SAINT: A Phase I/Expanded Phase II Study Using Safe Amounts of Ipilimumab, Nivolumab and Trabectedin as First-Line Treatment of Advanced Soft Tissue Sarcoma

BACKGROUND

This Phase 1/2 study is based on the hypothesis that immune checkpoint inhibitors are more effective when given earlier in the course of the disease for advanced soft tissue sarcoma.

METHODS

Phase I endpoints-maximum tolerated dose in previously treated patients; Phase II endpoints-best response, progression free survival and overall survival and incidence of adverse events in previously untreated patients; Phase I treatments-escalating doses of trabectedin (1.0, 1.2, 1.5 mg/m²) as continuous intravenous infusion over 24 h every 3 weeks, 1 mg/kg of ipilimumab given intravenously every 12 weeks, and 3 mg/kg of nivolumab given intravenously every 2 weeks; Phase II treatments-maximum tolerated dose of trabectedin and defined doses of ipilimumab and nivolumab.

RESULTS

Phase I (n = 9)-the maximum tolerated dose of trabectedin was 1.2 mg/m²; Phase II (n = 79)-6 complete responses, 14 partial responses, 49 stable disease, 25.3% best response rate, 87.3% disease control rate; median progression-free survival, 6.7 months (CI 95%: 4.4-7.9), median overall survival, 24.6 months (CI 95%: 17.0-.); Grade 3/4 therapy-related adverse events (n = 92)-increased ALT (25%), fatigue (8.7%), increased AST (8.7%), decreased neutrophil count (5.4%) and anemia (4.6%).

CONCLUSION

SAINT is a safe and effective first-line treatment for advanced soft tissue sarcoma.

IgE-neutralizing UB-221 mAb, distinct from omalizumab and ligelizumab, exhibits CD23-mediated IgE downregulation and relieves urticaria symptoms

Over the last 2 decades, omalizumab is the only anti-IgE antibody that has been approved for asthma and chronic spontaneous urticaria (CSU). Ligelizumab, a higher-affinity anti-IgE mAb and the only rival viable candidate in late-stage clinical trials, showed anti-CSU efficacy superior to that of omalizumab in phase IIb but not in phase III. This report features the antigenic-functional characteristics of UB-221, an anti-IgE mAb of a newer class that is distinct from omalizumab and ligelizumab. UB-221, in free form, bound abundantly to CD23-occupied IgE and, in oligomeric mAb-IgE complex forms, freely engaged CD23, while ligelizumab reacted limitedly and omalizumab stayed inert toward CD23; these observations are consistent with UB-221 outperforming ligelizumab and omalizumab in CD23-mediated downregulation of IgE production. UB-221 bound IgE with a strong affinity to prevent FcԑRI-mediated basophil activation and degranulation, exhibiting superior IgE-neutralizing activity to that of omalizumab. UB-221 and ligelizumab bound cellular IgE and effectively neutralized IgE in sera of patients with atopic dermatitis with equal strength, while omalizumab lagged behind. A single UB-221 dose administered to cynomolgus macaques and human IgE (ε, κ)-knockin mice could induce rapid, pronounced serum-IgE reduction. A single UB-221 dose administered to patients with CSU in a first-in-human trial exhibited durable disease symptom relief in parallel with a rapid reduction in serum free-IgE level.

ALDH7A1 inhibits the intracellular transport pathways during hypoxia and starvation to promote cellular energy homeostasis

The aldehyde dehydrogenase (ALDH) family of metabolic enzymes converts aldehydes to carboxylates. Here, we find that the reductive consequence of ALDH7A1 activity, which generates NADH (nicotinamide adenine dinucleotide, reduced form) from NAD, underlies how ALDH7A1 coordinates a broad inhibition of the intracellular transport pathways. Studying vesicle formation by the Coat Protein I (COPI) complex, we elucidate that NADH generated by ALDH7A1 targets Brefeldin-A ADP-Ribosylated Substrate (BARS) to inhibit COPI vesicle fission. Moreover, defining a physiologic role for the broad transport inhibition exerted by ALDH7A1, we find that it acts to reduce energy consumption during hypoxia and starvation to promote cellular energy homeostasis. These findings advance the understanding of intracellular transport by revealing how the coordination of multiple pathways can be achieved, and also defining circumstances when such coordination is needed, as well as uncovering an unexpected way that NADH acts in cellular energetics.

Effect of Anti-CD4 Antibody UB-421 on HIV-1 Rebound after Treatment Interruption

BACKGROUND

Administration of a single broadly neutralizing human immunodeficiency virus (HIV)-specific antibody to HIV-infected persons leads to the development of antibody-resistant virus in the absence of antiretroviral therapy (ART). It is possible that monotherapy with UB-421, an antibody that blocks the virus-binding site on human CD4+ T cells, could induce sustained virologic suppression without induction of resistance in HIV-infected persons after analytic treatment interruption.

METHODS

We conducted a nonrandomized, open-label, phase 2 clinical study evaluating the safety, pharmacokinetics, and antiviral activity of UB-421 monotherapy in HIV-infected persons undergoing analytic treatment interruption. All the participants had undetectable plasma viremia (<20 copies of HIV RNA per milliliter) at the screening visit. After discontinuation of ART, participants received eight intravenous infusions of UB-421, at a dose of either 10 mg per kilogram of body weight every week (Cohort 1) or 25 mg per kilogram every 2 weeks (Cohort 2). The primary outcome was the time to viral rebound (≥400 copies per milliliter).

RESULTS

A total of 29 participants were enrolled, 14 in Cohort 1 and 15 in Cohort 2. Administration of UB-421 maintained virologic suppression (<20 copies per milliliter) in all the participants (94.5% of measurements at study visits 2 through 9) during analytic treatment interruption, with intermittent viral blips (range, 21 to 142 copies per milliliter) observed in 8 participants (28%). No study participants had plasma viral rebound to more than 400 copies per milliliter. CD4+ T-cell counts remained stable throughout the duration of the study. Rash, mostly of grade 1, was a common and transient adverse event; one participant discontinued the study drug owing to a rash. A decrease in the population of CD4+ regulatory T cells was observed during UB-421 monotherapy.

CONCLUSIONS

UB-421 maintained virologic suppression (during the 8 to 16 weeks of study) in participants in the absence of ART. One participant discontinued therapy owing to a rash. (Funded by United Biomedical and others; ClinicalTrials.gov number, NCT02369146.).

Chemical interactions between Saturn’s atmosphere and its rings

The Pioneer and Voyager spacecraft made close-up measurements of Saturn’s ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn’s atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft’s Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.

Recent advances in the management of liposarcoma

Liposarcoma is the most common soft tissue sarcoma. With its various subtypes, the natural history of this disease can vary significantly from a locally recurrent tumor to a highly malignant one carrying a poor prognosis. Progress in the understanding of the specific molecular abnormalities in liposarcoma provides greater opportunity for new treatment modalities. Although surgical resection and radiation therapy remain the keystones for the management of primary liposarcoma, the inclusion of novel agents that target known abnormalities in advanced liposarcoma enhances the potential for improved outcomes.

Accuracy of preoperative percutaneous biopsy for the diagnosis of retroperitoneal liposarcoma subtypes

BACKGROUND

Surgery is the primary treatment for all subtypes of retroperitoneal liposarcoma, but neoadjuvant therapy may be warranted in cases of dedifferentiated liposarcoma (DDLS), which has an increased risk of recurrence and metastasis. Therefore, an accurate subtype-specific diagnosis is vital for appropriate consideration of neoadjuvant therapy. Previous studies assessing the subtype-specific accuracy of percutaneous biopsy are limited. We aimed to analyze the accuracy of preoperative percutaneous biopsy in the subtype-specific diagnosis of retroperitoneal liposarcoma and thus the reliability of percutaneous biopsy in guiding decisions about neoadjuvant treatment.

METHODS

We retrospectively reviewed the medical records, including the pathologic reports, interventional radiology reports, and operative reports, of patients registered in the retroperitoneal/well-differentiated liposarcoma (WDLS/DDLS) database at The University of Texas MD Anderson Cancer Center between 1993 and 2013.

RESULTS

We identified 120 patients who underwent 137 preoperative percutaneous biopsies followed by surgical resections. Pathologic examination following resection indicated that 74 of the patients had WDLS and 63 had DDLS. The overall diagnostic accuracy of percutaneous biopsy for identifying the subtype of liposarcoma was 62.8 % (86/137); the accuracy for identifying WDLS was significantly higher (85.1 %; 63/74) than that for identifying DDLS (36.5 %; 23/63) (p < 0.01).

CONCLUSIONS

Percutaneous biopsy has low accuracy in the diagnosis of retroperitoneal DDLS. This can potentially mislead physicians in the decision to implement neoadjuvant treatment. When developing treatment strategies, including clinical trials for patients with retroperitoneal liposarcoma, physicians should carefully consider the low accuracy of percutaneous biopsy in detecting DDLS.

Altered expression of protein tyrosine phosphatase, non-receptor type 22 isoforms in systemic lupus erythematosus

Introduction

A C-to-T single nucleotide polymorphism (SNP) located at position 1858 of human protein tyrosine phosphatase, non-receptor type 22 (PTPN22) complementary DNA (cDNA) is associated with an increased risk of systemic lupus erythematosus (SLE). How the overall activity of PTPN22 is regulated and how the expression of PTPN22 differs between healthy individuals and patients with lupus are poorly understood. Our objectives were to identify novel alternatively spliced forms of PTPN22 and to examine the expression of PTPN22 isoforms in healthy donors and patients with lupus.

Methods

Various human PTPN22 isoforms were identified from the GenBank database or amplified directly from human T cells. The expression of these isoforms in primary T cells and macrophages was examined with real-time polymerase chain reaction. The function of the isoforms was determined with luciferase assays. Blood samples were collected from 49 subjects with SLE and 15 healthy controls. Correlation between the level of PTPN22 isoforms in peripheral blood and clinical features of SLE was examined with statistical analyses.

Results

Human PTPN22 was expressed in several isoforms, which differed in their level of expression and subcellular localization. All isoforms except one were functionally interchangeable in regulating NFAT activity. SLE patients expressed higher levels of PTPN22 than healthy individuals and the levels of PTPN22 were negatively correlated with the Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index (SLICC-DI).

Conclusions

The overall activity of PTPN22 is determined by the functional balance among all isoforms. The levels of PTPN22 isoforms in peripheral blood could represent a useful biomarker of SLE.

PTPN22 modulates macrophage polarization and susceptibility to dextran sulfate sodium-induced colitis

PTPN22, a protein tyrosine phosphatase expressed mainly in hematopoietic cells, has been linked to many autoimmune diseases. A C-to-T single nucleotide polymorphism (SNP) at position 1858 of human PTPN22 cDNA decreases the risk of Crohn's disease. However, the function of PTPN22 and the mechanism by which this SNP reduces the risk of Crohn's disease are poorly understood. We find that PTPN22 is expressed in macrophages. It suppresses M1 macrophage polarization and reciprocally promotes the expression of M2-associated genes. PTPN22-deficient mice develop severe colitis induced by dextran sulfate sodium, and their intestinal macrophages express higher levels of M1 genes but lower levels of M2-associated genes. Furthermore, the protective T allele of the C1858T SNP is associated with attenuated expression of inflammatory cytokines and a higher level of PTPN22 in human M1 macrophages. This T allele-associated aberrant expression of PTPN22 is partly attributed to an autoinhibition mechanism, in which PTPN22 suppresses its own expression in M1 but not M2 macrophages. Our data not only demonstrate a critical role of PTPN22 in regulating macrophage polarization but also provide a molecular explanation for the protective effect of the C1858T SNP in Crohn's disease.

Ets-1 interacts with NRON complex and facilitates the nuclear entry of NFAT (P6335)

NFAT transcription factors are critical for the expression of IL-2 by Th cells. In resting Th cells, NFATs are phosphorylated and trapped in cytoplasm by NRON complex. Calcium dependent signals downstream of TCR lead to dephosphorylation of NFATs and disassembly of NROM complex, releasing NFAT into nucleus to transactivate IL-2. However, how the assembly and disassembly of NRON complex is regulated is not fully understood. Here we report that Ets-1, the prototype of the ETS family of transcription factors was critical for the expression of IL-2 in primary human Th cells. A sizable fraction of nuclear Ets-1 quickly exited nucleus in response to calcium dependent signals and interacted with protein components of NRON complex. Ets-1 deficiency resulted in impaired nuclear entry of NFAT proteins and their recruitment to the IL-2 promoter. To the best our knowledge, Ets-1 is the first transcription factor that is known to facilitate nuclear entry of NFAT proteins by interacting with NRON complex.

Orthotopic mouse model of colorectal cancer

The traditional subcutaneous tumor model is less than ideal for studying colorectal cancer. Orthotopic mouse models of colorectal cancer, which feature cancer cells growing in their natural location, replicate human disease with high fidelity. Two techniques can be used to establish this model. Both techniques are similar and require mouse anesthesia and laparotomy for exposure of the cecum. One technique involves injection of a colorectal cancer cell suspension into the cecal wall. Cancer cells are first grown in culture, harvested when subconfluent and prepared as a single cell suspension. A small volume of cells is injected slowly to avoid leakage. The other technique involves transplantation of a piece of subcutaneous tumor onto the cecum. A mouse with a previously established subcutaneous colorectal tumor is euthanized and the tumor is removed using sterile technique. The tumor piece is divided into small pieces for transplantation to another mouse. Prior to transplantation, the cecal wall is lightly damaged to facilitate tumor cell infiltration. The time to developing primary tumors and liver metastases will vary depending on the technique, cell line, and mouse species used. This orthotopic mouse model is useful for studying the natural progression of colorectal cancer and testing new therapeutic agents against colorectal cancer.

Trace eyeblink conditioning is hippocampally dependent in mice

The goal of this study was to determine whether trace eyeblink conditioning is a hippocampally dependent associative learning task in the mouse. First, we examined trace intervals of 0, 250, and 500 ms to determine a relatively long trace interval that would support eyeblink conditioning in young adult C57BL/6 mice. Mice rapidly acquired conditioned responses (CRs) with a 0-ms trace interval, acquired CRs with a 250-ms trace interval in approximately 2 days (2 sessions per day), and showed little acquisition with a 500-ms trace interval. Control mice were presented randomly unpaired stimuli and failed to show conditioning. We then determined the effect of lesioning dorsal hippocampal neurons on trace eyeblink conditioning. The hippocampus was injected bilaterally with vehicle (phosphate-buffered saline), 0.1% ibotenic acid, or 1% ibotenic acid. The vehicle group showed >60% CRs. The 0.1% group showed significantly fewer CRs (35-45%). The 1% group showed a level of CRs similar to that of the control mice. All the lesioned mice exhibited >60% CRs when subsequently trained with a 0-ms trace interval. A regression analysis indicated that the volume of area CA1 lesioned was more predictive of the behavioral impairment than the lesion volume of either CA3 or dentate gyrus, or even the total lesion volume. We conclude that dorsal hippocampal neurons play a critical role in eyeblink conditioning when a 250-ms trace interval is used with the C57BL/6 mouse, and that this paradigm will be useful for studying behavior and the in vivo and in vitro electrophysiology of hippocampal neurons in normal and transgenic or knockout mice.

Neurotoxic lesions of the dorsal hippocampus disrupt auditory-cued trace heart rate (fear) conditioning in rabbits

The first experiment in this study used the classical heart rate (HR) conditioning paradigm to determine if rabbits could associate an auditory conditioned stimulus (CS) and a fear-producing shock-unconditioned stimulus (US) separated by an empty 10-s trace interval. Trace conditioned rabbits (n = 7) acquired significant bradycardiac conditioned HR responses on CS-alone test trials during a single 35-trial conditioning session. Control animals (n = 7) which received unpaired CSs and USs did not show HR conditioning. During a retention session of CS-alone trials 24 h after the conditioning session, some trace-conditioned animals showed conditioned HR responses immediately following CS onset (n = 3), while others showed responses appropriately timed to the US onset (n = 4) used in trace conditioning 24 h earlier. Thus, rabbits remember the duration of the long 10-s trace interval 24 h after a single day of training. The second part of this study sought to determine if cells in the dorsal hippocampus play a role in trace HR conditioning. Rabbits were given bilateral ibotenic acid lesions in the neocortex (n = 7) or dorsal hippocampus (n = 8). During trace conditioning and retention, neocortical animals showed conditioned HR responses to the CS, whereas the hippocampal group showed no significant HR conditioning. One week after trace conditioning, the same animals received a delay HR conditioning session where no trace interval separated the CS and US. During delay conditioning, hippocampal animals showed significant conditioned HR responses to the CS that were similar to the neocortical group. Thus, the dorsal hippocampus plays a critical role in rabbit HR conditioning when the CS and US are separated by a 10-s trace interval. This paradigm may be ideal for in vivo electrophysiological recording studies because rabbits are easily immobilized during the testing procedure, and learning occurs during a single day of training.

Human carbonyl reductase overexpression in the heart advances the development of doxorubicin-induced cardiotoxicity in transgenic mice

Doxorubicinol (dxol) is the major metabolite formed in the hearts of cancer patients being treated with the widely used chemotherapeutic agent, doxorubicin (dox). The well-documented cardiomyopathy associated with dox treatment has been studied in vitro and ex vivo providing evidence that the C-13 hydroxy metabolite, dxol, might play a key role in the development of dox-induced cardiotoxicity. In this report, we have developed transgenic mice with heart-specific expression of human carbonyl reductase (HCBR), an enzyme that metabolizes dox to dxol. Dox was rapidly converted to dxol in the hearts of the transgenic expressers, which led to advanced development of both acute and chronic cardiotoxicity. Acute cardiotoxicity was evident by a 60% increase in serum creatine kinase activity and a 5-fold increase in cardiac damage measured by electron microscopy. Myofibril degeneration was the major damage observed in acute dox toxicity. Electrocardiograph telemetry, survival data, and electron microscopy were monitored during chronic dox-induced cardiotoxicity. HCBR expressers developed cardiotoxicity 6-7 weeks before the nonexpressers. The HCBR expressers survived for 5 weeks compared with 12 weeks for the controls. Electrocardiograph profiles and necropsies showed the cause of death to be the development of cardiomyopathies leading to congestive heart failure. Levels of dxol were four times higher in the HCBR expresser hearts than in the nonexpressers. Electron microscopy data showed swelling and major structural damage of the mitochondria in the HCBR expressers. These data demonstrate that the C-13 hydroxy metabolite of dox advances the development of dox-induced cardiotoxicity in an in vivo system and suggest that heart carbonyl reductase activity may contribute to dox-induced cardiotoxicity in humans.

Characteristics of a sporulation stimulating factor from Clostridium perfringens type A

A product in the culture supernatant fluid of Clostridium perfringens NCTC 8239 stimulated the sporulation of a test strain, NCTC 8679, of the same organism. The responsible factor, termed sporulation factor (SF), was present in seven cultures of Cl. perfringens grown in either a defined or complex medium. The SF reversed glucose-mediated catabolite repression of sporulation by this organism. Preliminary characterization of the SF demonstrated a resistance to elevated temperatures and proteases and a molecular weight of less than 500 Da. The known association of Cl. perfringens enterotoxin with sporulation highlights the importance of interactions between strains of this organism as may occur in the human intestine during foodborne illness.

Airway epithelial tight junctions and binding and cytotoxicity of Pseudomonas aeruginosa

The role of tight junctions in the binding and cytoxicity of Pseudomonas aeruginosa to apical or basolateral membranes of lung airway epithelial cells was tested with fluorescence microscopy on living cells. Binding of noncytotoxic P. aeruginosa strain O1 was assessed with P. aeruginosa that expressed green fluorescent protein. Binding of cytotoxic P. aeruginosa strain 6206 was assessed with FITC-labeled P. aeruginosa; cytotoxicity was determined from nuclear uptake of the impermeant dye propidium iodide. The role of direct contact of P. aeruginosa to epithelial cells was tested with filters with small (0.45-micrometer) or large (2.0-micrometer) pores. High transepithelial resistance (R(t)) Calu-3 and cultured bovine tracheal monolayers (R(t) > 1,000 Omega. cm(2)) bound P. aeruginosa very infrequently (<1 P. aeruginosa/100 cells) at the apical membrane, but P. aeruginosa bound frequently to cells near "free edges" at holes, wounds, islands, and perimeters; cytotoxicity required direct interaction with basolateral membranes. Wounded high R(t) epithelia showed increased P. aeruginosa binding and cytotoxicity at the free edges because basolateral membranes were accessible to P. aeruginosa, and dead and living cells near the wound bound P. aeruginosa similarly. Compared with high R(t) epithelia, low R(t) CFT1 (R(t) = 100-200 Omega. cm(2)) and EGTA-treated Calu-3 monolayers were 25 times more susceptible to P. aeruginosa binding throughout the monolayer. Cytotoxicity to CFT1 cells (throughout the confluent monolayer, not only at the free edge) occurred after a shorter delay (0.25 vs. 2.0 h) and then five times faster than to Calu-3 cells, indicating that the time course of P. aeruginosa cytotoxicity may be limited by the rate of gaining access through tight junctions and that this occurred faster in low R(t) than in high R(t) airway epithelia. Cytotoxicity appeared to occur in a sequential process that led first to a loss of fura 2 and a later uptake of propidium iodide. P. aeruginosa bound three times more frequently to regions between cells (tight junctions?) than to cell membranes of low R(t) CFT1 cells.

Hippocampectomy disrupts auditory trace fear conditioning and contextual fear conditioning in the rat

The hippocampus is believed to be an important structure for learning tasks that require temporal processing of information. The trace classical conditioning paradigm requires temporal processing because the conditioned stimulus (CS) and the unconditioned stimulus (US) are temporally separated by an empty trace interval. The present study sought to determine whether the hippocampus was necessary for rats to perform a classical trace fear conditioning task in which each of 10 trials consisted of an auditory tone CS (1 5-s duration) followed by an empty 30-s trace interval and then a fear-producing floor-shock US (0.5-s duration). Several weeks prior to training, animals were anesthetized and given aspiration lesions of the neocortex (NEO; n = 6), hippocampus and overlying neocortex (HIPP; n = 7), or no lesions at all (control; n = 6). Approximately 24 h after trace conditioning, NEO and control animals showed a significant decrease in movement to a CS-alone presentation that was indicative of a conditioned fear response. Animals in the HIPP group did not show conditioned fear responses to the CS alone, nor did a pseudoconditioning group (n = 7) that was trained with unpaired CSs and USs. Furthermore, all groups except the HIPP group showed conditioned fear responses to the original context in which they received shock USs. One week later, HIPP, NEO, and control animals received delay fear-conditioning trials with no trace interval separating the CS and US. Six of seven HIPP animals could perform the delay version, but none could perform the trace version. This result suggests that the trace fear task is a reliable and useful model for examining the neural mechanisms of hippocampally dependent learning.

Hippocampectomy disrupts auditory trace fear conditioning and contextual fear conditioning in the rat

The hippocampus is believed to be an important structure for learning tasks that require temporal processing of information. The trace classical conditioning paradigm requires temporal processing because the conditioned stimulus (CS) and the unconditioned stimulus (US) are temporally separated by an empty trace interval. The present study sought to determine whether the hippocampus was necessary for rats to perform a classical trace fear conditioning task in which each of 10 trials consisted of an auditory tone CS (1 5-s duration) followed by an empty 30-s trace interval and then a fear-producing floor-shock US (0.5-s duration). Several weeks prior to training, animals were anesthetized and given aspiration lesions of the neocortex (NEO; n = 6), hippocampus and overlying neocortex (HIPP; n = 7), or no lesions at all (control; n = 6). Approximately 24 h after trace conditioning, NEO and control animals showed a significant decrease in movement to a CS-alone presentation that was indicative of a conditioned fear response. Animals in the HIPP group did not show conditioned fear responses to the CS alone, nor did a pseudoconditioning group (n = 7) that was trained with unpaired CSs and USs. Furthermore, all groups except the HIPP group showed conditioned fear responses to the original context in which they received shock USs. One week later, HIPP, NEO, and control animals received delay fear-conditioning trials with no trace interval separating the CS and US. Six of seven HIPP animals could perform the delay version, but none could perform the trace version. This result suggests that the trace fear task is a reliable and useful model for examining the neural mechanisms of hippocampally dependent learning.