Pembrolizumab as first-line therapy for patients with PD-L1-positive advanced non-small cell lung cancer: a phase 1 trial

Background

Pembrolizumab improved survival as first- and second-line therapy compared with chemotherapy in patients with highly programmed death ligand 1 (PD-L1) expressing advanced non-small cell lung cancer (NSCLC). We report the long-term safety and clinical activity of pembrolizumab as first-line therapy for patients with advanced NSCLC and the correlation between PD-L1 expression and efficacy.

Patients and methods

In the open-label phase 1b KEYNOTE-001 trial, treatment-naive patients with advanced NSCLC whose tumors expressed PD-L1 (≥1% staining, assessed using a prototype assay) were randomly assigned to intravenous pembrolizumab 2 or 10 mg/kg every 3 (Q3W) or 2 (Q2W) weeks. Response was assessed per central RECIST v1.1 every 9 weeks in all patients who received ≥1 pembrolizumab dose. Using pre-treatment tumor tissue, a clinical assay quantified the percentage of tumor cells expressing PD-L1 as tumor proportion score (TPS).

Results

Between 1 March 2013 and 18 September 2015, 101 patients received pembrolizumab 2 mg/kg Q3W (n = 6), 10 mg/kg Q3W (n = 49), or 10 mg/kg Q2W (n = 46). Of these, 27 (26.7%) had TPS ≥50%, 52 (51.5%) had TPS 1%-49%, and 12 (11.9%) had TPS <1%. The objective response rate (ORR) was 27% (27/101, 95% CI 18-37) and median overall survival was 22.1 months (95% CI 17.1-27.2). In patients with PD-L1 TPS ≥50%, ORR, 12-month PFS, and 12-month OS were higher [14/27 (51.9%; 95% CI 32%-71%), 54%, and 85%, respectively] than the overall population [27/101 (26.7%; 95% CI 18.4%-36.5%), 35%, 71%]. Pembrolizumab was well tolerated, with only 12 (11.9%) patients experiencing grade 3/4 treatment-related adverse events and no treatment-related deaths.

Conclusions

Pembrolizumab provides promising long-term OS benefit with a manageable safety profile for PD-L1-expressing treatment-naive advanced NSCLC, with greatest efficacy observed in patients with TPS ≥50%.

Clinical trial name and number

KEYNOTE-001 (ClinicalTrials.gov, NCT01295827).

Aldolase provides an unusual binding site for thrombospondin-related anonymous protein in the invasion machinery of the malaria parasite

An actomyosin motor located underneath the plasma membrane drives motility and host-cell invasion of apicomplexan parasites such as Plasmodium falciparum and Plasmodium vivax, the causative agents of malaria. Aldolase connects the motor actin filaments to transmembrane adhesive proteins of the thrombospondin-related anonymous protein (TRAP) family and transduces the motor force across the parasite surface. The TRAP-aldolase interaction is a distinctive and critical trait of host hepatocyte invasion by Plasmodium sporozoites, with a likely similar interaction crucial for erythrocyte invasion by merozoites. Here, we describe 2.4-A and 2.7-A structures of P. falciparum aldolase (PfAldo) obtained from crystals grown in the presence of the C-terminal hexapeptide of TRAP from Plasmodium berghei. The indole ring of the critical penultimate Trp-residue of TRAP fits snugly into a newly formed hydrophobic pocket, which is exclusively delimited by hydrophilic residues: two arginines, one glutamate, and one glutamine. Comparison with the unliganded PfAldo structure shows that the two arginines adopt new side-chain rotamers, whereas a 25-residue subdomain, forming a helix-loop-helix unit, shifts upon binding the TRAP-tail. The structural data are in agreement with decreased TRAP binding after mutagenesis of PfAldo residues in and near the induced TRAP-binding pocket. Remarkably, the TRAP- and actin-binding sites of PfAldo seem to overlap, suggesting that both the plasticity of the aldolase active-site region and the multimeric nature of the enzyme are crucial for its intriguing nonenzymatic function in the invasion machinery of the malaria parasite.

Crystal structures, metal activation, and DNA-binding properties of two-domain IdeR from Mycobacterium tuberculosis

The iron-dependent regulator IdeR is a key transcriptional regulator of iron uptake in Mycobacterium tuberculosis. In order to increase our insight into the role of the SH3-like third domain of this essential regulator, the metal-binding and DNA-binding properties of two-domain IdeR (2D-IdeR) whose SH3-like domain has been truncated were characterized. The equilibrium dissociation constants for Co2+ and Ni2+ activation of 2D-IdeR for binding to the fxbA operator and the DNA-binding affinities of 2D-IdeR in the presence of excess metal ions were estimated using fluorescence spectroscopy. 2D-IdeR binds to fxbA operator DNA with similar affinity as full-length IdeR in the presence of excess metal ion. However, the Ni2+ concentrations required to activate 2D-IdeR for DNA binding appear to be smaller than that for full-length IdeR while the concentration of Co2+ required for activation remains the same. We have determined the crystal structures of Ni2+-activated 2D-IdeR at 1.96 A resolution and its double dimer complex with the mbtA-mbtB operator DNA in two crystal forms at 2.4 A and 2.6 A, the highest resolutions for DNA complexes for any structures of iron-dependent regulator family members so far. The 2D-IdeR-DNA complex structures confirm the specificity of Ser37 and Pro39 for thymine bases and suggest preferential contacts of Gln43 to cytosine bases of the DNA. In addition, our 2D-IdeR structures reveal a remarkable property of the TEV cleavage sequence remaining after removal of the C-terminal His6. This C-terminal tail promotes crystal contacts by forming a beta-sheet with the corresponding tail of neighboring subunits in two unrelated structures of 2D-IdeR, one with and one without DNA. The contact-promoting properties of this C-terminal TEV cleavage sequence may be beneficial for crystallizing other proteins.

Structure of the MTIP-MyoA complex, a key component of the malaria parasite invasion motor

The causative agents of malaria have developed a sophisticated machinery for entering multiple cell types in the human and insect hosts. In this machinery, a critical interaction occurs between the unusual myosin motor MyoA and the MyoA-tail Interacting Protein (MTIP). Here we present one crystal structure that shows three different conformations of Plasmodium MTIP, one of these in complex with the MyoA-tail, which reveal major conformational changes in the C-terminal domain of MTIP upon binding the MyoA-tail helix, thereby creating several hydrophobic pockets in MTIP that are the recipients of key hydrophobic side chains of MyoA. Because we also show that the MyoA helix is able to block parasite growth, this provides avenues for designing antimalarials.

Structural biology and structure-based inhibitor design of cholera toxin and heat-labile enterotoxin

Structural biology studies on cholera toxin and the closely related heat-labile enterotoxin from enterotoxigenic Escherichia coli over the past decade have shed light on the mechanism of toxin action at molecular and atomic levels. Also, components of the extracellular protein secretion apparatus that translocate the toxins across the outer membrane are being investigated. At the same time, structure-based design has led to various classes of compounds targeting different toxin sites, including highly potent multivalent inhibitors that block the toxin receptor-binding process.

Analysis of the sequence motifs responsible for the interactions of peroxins 14 and 5, which are involved in glycosome biogenesis in Trypanosoma brucei

Glycosome biogenesis in trypanosomatids occurs via a process that is homologous to peroxisome biogenesis in other eukaryotes. Glycosomal matrix proteins are synthesized in the cytosol and imported posttranslationally. The import process involves a series of protein-protein interactions starting by recognition of glycosomal matrix proteins by a receptor in the cytosol. Most proteins to be imported contain so-called PTS-1 or PTS-2 targeting sequences recognized by, respectively, the receptor proteins PEX5 and PEX7. PEX14, a protein associated with the peroxisomal membrane, has been identified as a component of the docking complex and a point of convergence of the PEX5- and PEX7-dependent import pathways. In this paper, the strength of the interactions between Trypanosoma brucei PEX14 and PEX5 was studied by a fluorescence assay, using (i) a panel of N-terminal regions of TbPEX14 protein variants and (ii) a series of different peptides derived from TbPEX5, each containing one of the three WXXXF/Y motifs present in this receptor protein. On the PEX14 side, the N-terminal region of TbPEX14 including residues 1-84 appeared to be responsible for TbPEX5 binding. The results from PEX14 mutants identified specific residues in the N-terminal region of TbPEX14 involved in PEX5 binding and showed that in particular hydrophobic residues F35 and F52 are critical. On the PEX5 side, 13-mer peptides incorporating the first or the third WXXXF/Y motif bind to PEX14 with an affinity in the nanomolar range. However, the second WXXXF/Y motif peptide did not show any detectable affinity. Studies using variants of second and third motif peptides suggest that the alpha-helical content of the peptides as well as the charge of a residue at position 9 in the motif may be important for PEX14 binding. Assays with 7-, 10-, 13-, and 16-mer third motif peptides showed that 16-mers and 13-mers have comparable binding affinity for PEX14, whereas 10-mers and 7-mers have about 10- and 100-fold lower affinity than the 16-mers, respectively. The low sequence identities of PEX14 and PEX5 between parasite and its human host, and the vital importance of proper glycosome biogenesis to the parasite, render these peroxins highly promising drug targets.

Solution and crystallographic studies of branched multivalent ligands that inhibit the receptor-binding of cholera toxin

The structure-based design of multivalent ligands offers an attractive strategy toward high affinity protein inhibitors. The spatial arrangement of the receptor-binding sites of cholera toxin, the causative agent of the severe diarrheal disease cholera and a member of the AB(5) bacterial toxin family, provides the opportunity of designing branched multivalent ligands with 5-fold symmetry. Our modular synthesis enabled the construction of a family of complex ligands with five flexible arms each ending with a bivalent ligand. The largest of these ligands has a molecular weight of 10.6 kDa. These ligands are capable of simultaneously binding to two toxin B pentamer molecules with high affinity, thus blocking the receptor-binding process of cholera toxin. A more than million-fold improvement over the monovalent ligand in inhibitory power was achieved with the best branched decavalent ligand. This is better than the improvement observed earlier for the corresponding nonbranched pentavalent ligand. Dynamic light scattering studies demonstrate the formation of concentration-dependent unique 1:1 and 1:2 ligand/toxin complexes in solution with no sign of nonspecific aggregation. This is in complete agreement with a crystal structure of the branched multivalent ligand/toxin B pentamer complex solved at 1.45 A resolution that shows the specific 1:2 ligand/toxin complex formation in the solid state. These results reiterate the power of the structure-based design of multivalent protein ligands as a general strategy for achieving high affinity and potent inhibition.

An unexpected extended conformation for the third TPR motif of the peroxin PEX5 from Trypanosoma brucei

A number of helix-rich protein motifs are involved in a variety of critical protein-protein interactions in living cells. One of these is the tetratrico peptide repeat (TPR) motif that is involved, amongst others, in cell cycle regulation, chaperone function and post-translation modifications. So far, these helix-rich TPR motifs have always been observed to be a compact unit of two helices interacting with each other in antiparallel fashion. Here, we describe the structure of the first three TPR-motifs of the peroxin PEX5 from Trypanosoma brucei, the causative agent of sleeping sickness. Peroxins are proteins involved in peroxisome, glycosome and glyoxysome biogenesis. PEX5 is the receptor of the proteins targeted to these organelles by the "peroxisomal targeting signal-1", a C-terminal tripeptide called PTS-1. The first two of the three TPR-motifs of T. brucei PEX5 appear to adopt the canonical antiparallel helix hairpin structure. In contrast, the third TPR motif of PEX5 has a dramatically different conformation in our crystals: the two helices that were supposed to form a hairpin are folded into one single 44 A long continuous helix. Such a conformation has never been observed before for a TPR motif. This raises interesting questions including the potential functional importance of a "jack-knife" conformational change in TPR motifs.

Exploration of the GM1 receptor-binding site of heat-labile enterotoxin and cholera toxin by phenyl-ring-containing galactose derivatives

Cholera toxin (CT) and the closely related heat-labile enterotoxin of Escherichia coli (LT) are responsible for numerous cases of diarrhea worldwide, leading to considerable morbidity and mortality. The B subunits of these heterohexameric AB(5) toxins form a pentameric arrangement which is responsible for binding to the receptor GM1 of the target epithelial cells of the host. Blocking these B pentamer-receptor interactions forms an avenue for therapeutic intervention. Here, the structural characterization of potential receptor-blocking compounds are described based on the previously identified inhibitor m-nitrophenyl-alpha-D-galactoside (MNPG). The structure of a CTB-MNPG complex confirms that the binding mode of this inhibitor is identical in the two homologous toxins CT and LT and is characterized by a glycosyl linkage geometry that leads to displacement of a well ordered water molecule near the amide group of Gly33 by the O1-substituent of MNPG. This glycosyl geometry is not maintained in the absence of a substituent that can displace this water, as shown by a complex of LTB with p-aminophenyl-alpha-D-galactoside (PAPG). New compounds were synthesized to investigate the feasibility of maintaining the favorable binding interactions exhibited by MNPG while gaining increased affinity through the addition of hydrophobic substituents complementary to either of two hydrophobic regions of the receptor-binding site. The structural characterization of complexes of LTB with two of these compounds, 3-benzylaminocarbonylphenyl-alpha-D-galactoside (BAPG) and 2-phenethyl-7-(2,3-dihydrophthalazine-1,4-dione)-alpha-D-galactoside (PEPG), demonstrates a partial success in this goal. Both compounds exhibit a mixture of binding modes, some of which are presumably influenced by the local packing environment at multiple crystallographically independent binding sites. The terminal phenyl ring of BAPG associates either with the phenyl group of Tyr12 or with the hydrophobic patch formed by Lys34 and Ile58. The latter interaction is also made by the terminal phenyl substituent of PEPG, despite a larger ring system linking the galactose moiety to the terminal phenyl. However, neither BAPG nor PEPG displaces the intended target water molecule. Both of the designed compounds exhibit increased affinity relative to the galactose and to PAPG notwithstanding the failure to displace a bound water, confirming that additional favorable hydrophobic interactions can be gained by extending the starting inhibitor by a hydrophobic tail. The insight gained from these structures should allow the design of additional candidate inhibitors that retain both the glycosyl geometry and water displacement exhibited by MNPG and the favorable hydrophobic interactions exhibited by BAPG and PEPG.

Lovastatin biosynthesis in Aspergillus terreus: characterization of blocked mutants, enzyme activities and a multifunctional polyketide synthase gene

BACKGROUND

Lovastatin, an HMG-CoA reductase inhibitor produced by the fungus Aspergillus terreus, is composed of two polyketide chains. One is a nonaketide that undergoes cyclization to a hexahydronaphthalene ring system and the other is a simple diketide, 2-methylbutyrate. Fungal polyketide synthase (PKS) systems are of great interest and their genetic manipulation should lead to novel compounds.

RESULTS

An A. terreus mutant (BX102) was isolated that could not synthesize the nonaketide portion of lovastatin and was missing a approximately 250 kDa polypeptide normally present under conditions of lovastatin production. Other mutants produced lovastatin intermediates without the methylbutyryl sidechain and were missing a polypeptide of approximately 220 kDa. The PKS inhibitor cerulenin reacted covalently with both polypeptides. Antiserum raised against the approximately 250 kDa polypeptide was used to isolate the corresponding gene, which complemented the BX102 mutation. The gene encodes a polypeptide of 269 kDa containing catalytic domains typical of vertebrate fatty acid and fungal PKSs, plus two additional domains not previously seen in PKSs: a centrally located methyltransferase domain and a peptide synthetase elongation domain at the carboxyl terminus.

CONCLUSIONS

The results show that the nonaketide and diketide portions of lovastatin are synthesized by separate large multifunctional PKSs. Elucidation of the primary structure of the PKS that forms the lovastatin nonaketide, as well as characterization of blocked mutants, provides new details of lovastatin biosynthesis.

Structure-based exploration of the ganglioside GM1 binding sites of Escherichia coli heat-labile enterotoxin and cholera toxin for the discovery of receptor antagonists

Ganglioside GM1 is the natural receptor for cholera toxin (CT) and heat-labile enterotoxin (LT), which are the causative agents of cholera and traveler's diarrhea, respectively. This observation suggests that small molecules interfering with this recognition process may prevent entry of the toxins into intestinal cells, thereby averting their devastating effects. Here, the terminal sugar of ganglioside GM1, galactose, was chosen as a lead in designing such receptor antagonists. Guided by the experimentally determined binding mode of galactose, we selected a "substructure" for searching the Available Chemicals Database, which led to the purchase of 35 galactose derivatives. Initial screening of these compounds in an LT ELISA revealed that 22 of them have a higher affinity for LT than galactose itself. A structurally diverse subset of these galactose derivatives was selected for determination of IC50 values in the LT ELISA and IC50 values in a CT assay, as well as for the determination of Kd's using the intrinsic fluorescence of LT. The best receptor antagonist found in this study was m-nitrophenyl alpha-galactoside with an IC50 of 0.6 (2) mM in the LT ELISA and 0.72 (4) mM in the CT assay, 100-fold lower than both IC50 values of galactose. Careful analysis of our binding data and comparison with crystal structures led to the derivation of correlations between the structure and affinity of the galactose derivatives. These characteristics will be used in the design of a second round of LT and CT receptor antagonists.

Crystal structure of heat-labile enterotoxin from Escherichia coli with increased thermostability introduced by an engineered disulfide bond in the A subunit

Cholera toxin (CT) produced by Vibrio cholerae and heat-labile enterotoxin (LT-I), produced by enterotoxigenic Escherichia coli, are AB5 heterohexamers with an ADP-ribosylating A subunit and a GM1 receptor binding B pentamer. These toxins are among the most potent mucosal adjuvants known and, hence, are of interest both for the development of anti-diarrheal vaccines against cholera or enterotoxigenic Escherichia coli diarrhea and also for vaccines in general. However, the A subunits of CT and LT-I are known to be relatively temperature sensitive. To improve the thermostability of LT-I an additional disulfide bond was introduced in the A1 subunit by means of the double mutation N40C and G166C. The crystal structure of this double mutant of LT-I has been determined to 2.0 A resolution. The protein structure of the N40C/G166C double mutant is very similar to the native structure except for a few local shifts near the new disulfide bond. The introduction of this additional disulfide bond increases the thermal stability of the A subunit of LT-I by 6 degrees C. The enhancement in thermostability could make this disulfide bond variant of LT-I of considerable interest for the design of enterotoxin-based vaccines.

Mutants of Streptomyces cattleya defective in the synthesis of a factor required for thienamycin production

Thienamycin non-producing mutants of Streptomydes cattleya were identified that displayed a cross-feeding relationship. A diffusible product from one of these mutants (RK-11) resulted in restoration of thienamycin production when fed to cultures of another mutant (RK-4). In vivo radiolabeling experiments were conducted to test whether the RK-11 mutant produced a late biosynthetic intermediate which contained a carbapenem ring and a cysteaminyl and/or a hydroxyethyl side chain. Both [35S]cystine and [methyl-3H]methionine were used to label the RK-11 product which was then fed to RK-4 cultures. None of the thienamycin subsequently produced by RK-4 converter cells was labeled, implying the lack of either side chain of the thienamycin molecule in the RK-11 product. Further stability studies suggested that the RK-11 product does not contain a carbapenem ring. Additional feeding experiments with RK-4 cells also ruled out the possibility that the RK-11 product is a co-factor necessary for thienamycin production. It is concluded that the RK-11 product may regulate expression of the thienamycin gene cluster.

Management of dizziness in primary care

BACKGROUND

We sought to determine the types of dizziness problems that are commonly seen in primary care practices, and to bring to light clinical and demographic factors that predict management decisions.

METHODS

We undertook a prospective cohort study with a 6-month follow-up using data gathered in nine primary care practices in two North Carolina counties. Subjects were 144 dizziness patients examined by primary care physicians. Data collected included demographic characteristics, a standardized dizziness history, physician estimation of symptom severity and diagnostic certainty, and physician "worry" about arrhythmia, transient ischemic attack, and brain tumor. Physicians reported their management decisions and diagnosis (or differential diagnosis) by responding to a questionnaire after completing the patient encounter. A 6-month follow-up chart review and physician interview were completed on 140 patients (97.2 percent); information obtained included changes in diagnosis and patient mortality.

RESULTS

The most common diagnoses were labyrinthitis, otitis media, benign positional vertigo, unspecified presyncope, sinusitis, and transient ischemic attack. The initial diagnosis changed during the 6-month follow-up period in 34 (24.3 percent) of patients. The overall course of these patients was benign, however, with only one death occurring during the 6-month follow-up period. Patients' dizziness tended to be managed using a combination of strategies, including office laboratory testing (33.6 percent), advanced testing (11.4 percent), referral to a specialist (9.3 percent), medication (61.3 percent), observation (71.8 percent), reassurance (41.6 percent), and behavioral recommendations (15.0 percent). Office laboratory testing was associated with younger patient age, a suspected metabolic or endocrine disorder, and physician worry about a cardiac arrhythmia; advanced laboratory testing was associated with suspected cardiovascular or neurologic disorders. Medication tended to be prescribed for vertigo and severe symptoms and avoided when physicians were worried about a cardiac arrhythmia. Referral to a specialist was associated with suspected neurologic disease. Observation, behavior change, and reassurance were avoided in patients with poorly defined dizziness and tended to be used in older patients. The management approaches employed by the 4 physicians who referred the most subjects to the study varied considerably.

CONCLUSIONS

Dizziness in primary care represents an extremely broad spectrum of diagnoses. The generally conservative management approach of primary care physicians in this study is consistent with basic clinical and epidemiologic principles, and patient mortality with this approach is low.

Diazepam loading: simplified treatment of alcohol withdrawal

Alcohol withdrawal therapy can be simplified with a loading dose of diazepam, taking advantage of the kinetic tapering afforded by the drug's long t 1/2s and its metabolites, and of the effectiveness of nonpharmacologic maneuvers. In a double-blind trial, 50 inpatients in moderate to severe alcohol withdrawal received 20 mg oral diazepam and supportive care (n = 25) or placebo and supportive care (n = 25) every 2 hr until they were asymptomatic. Fifty-six percent of patients responded to placebo within 5 +/- 2.9 hr (mean +/- SD), whereas 72% responded to initial diazepam within 6.3 +/- 3.9 hr. Patients treated with diazepam had more rapid and greater improvement than those treated with placebo. Patients who did not respond to six doses of diazepam received further (unblinded) diazepam, 20 mg, every 1 to 2 hr. All patients who did not initially respond (n = 18) improved after more diazepam. Thus all patients who received diazepam (n = 36), during the experimental phase or subsequently, were effectively treated. There were no adverse effects. The median number of 20-mg diazepam doses to treat alcohol withdrawal were three, given over a period of 7.6 hr (range = 1 to 12 and 0.33 to 45 hr). Complications occurred only in those who received placebo during the experimental phase, indicating that delay in therapy may be responsible for the appearance of complications in alcohol withdrawal.

Nonpharmacologic intervention in acute alcohol withdrawal

The importance of nonpharmacologic and pharmacologic interventions in the treatment of alcohol withdrawal is not known. A randomized, double-blind, placebo-controlled trial was conducted with 41 patients in alcohol withdrawal in an emergency department. The patients received either supportive care (10 min of standardized assessments, reassurance, reality orientation, and nursing care an hour) with three doses of sublingual lorazepam 2 mg every 2 hr (21 patients, drug group) or supportive care with three doses of sublingual placebo every 2 hr (20 patients, no-drug group). Immediately before each drug dose, the clinical course of alcohol withdrawal was assessed hourly by the Clinical Institute Withdrawal Assessment for Alcohol (CIWA-A). Interraters reliability in using CIWA-A was high. After each assessment, supportive care was given for 10 min before each dose. After completion of a 7-hr initial phase, patients were discharged and reassessed daily for 5 days. Thirty-seven patients (90.2%) improved in the initial phase. Treatment failures (CIWA-A greater than 10) were more common in the patients treated without drug (3/20, 15%) than in those treated with drug (1/21, 4.8%). Overall variations in intergroup CIWA-A scores during the initial phase were not significant. The rate of improvement of CIWA-A scores over the first 2 hr after drug was slightly faster in patients receiving lorazepam than in the control group. CIWA-A scores were the same during follow-up. These results indicate that most outpatients in mild to moderate alcohol withdrawal without medical complications improve without drug therapy in the emergency department setting.

Pavlovian control of cross-tolerance between pentobarbital and ethanol

Tolerance to several effects of a number of drugs has been shown to depend on Pavlovian conditioning processes. Experiment I extended the compensatory conditioning model (Siegel 1975) to tolerance to the hypothermic effect of pentobarbital (30 mg/kg). In Experiment I, rats that acquired hypothermic tolerance in one environment did not display tolerance when tested in an environment not previously associated with drug administration. In Experiment II, rats were made tolerant to the hypothermic effect of pentobarbital (30 mg/kg) and tested for cross-tolerance to ethanol (2.5 g/kg). Cross-tolerance was observed, but it was significantly reduced if the test was in an environment different from the one in which tolerance to pentobarbital was originally acquired. Thus, the compensatory conditioning model accounts for at least part of the tolerance and cross-tolerance to the thermic effects of alcohol and pentobarbital. The physiological processes in the CNS underlying tolerance and cross-tolerance for these drugs, therefore, are controlled by associative processes.