificity of a G-protein signal, receptors activated solely by synthetic ligands (RASSLs) have been developed. These engineered receptors no longer respond to endogenous peptides, but can still be activated by a specific small-molecule drug. Further control over the location of the signal can be achieved by using RASSLs in conjunction with tissue-specific expression systems in vivo. Existing RASSLs have clarified the role of G(i) signaling in cardiac physiology and are currently being used to study cardiomyopathy, muscle remodeling, sensory transduction and complex neurobehavioral responses.

Modulation of the effects of cocaine by 5-HT1B receptors: a comparison of knockouts and antagonists

Serotonergic transmission has been suggested to modulate the effects of cocaine. However, the specific receptors underlying this phenomenon have not been identified. To evaluate the role of the 5-HT1B receptor in mediating the actions of cocaine, we used two model systems: knockout (KO) mice lacking the 5-HT1B receptor and an acute treatment with the 5-HT1B/1D antagonist GR127935. GR127935 attenuated the ability of cocaine to stimulate locomotion and induce c-fos expression in the striatum. However, GR127935 had no apparent effect on the rewarding or sensitizing effects of cocaine. In contrast, as demonstrated previously, the 5-HT1B receptor KO mice showed a heightened locomotor response to cocaine, as well as an increased propensity to self-administer cocaine. Thus, an acute pharmacological blockade of the 5-HT1B receptor decreases some effects of cocaine, while a constitutive genetic KO of the same receptor has opposite effects. These results suggest that compensatory changes have taken place during the development of the 5-HT1B KO mice, which may have rendered these mice more vulnerable to cocaine. The 5-HT1B KO mice should therefore be considered as a genetic model of vulnerability to drug abuse rather than a classic pharmacological tool.

Absence of cocaine-induced place conditioning in serotonin 1B receptor knock-out mice

A large body of evidence suggests that genetic factors may affect the reinforcing properties of drugs of abuse. This study investigated the involvement of the serotonin 1B (5-HT1B) receptor in modulating cocaine-induced place conditioning by comparing the response of 5-HT1B receptor gene knock-out mice with wild type 129/Sv-ter mice. The rewarding effects of various doses of cocaine (0, 2.5, 5, 10, 20, and 40 mg/kg) were examined in both strains. Results clearly show that 5-HT1B receptor knock-out mice failed to display a conditioned place preference for stimuli paired with cocaine while wild type mice exhibited a conditioned place preference for the compartment paired with cocaine (5 and 20 mg/kg). As other studies showed that 5-HT1B knock-out mice self-administer cocaine, these results suggest a dissociation between the psychologic state linked to self-administration and the one measured in conditioned place preference.

5-HT receptor knockout mice: pharmacological tools or models of psychiatric disorders

The molecular diversity of cloned serotonin receptor subtypes in the brain makes it difficult to understand the specific modulatory roles played by different receptors. In order to understand the role of the 5-HT1B receptor subtype in behavior and neuropsychiatric disorders, we have been studying genetic knockout mice lacking the 5-HT1B receptor. The 5-HT1B knockout mice show evidence of increased aggression and impulsivity, behavioral patterns that are also associated with reduced 5-HT function. They also show reduced or absent locomotor stimulation to some serotoninergic drugs, indicating that the locomotor effects of these drugs require the 5-HT1B receptor. However, in some cases, data obtained with knockout mice conflicts with the pharmacological data. The 5-HT1B receptor knockout mice show a phenotype of increased vulnerability to drugs of abuse such as cocaine. However, pharmacological studies suggest that 5-HT1B stimulation enhances the effects of cocaine, while 5-HT1B blockade can attenuate some of the effects of cocaine. Compensations that enhance dopamine function appear to be responsible for the drug-vulnerable phenotype of 5-HT1B receptor knockout mice. By studying these compensations and changes in neural function, we can learn more about the fundamental mechanisms underlying addiction. The 5-HT1B knockout mice should be considered a model for the disease state of vulnerability to drugs of abuse, rather than a direct pharmacological model of 5-HT1B receptor function.

Locomotor response to MDMA is attenuated in knockout mice lacking the 5-HT1B receptor

3,4-Methylenedioxymethamphetamine (MDMA) is a psychoactive drug of abuse which is increasingly popular in human recreational drug use. In rats, the drug has been shown to stimulate locomotion while decreasing exploratory behavior. MDMA acts as an indirect agonist of serotonin (5-HT) receptors by inducing 5-HT release by a 5-HT reuptake transporter-dependent mechanism, although it is not known which 5-HT receptors are important for the behavioral effects of the drug. In order to examine the role of specific 5-HT receptors, we assessed the behavioral effects of MDMA on knockout mice lacking the 5-HT1B receptor. Knockout animals show a reduced locomotor response to MDMA, although delayed locomotor stimulation is present in these animals. This finding indicates that the locomotor effects of MDMA are dependent upon the 5-HT1B receptor, at least in part. In contrast, MDMA eliminates exploratory behavior in both normal and knockout mice, suggesting that the exploratory suppression induced by MDMA occurs through mechanisms other than activation of the 5-HT1B receptor. To confirm these findings, we tested the effects of MDMA on the locomotor and exploratory behavior of wild-type mice pretreated with GR 127935, a 5-HT1B/1D receptor antagonist. These mice had an attenuated locomotor response to MDMA, but still exhibited the drug-induced suppression of exploration.

5-HT1B receptor modulation of prepulse inhibition: recent findings in wild-type and 5-HT1B knockout mice

Sensorimotor gating of the startle reflex occurs when the presentation of a weak "prepulse" 30-500 msec prior to a startling stimulus inhibits the reflex, and is called prepulse inhibition (PPI). The study of PPI has recently been extended to mice to take advantage of recent advances in molecular genetics, because several neuropsychiatric disorders including schizophrenia, obsessive compulsive disorder, and schizotypal personality disorder are characterized by PPI deficits. Studies in wild-type and 5-HT1B knockout mice suggest that activation of 5-HT1B receptors decreases PPI. The direct 5-HT1A/1B agonist RU24969 decreases PPI in wild-type but not 5-HT1B knockout mice. Likewise, the serotonin releasing compounds MDMA(+), MBDB(+/-), and alpha-ethyltryptamine (AET) have no effect on PPI in wild-type mice, but increase PPI in 5-HT1B knockout mice. As the direct 5-HT1A agonist 8-OH-DPAT increases PPI in mice, the unmasking of these effects may also contribute to the PPI-increasing effects of 5-HT releasers in 5-HT1B knockout mice.

Absence of fenfluramine-induced anorexia and reduced c-Fos induction in the hypothalamus and central amygdaloid complex of serotonin 1B receptor knock-out mice

Fenfluramine, a serotonin releaser and uptake inhibitor, has been widely prescribed as an appetite suppressant. Despite its popular clinical use, however, the precise neural pathways and specific 5-HT receptors that account for its anorectic effect have yet to be elucidated. To test the hypothesis that stimulation of 5-HT1B receptors is required for the anorectic effect of fenfluramine, we assessed food intake in wild-type and 5-HT1B knock-out mice. Next, to determine possible brain structures and pathways that may contribute to the 5-HT1B-mediated effects of fenfluramine, we studied by immunohistochemistry the induction of the immediate early gene c-fos. Although the effect of fenfluramine on locomotion was indistinguishable between both wild-type and 5-HT1B knock-out mice, the anorectic effect of the drug was absent in only the knock-out mice. Furthermore, the induction of c-Fos immunoreactivity found in the paraventricular nucleus of the hypothalamus (PVN) of wild-type mice was substantially reduced in the knock-outs. Induction in the central amygdaloid nucleus (CeA) and in the bed nucleus of the stria terminalis (BNST), although robust in wild-type animals, was completely absent in knock-out animals. The mixed 5-HT1A/1B agonist RU24969 was able to mimic both the hypophagia and c-fos induction elicited by fenfluramine in wild-type mice, but not in the 5-HT1B knock-out mice. Our results thus demonstrate that stimulation of 5-HT1B receptors is required for fenfluramine-induced anorexia and suggest a role for the PVN, CeA, and BNST in mediating this effect.

Increased vulnerability to cocaine in mice lacking the serotonin-1B receptor

There is increasing evidence that genetic factors can influence individual differences in vulnerability to drugs of abuse. Serotonin (5-hydroxytryptamine, 5-HT), acting through many receptors can modulate the activity of neural reward pathways and thus the effects of various drugs of abuse. Here we examine the effects of cocaine in mice lacking one of the serotonin-receptor subtypes, the 5-HT1B receptor. We show that mice lacking 5-HT1B display increased locomotor responses to cocaine and that they are more motivated to self-administer cocaine. We propose that even drug-naive 5-HT1B-knockout mice are in a behavioural and biochemical state that resembles that of wild-type mice sensitized to cocaine by repeated exposure to the drug. This altered state might be responsible for their increased vulnerability to cocaine.

Serotonin1B receptor modulation of startle reactivity, habituation, and prepulse inhibition in wild-type and serotonin1B knockout mice

Two operational measures of central information processing mechanisms are habituation and prepulse inhibition (PPI) of the startle response. Both measures can be assessed reliably in humans and other animals, and have been shown to be deficient in patients with schizophrenia. The three present experiments assessed the involvement of the serotonin1B (5-HT1B) receptor in modulating startle reactivity, habituation, and PPI by comparing 5-HT1B receptor gene knockout (5-HT1B knockout) with wild-type 129/Sv mice. In experiment I, female mice received saline, 2.0 mg/kg 5-methoxy-3(1,2,3,6)tetrahydropyridin-4-yl-1H-indole (RU24969), a 5-HT1A/1B agonist, and 1.0 mg/kg 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A agonist. Female mice received saline, 10.0 mg/kg RU24969, and 5.0 mg/kg 8-OH-DPAT in experiment 2, and male mice received saline, 10.0 mg/kg RU24969, and 5.0 mg/kg 8-OH-DPAT in experiment 3. All three studies used identical within-subjects designs. Two phenotypic differences were observed following saline treatment: 5-HT1B knockout mice consistently exhibited a small increase in PPI that achieved significance in experiment 1; and 5-HT1B knockout male mice exhibited robust decreases in startle reactivity. Habituation was disrupted consistently by RU24969 in wild-type but not in 5-HT1B knockout mice, while 8-OH-DPAT had no effect on habituation. Consistent with the phenotypic difference in PPI, the high dose of RU24969 significantly and consistently reduced PPI in wild-type but not in 5-HT1B knockout mice. 8-OH-DPAT increased PPI in both wild-type and 5-HT1B knockout mice in every experiment. These findings suggest that 5-HT1B receptors modulate startle reactivity, habituation, and PPI in mice. Additionally, a potential species difference may exist in the behavioral effects of 5-HT1A receptor activation on PPI.