Assessing in vitro stability of remdesivir (GS-5734) and conversion to GS-441524 in feline plasma and whole blood

Feline infectious peritonitis (FIP) is a potentially fatal coronavirus-driven disease of cats. Treatment with nucleoside analogue GS-441524 and or prodrug remdesivir (RDV) have produced remission in both experimentally induced and naturally occurring FIP, yet information regarding metabolism of RDV into GS-441524 in cats is scarce. This study assessed possible phase I metabolism of RDV in cats, utilising an in vitro feline microsome model with in vitro t1/2 and in vitro Clint calculated using the substrate depletion method. A previously validated high-performance liquid chromatography (HPLC) fluorescence method was utilised for detection and analysis of RDV and GS-441524. Qualitative yield of RDV and intermediate metabolite GS-441524 were determined following microsome incubation, then compared to whole blood and plasma incubations. In vitro microsome incubation resulted in rapid depletion of RDV, though it did not appear to resemble a conventional phase I-dependent reaction in cats, as it is in humans and dogs. Depletion of RDV into GS-441524 was demonstrated in whole blood in vitro, suggesting cats convert RDV to GS-441524, likely via blood esterases, as observed in mice and rats. RDV metabolism is unlikely to be impacted by impaired liver function in cats. Furthermore, as RDV depletes within minutes, whereas GS-441524 is very stable, whole blood or plasma GS-441524 concentrations, rather than plasma RDV concentrations, are more appropriate for therapeutic drug monitoring (TDM) in cats receiving RDV.

Brain magnetic resonance imaging and computed tomography findings in cats and dogs with central nervous system cryptococcosis in Australia: 23 cases (2009-2020)

OBJECTIVE

To describe the imaging findings in Australian cats and dogs with CNS cryptococcosis.

ANIMALS

23 cases (10 cats; 13 dogs) with CNS cryptococcosis and brain MRI or CT studies available to review.

METHODS

Retrospective, multi-institutional case series. Brain MRI or CT studies were reviewed by a board-certified radiologist. Imaging findings were described and the differences between cats and dogs explored.

RESULTS

Morphologic features were consistent with extra-axial lesions in all (n = 13) dogs and either intra-axial (5/10) or extra-axial (4/10) lesions in cats, with 1 cat having no detectable lesions in low-field brain MRI scans. Meningeal abnormalities were most common, followed by forebrain and cerebellar lesions. Intracranial MRI lesions were typically T2 hyperintense and T1 hypo- to isointense. Four cases had T2 hypointense lesions affecting the brain, sinonasal cavity, or regional lymph nodes. Intracranial CT lesions were mostly soft tissue attenuating. Contrast enhancement was present in all cases with contrast series available, with ring enhancement shown only in cats. Osteolysis was more common in dogs than cats, particularly affecting the cribriform plate. All 13 dogs and many (6/10) cats had at least 1 lesion affecting sinonasal or contiguous tissues, and locoregional lymphadenomegaly was common (7/10 cats; 11/13 dogs).

CLINICAL RELEVANCE

Imaging lesions in cryptococcal meningoencephalitis were extra-axial in dogs but could be intra-axial or extra-axial in cats. Careful examination for extracranial lesions (sinonasal, retrobulbar, facial soft tissue, tympanic bullae, or locoregional lymph nodes) is important to provide alternative safe biopsy sites. T2 hypointense lesions, while rare, should prompt consideration of cryptococcosis.

Quantification of GS-441524 concentration in feline plasma using high performance liquid chromatography with fluorescence detection

The adenosine analogue GS-441524 has demonstrated efficacy in treatment of feline infectious peritonitis (FIP). With no commercially registered formulations of GS-441524 available, global focus shifted to its pro-drug remdesivir, as it became more accessible throughout the COVID-19 pandemic. This study developed and validated a simple liquid chromatography equipped with a fluorescence detector to quantify plasma concentrations of GS-441524 applicable for routine therapeutic monitoring of remdesivir or GS-441524 therapy for FIP infected cats. A Waters X-Bridge C18, 5 µm, 150 × 4.6 mm, column was used and mixtures of 20 mM ammonium acetate (pH 4.5) with acetonitrile of 5% and 70% were prepared for gradient mobile phase. With a simple protein precipitation using methanol to clean plasma sample, GS-441524 was monitored at excitation and emission wavelengths of 250 nm and 475 nm, respectively. Using an external standard, the lowest and highest limits of quantification were 19.5 ng/mL to 10,000 ng/mL, respectively. The intra- and inter day trueness of the quality controls (QCs) were within 10% of their nominal concentrations and intra- and inter day precision of the QCs (expressed as the coefficient of variation) ranged from 1.7 to 5.7%, This assay was able to quantify plasma trough levels of GS-441524 (23.7-190.1 ng/mL) after the administration of remdesivir (9.9-15.0 mg/kg BW, IV or SC) in FIP cats (n = 12). Accordingly, this study generated an alternative and cost-effective way to quantify GS-441524 in feline biological fluids at least up to 24 hr after administrations of remdesivir.

Outcomes of treatment of cats with feline infectious peritonitis using parenterally administered remdesivir, with or without transition to orally administered GS-441524

BACKGROUND

Nucleoside analog GS-441524 is effective in treating cats with feline infectious peritonitis (FIP). Investigation into the use of parent nucleotide analog remdesivir (GS-5734) is needed.

OBJECTIVES

To assess efficacy and tolerability of remdesivir with or without transition to GS-441524 in cats with FIP and document clinical and clinicopathologic progression over 6 months.

ANIMALS

Twenty-eight client-owned cats with FIP.

METHODS

Cats were prospectively recruited between May 2021 and May 2022. An induction dosage of remdesivir 10 to 15 mg/kg intravenously or subcutaneously q24h was utilized for 4 doses, with a maintenance dosage of remdesivir (6-15 mg/kg SC) or GS-441524 (10-15 mg/kg per os) every 24 hours continued for at least 84 days. Laboratory testing, veterinary, and owner assessments were recorded.

RESULTS

Twenty-four cats survived to 6 months (86%). Three cats died within 48 hours. Excluding these, survival from 48 hours to 6 months was 96% (24/25). Remission was achieved by day 84 in 56% (14/25). Three cats required secondary treatment for re-emergent FIP. Remission was achieved in all 3 after higher dosing (15-20 mg/kg). Adverse reactions were occasional site discomfort and skin irritation with remdesivir injection. Markers of treatment success included resolution of pyrexia, effusions, and presenting signs of FIP in the first half of treatment and normalization of globulin concentration, and continued body weight gains in the latter half of the treatment period.

CONCLUSIONS AND CLINICAL IMPORTANCE

Parenteral administration of remdesivir and oral administration of GS-441524 are effective and well-tolerated treatments for FIP. Early emphasis on clinical, and later emphasis on clinicopathologic response, appears prudent when monitoring treatment efficacy.

Clinical features, outcomes, and long-term survival times of cats and dogs with central nervous system cryptococcosis in Australia: 50 cases (2000-2020)

OBJECTIVE

To describe the clinical findings and outcomes of Australian cats and dogs with CNS cryptococcosis.

ANIMALS

19 cats and 31 dogs with CNS cryptococcosis diagnosed between 2000 and 2020.

PROCEDURES

A case series and cohort study were performed using the same 50 animals. Both studies were multi-institutional and both retrospective and prospective. Disease features were compared between cats and dogs, and associations between putative risk factors and survival time (ST) were assessed.

RESULTS

Dogs were younger at initial presentation than cats and had lower latex cryptococcal antigen agglutination titers. Extraneurologic signs were common and frequently involved sinonasal and contiguous tissues. Neuroanatomic localization was predominantly forebrain, central vestibular (including cerebellum), multifocal, or diffuse. CSF analysis predominantly showed pleocytosis, with eosinophilic inflammation common in dogs. Seventy-eight percent (39/50) of patients received antifungal treatment. Median STs (from presentation) in treated patients were 1,678 days for cats and 679 days for dogs. Abnormal mentation at presentation (in dogs) and CSF collection (in cats) were associated with shorter STs. In treated dogs, those that received glucocorticoids prior to diagnosis, or single rather than multiple antifungal agents, had shorter STs.

CLINICAL RELEVANCE

The prognosis for feline and canine CNS cryptococcosis is guarded, yet long STs are possible with appropriate treatment. Presence of subtle upper respiratory tract signs may suggest cryptococcosis in patients with neurologic signs, while the absence of neurologic signs does not preclude CNS involvement.

Synergy Between Gαz Deficiency and GLP-1 Analog Treatment in Preserving Functional β-Cell Mass in Experimental Diabetes

A defining characteristic of type 1 diabetes mellitus (T1DM) pathophysiology is pancreatic β-cell death and dysfunction, resulting in insufficient insulin secretion to properly control blood glucose levels. Treatments that promote β-cell replication and survival, thus reversing the loss of β-cell mass, while also preserving β-cell function, could lead to a real cure for T1DM. The α-subunit of the heterotrimeric Gz protein, Gαz, is a tonic negative regulator of adenylate cyclase and downstream cAMP production. cAMP is one of a few identified signaling molecules that can simultaneously have a positive impact on pancreatic islet β-cell proliferation, survival, and function. The purpose of our study was to determine whether mice lacking Gαz might be protected, at least partially, from β-cell loss and dysfunction after streptozotocin treatment. We also aimed to determine whether Gαz might act in concert with an activator of the cAMP-stimulatory glucagon-like peptide 1 receptor, exendin-4 (Ex4). Without Ex4 treatment, Gαz-null mice still developed hyperglycemia, albeit delayed. The same finding held true for wild-type mice treated with Ex4. With Ex4 treatment, Gαz-null mice were protected from developing severe hyperglycemia. Immunohistological studies performed on pancreas sections and in vitro apoptosis, cytotoxicity, and survival assays demonstrated a clear effect of Gαz signaling on pancreatic β-cell replication and death; β-cell function was also improved in Gαz-null islets. These data support our hypothesis that a combination of therapies targeting both stimulatory and inhibitory pathways will be more effective than either alone at protecting, preserving, and possibly regenerating β-cell mass and function in T1DM.

The gastrin-releasing peptide analog bombesin preserves exocrine and endocrine pancreas morphology and function during parenteral nutrition

Stimulation of digestive organs by enteric peptides is lost during total parental nutrition (PN). Here we examine the role of the enteric peptide bombesin (BBS) in stimulation of the exocrine and endocrine pancreas during PN. BBS protects against exocrine pancreas atrophy and dysfunction caused by PN. BBS also augments circulating insulin levels, suggesting an endocrine pancreas phenotype. While no significant changes in gross endocrine pancreas morphology were observed, pancreatic islets isolated from BBS-treated PN mice showed a significantly enhanced insulin secretion response to the glucagon-like peptide-1 (GLP-1) agonist exendin-4, correlating with enhanced GLP-1 receptor expression. BBS itself had no effect on islet function, as reflected in low expression of BBS receptors in islet samples. Intestinal BBS receptor expression was enhanced in PN with BBS, and circulating active GLP-1 levels were significantly enhanced in BBS-treated PN mice. We hypothesized that BBS preserved islet function indirectly, through the enteroendocrine cell-pancreas axis. We confirmed the ability of BBS to directly stimulate intestinal enteroid cells to express the GLP-1 precursor preproglucagon. In conclusion, BBS preserves the exocrine and endocrine pancreas functions during PN; however, the endocrine stimulation is likely indirect, through the enteroendocrine cell-pancreas axis.

Cost-effectiveness analysis of neoadjuvant pertuzumab and trastuzumab therapy for locally advanced, inflammatory, or early HER2-positive breast cancer in Canada

OBJECTIVE

The NeoSphere trial demonstrated that the addition of pertuzumab to trastuzumab and docetaxel for the neoadjuvant treatment of HER2-positive locally advanced, inflammatory, or early breast cancer (eBC) resulted in a significant improvement in pathological complete response (pCR). Furthermore, the TRYPHAENA trial supported the benefit of neoadjuvant dual anti-HER2 therapy. Survival data from these trials is not yet available; however, other studies have demonstrated a correlation between pCR and improved event-free survival (EFS) and overall survival (OS) in this patient population. This study represents the first Canadian cost-effectiveness analysis of pertuzumab in the neoadjuvant treatment of HER2-positive eBC.

METHODS

A cost-utility analysis (CUA) was conducted using a three health state Markov model ('event-free', 'relapsed', and 'dead'). Two separate analyses were conducted; the first considering total pCR (ypT0/is ypN0) data from NeoSphere, and the second from TRYPHAENA. Published EFS and OS data partitioned for patients achieving/not achieving pCR were used in combination with the percentage achieving pCR in the pertuzumab trials to estimate survival. This CUA included published utility values and direct medical costs including drugs, treatment administration, management of adverse events, supportive care, and subsequent therapy. To address uncertainty, a probabilistic sensitivity analysis (PSA) and alternative scenarios were explored.

RESULTS

Both analyses suggested that the addition of pertuzumab resulted in increased life-years and quality-adjusted life-years (QALYs). The incremental cost per QALY ranged from $25,388 (CAD; NeoSphere analysis) to $46,196 (TRYPHAENA analysis). Sensitivity analyses further support the use of pertuzumab, with cost-effectiveness ratios ranging from $9230-$64,421. At a threshold of $100,000, the addition of pertuzumab was cost-effective in nearly all scenarios (93% NeoSphere; 79% TRYPHAENA).

CONCLUSION

Given the improvement in clinical efficacy and a favorable cost per QALY, the addition of pertuzumab in the neoadjuvant setting represents an attractive treatment option for HER2-positive eBC patients.

Acquired proximal renal tubulopathy in dogs exposed to a common dried chicken treat: retrospective study of 108 cases (2007-2009)

BACKGROUND

Proximal renal tubulopathy was reported in Australian dogs with markedly increased frequency from September 2007.

METHODS

Two veterinarian-completed surveys were launched in response to an increased incidence of acquired proximal renal tubulopathy in dogs. The selection criterion for inclusion was glucosuria with blood glucose < 10 mmol/L. Data collected included signalment, presenting signs, history of feeding treats, results of urinalysis and blood tests, treatment and time to resolution of clinical signs.

RESULTS

A total of 108 affected dogs were studied. All had been fed the same brand of dried chicken treats, made in China, for a median of 12 weeks (range, 0.3-78 weeks). Small breeds (< 10 kg) accounted for 88% of cases. Common presenting signs included polyuria/polydipsia (76%), lethargy (73%), inappetence (65%) and vomiting (54%). Common biochemical findings included euglycaemia (74%; 71/96), hypoglycaemia (23%; 22/96), acidosis (77%; 20/26), hypokalaemia (45%; 38/84), hypophosphataemia (37%; 28/75) and azotaemia (27%; 23/85). In addition to discontinuation of treats, 64 dogs received medical treatment, including intravenous fluids (52%) and oral electrolyte, amino acid or vitamin supplements. Six dogs died or were euthanased. Two dogs were necropsied. Histopathological findings consisted of proximal tubular necrosis accompanied by regeneration. Time to resolution of clinical signs in 35 survivors available for follow-up was < 2 weeks (n = 8), 2-4 weeks (n = 2), 5-7 weeks (n = 5) and 2-6 months (n = 10).

CONCLUSION

Of the 108 dogs with acquired proximal renal tubulopathy contemporaneous with chicken treat consumption, most survived but many required aggressive supportive care. The treats likely contained a toxin targeting the proximal renal tubules. Diet history and urinalysis were vital for diagnosis.

Canine vector-borne disease pathogens in dogs from south-east Queensland and north-east Northern Territory

OBJECTIVES

To determine the prevalence of canine vector-borne diseases (CVBD: Babesia spp., Anaplasma spp., Ehrlichia spp., haemotropic mycoplasmas and Hepatozoon) in Australian dogs; namely, dogs from pounds in south-east Queensland and an indigenous Aboriginal community in the north-east of the Northern Territory.

DESIGN AND PROCEDURE

Blood samples were collected from 100 pound dogs and 130 Aboriginal community dogs and screened for the CVBD pathogens using polymerase chain reaction (PCR). All positive PCR products were sequenced for species confirmation.

RESULTS

In total, 3 pound dogs and 64 Aboriginal community dogs were infected with at least one CVBD pathogen. Overall, B. vogeli was detected in 13 dogs, A. platys in 49, M. haemocanis in 23, Candidatus Mycoplasma haematoparvum in 3 and C. M. haemobos in 1 dog. Co-infections were detected in 22 Aboriginal community dogs.

CONCLUSIONS

This study found B. vogeli, A. platys and haemotropic mycoplasma infections to be common in dogs in subtropical and tropical areas of Australia. This study also reports for the first time the prevalence and genetic characterisation of haemotropic mycoplasmas in dogs in Australia.

A modified three-dose protocol for colonization of the canine urinary tract with the asymptomatic bacteriuria Escherichia coli strain 83972

Establishment of asymptomatic bacteriuria is a novel alternative to antimicrobial therapy for management of recurrent bacterial urinary tract infection in humans and may also be useful for dogs if it can be shown that colonization of the canine bladder can be achieved. A three-dose protocol for Escherichia coli strain 83972 inoculation was developed to attempt induction of persistent bacteriuria in healthy dogs. A previous study using a single inoculation colonized dogs for no longer than 10 days and multi-dose protocols have been used to establish persistent bacteriuria in human patients. Three doses of approximately 10(9)E. coli 83972 bacteria were introduced into the bladder of eight healthy female dogs over 24h via an indwelling sterile urinary catheter. Three additional dogs were sham-inoculated. Duration of colonization ranged from 1 to 28 days (median 2 days) with no discernible reason for the prolonged colonization in one dog. Multi-dose inoculation of healthy dogs was not obviously superior to our previous use of single-dose inoculation apart from one dog remaining colonized for 28 days following the three-dose inoculation protocol.

Molecular evidence of Rickettsia felis infection in dogs from Northern Territory, Australia

The prevalence of spotted fever group rickettsial infection in dogs from a remote indigenous community in the Northern Territory (NT) was determined using molecular tools. Blood samples collected from 130 dogs in the community of Maningrida were subjected to a spotted fever group (SFG)-specific PCR targeting the ompB gene followed by a Rickettsia felis-specific PCR targeting the gltA gene of R. felis. Rickettsia felis ompB and gltA genes were amplified from the blood of 3 dogs. This study is the first report of R. felis infection in indigenous community dogs in NT.

Molecular evidence supports the role of dogs as potential reservoirs for Rickettsia felis

Rickettsia felis causes flea-borne spotted fever in humans worldwide. The cat flea, Ctenocephalides felis, serves as vector and reservoir host for this disease agent. To determine the role of dogs as potential reservoir hosts for spotted fever group rickettsiae, we screened blood from 100 pound dogs in Southeast Queensland by using a highly sensitive genus-specific PCR. Nine of the pound dogs were positive for rickettsial DNA and subsequent molecular sequencing confirmed amplification of R. felis. A high prevalence of R. felis in dogs in our study suggests that dogs may act as an important reservoir host for R. felis and as a potential source of human rickettsial infection.

Canine bacterial urinary tract infections: new developments in old pathogens

Uncomplicated bacterial urinary tract infections (UTIs) occur commonly in dogs. Persistent or recurrent infections are reported less frequently. They typically occur in dogs with an underlying disease and are sometimes asymptomatic, especially in dogs with predisposing chronic disease. Escherichia coli is the organism most frequently cultured in both simple and complicated UTIs. Organisms such as Enterococcus spp. and Pseudomonas spp. are less common in uncomplicated UTI, but become increasingly prominent in dogs with recurrent UTI. The ability of bacteria to acquire resistance to antimicrobials and/or to evade host immune defence mechanisms is vital for persistence in the urinary tract. Antimicrobial therapy limitations and bacterial strains with such abilities require novel control strategies. Sharing of resistant bacteria between humans and dogs has been recently documented and is of particular concern for E. coli O25b:H4-ST131 strains that are both virulent and multi-drug resistant. The epidemiology of complicated UTIs, pathogenic traits of uropathogens and new therapeutic concepts are outlined in this review.

Experimental colonization of the canine urinary tract with the asymptomatic bacteriuria Escherichia coli strain 83972

Establishment of asymptomatic bacteriuria (ABU) with Escherichia coli 83972 is a viable prophylactic alternative to antibiotic therapy for the prevention of recurrent bacterial urinary tract infection in humans. Approximately 2 × 10(8) viable E. coli 83972 cells were introduced into the bladder of six healthy female dogs via a sterile urinary catheter. The presence of pyuria, depression, stranguria, pollakiuria and haematuria was documented for 6 weeks and urinalysis and aerobic bacterial cultures were performed every 24-72 h. Pyuria was present in all dogs on day 1 post-inoculation and 4/6 dogs (67%) had a positive urine culture on this day. Duration of colonization ranged from 0 to 10 days (median 4 days). Four dogs were re-inoculated on day 20. Duration of colonization following the second inoculation ranged from 1 to 3 days. No dog suffered pyrexia or appeared systemically unwell but all dogs initially exhibited mild pollakiuria and a small number displayed gross haematuria and/or stranguria. By day 3 of each trial all clinical signs had resolved. Persistent bacteriuria was not achieved in any dog but two dogs were colonized for 10 days following a single inoculation. Further research is required to determine whether establishment of ABU in dogs with recurrent urinary tract infection is a viable alternative to repeated doses of antimicrobial agents.

Greater first year effectiveness drives favorable cost-effectiveness of brand risedronate versus generic or brand alendronate: modeled Canadian analysis

The RisedronatE and ALendronate (REAL) study provided a unique opportunity to conduct cost-effectiveness analyses based on effectiveness data from real-world clinical practice. Using a published osteoporosis model, the researchers found risedronate to be cost-effective compared to generic or brand alendronate for the treatment of Canadian postmenopausal osteoporosis in patients aged 65 years or older.

INTRODUCTION

The REAL study provides robust data on the real-world performance of risedronate and alendronate. The study used these data to assess the cost-effectiveness of brand risedronate versus generic or brand alendronate for treatment of Canadian postmenopausal osteoporosis patients aged 65 years or older.

METHODS

A previously published osteoporosis model was populated with Canadian cost and epidemiological data, and the estimated fracture risk was validated. Effectiveness data were derived from REAL and utility data from published sources. The incremental cost per quality-adjusted life-year (QALY) gained was estimated from a Canadian public payer perspective, and comprehensive sensitivity analyses were conducted.

RESULTS

The base case analysis found fewer fractures and more QALYs in the risedronate cohort, providing an incremental cost per QALY gained of $3,877 for risedronate compared to generic alendronate. The results were most sensitive to treatment duration and effectiveness.

CONCLUSIONS

The REAL study provided a unique opportunity to conduct cost-effectiveness analyses based on effectiveness data taken from real-world clinical practice. The analysis supports the cost-effectiveness of risedronate compared to generic or brand alendronate and the use of risedronate for the treatment of osteoporotic Canadian women aged 65 years or older with a BMD T-score < or =-2.5.

Effect of a Single Plasma Transfusion on Thromboembolism in 13 Dogs With Primary Immune-Mediated Hemolytic Anemia

Thirteen dogs with primary immune-mediated hemolytic anemia received fresh-frozen plasma within 12 hours of admission, in addition to unfractionated heparin and other therapies, such as prednisone, azathioprine, and packed red blood cell transfusion. Antithrombin activity was quantified prior to transfusion and at 30 minutes and 48 hours after transfusion. Plasma antithrombin activity did not change significantly after a single plasma transfusion. There were no deaths in the first 48 hours of treatment. Thromboembolism was identified at necropsy in six of 10 dogs that died within 12 months of admission. There was no significant difference in the incidence of thromboembolism between the current treatment group and a historical control group.

Hyperadrenocorticism associated with excessive sex hormone production by an adrenocortical tumor in two dogs

An 11-year-old spayed female Labrador Retriever and a 9-year-old castrated male miniature Poodle were evaluated because of clinical signs of hyperadrenocorticism. Cortisol testing did not support a diagnosis of hypercortisolemia in either dog; however, imaging studies revealed unilateral adrenal tumors in both dogs. Serum concentrations of 17-hydroxyprogesterone, progesterone, and estradiol were high in both dogs, and androstenedione concentrations were also high in 1 dog. It is suspected that sex hormone secretion by the adrenal tumors in these dogs resulted in clinical signs of hyperadrenocorticism. Clinical signs and hormonal abnormalities resolved in the male dog after surgical resection of the tumor. There was no improvement in clinical signs after treatment with mitotane in the female dog, which died 2 months after diagnosis. Histologic evaluation confirmed the presence of adrenocortical carcinoma in both dogs.

Symptomatic upper extremity venous thrombosis associated with permanent transvenous pacemaker electrodes

Symptomatic arm vein thrombosis secondary to permanent transvenous cardiac pacing electrodes is an uncommon occurrence with an incidence of 1 to 3 percent. Two patients with this problem are presented who were treated with streptokinase followed by conventional anticoagulation therapy with heparin and warfarin. Near total resolution of the thromboses was accomplished in both patients. In follow-up periods of nine and 20 months, the patients have remained asymptomatic. An aggressive approach to this problem may decrease the current high morbidity.

The kinetic properties and reaction mechanism of histamine methyltransferase from human skin

The substrate kinetic properties of histamine methyltransferase from human skin were studied at limiting concentrations of both histamine and S-adenosylmethionine. Substrate inhibition by histamine was observed at concentrations above 10 microM. Primary plots showed evidence of a sequential reaction mechanism. The Michaelis constants were derived from secondary plots of slopes from the primary plots ([S]/v versus [S]) versus reciprocal of the second substrate concentration. The mean Km values for histamine and S-adenosylmethionine were 4.2 and 1.8 microM respectively. Histamine in concentrations of 25-100 microM inhibited enzyme activity uncompetitively with respect to S-adenosylmethionine. No substrate inhibition was observed with S-adenosylmethionine. To elucidate the reaction mechanism further, inhibition by the two products, S-adenosylhomocysteine and 1-methylhistamine, was studied. S-Adenosylhomocysteine inhibited non-competitively with respect to histamine and competitively with respect to S-adenosylmethionine. 1-Methylhistamine inhibited non-competitively with respect to histamine and to S-adenosylmethionine. These results are interpreted as providing evidence for an ordered sequential Bi Bi reaction mechanism, with the methyl-group donor S-adenosylmethionine as the first substrate that adds to the enzyme and histamine as the second substrate. 1-Methylhistamine is the first product to leave the enzyme and S-adenosylhomocysteine is the second. The results are discussed in terms of the possible role that this enzyme could play in the modulation of histamine-mediated reactions in skin.

Differences in catalytic properties between cerebral cytoplasmic and mitochondrial hexokinases

1. Clear kinetic differences between cytoplasmic and mitochondrial forms of type-I cerebral hexokinase were demonstrated from experiments performed under identical conditions on three (cytoplasmic, bound mitochondrial and solubilized mitochondrial) preparations of the enzyme. 2. Whereas the Michaelis constant for glucose (KmGlc) was consistent, that for MgATP2- (KmATP) was lower in the cytoplasmic than in the two mitochondrial preparations. The substrate dissociation constants (KsGlc and KsATP) were both higher in the cytoplasmic than in the mitochondrial preparations. A further difference in the substrate kinetic patterns was that KmATP=KmATP for the cytoplasmic enzyme, in contrast with the mitochondrial enzyme, where KmATP was clearly not equal to KsATP [Bachelard et al. (1971) Biochem. J. 123, 707-715]. 3. Dead-end inhibition produced by N-acetyl-glucosamine and by AMP also exhibited different quantitative kinetic patterns for the two enzyme sources. Both inhibitions gave Ki values similar or equal to those of Ki' for the cytoplasmic activity, whereas Ki was clearly not equal to Ki' for the mitochondrial activity. 4. All of these studies demonstrated the similarity of the two mitochondrial activities (particulate and solubilized), which were both clearly different from the cytoplasmic activity. 5. The analysis gives a practical example of our previous theoretical treatment on the derivation of true inhibition constants. 6. The results are discussed in terms of the function of cerebral hexokinases.

Kinetics of methoxyflurane biotransformation with reference to substrate inhibition

The kinetics of biotransformation of methoxyflurane by rat hepatic microsomes in vitro was studied. The rate of biotransformation as measured by analysis of metabolites continued to increase even at near-saturation concentrations of the anesthetic. Methoxyflurane biotransformation followed either an unbounded curve with empirical formula y = a ln(bx + 1) or an asymptotic curve with formula (see article) No substrate inhibition was observed. Total fluoride Vmax of 135.1 mmumol F-/mg protein/30' was increased to 931.9 by phenobarbital induction; free fluoride Vmax from 39.2 to 403.2. Thus, enzyme induction shifted biotransformation to the production of greater amounts of inorganic free fluoride metabolites than organic fluoride-containing metabolites. Phenobarbital induction caused qualitative as well as quantitative alteration in the biotransformation of methoxyflurane.

Kinetic properties of hexokinase in resected temporal lobes of patients with drug-resistant epilepsy

Properties of the cerebral glycolytic enzyme, hexokinase, were studied in biopsy samples of human temporal lobe, obtained during lobectomy for drug-resistant epilepsy and compared "blind" with contol biopsy samples of human cerebral cortex. No significant changes in the total activity or subcellular distribution of the enzyme were observed but the Km value for glucose was altered. The 17 control samples gave a normal mean value for Km (glucose) of 0.05 mM and the 14 epileptic samples gave a significantly higher mean value of 0.09 mM. The drugs used in previous treatment of the epilepsies were "scored" with respect to type and dose; analysis of these in relation to the kinetic results eliminated the possibility that the increase in Km value was an artifact due to the drugs. The observed change in enzyme kinetic properties is discussed in terms of potential interactions of small molecules with the isoenzymes of cerebral hexokinase.

Cerebral-cortex hexokinase. Elucidation of reaction mechanisms by substrate and dead-end inhibitor kinetic analysis

1. The substrate kinetic properties of cerebral hexokinases (mitochondrial and cytoplasmic) were studied at limiting concentrations of both glucose and MgATP(2-). Primary plots of the enzymic activity gave no evidence of a Ping Pong mechanism in three types of mitochondrial preparation tested (intact and osmotically disrupted mitochondria, and the purified mitochondrial enzyme), nor in the purified cytoplasmic preparation. 2. Secondary plots of intercepts from the primary plots (1/v versus 1/s) versus reciprocal of second substrate of the mitochondrial activity gave kinetic constants which differed from those obtained directly from the plots of 1/v versus 1/s or of s/v versus s, although the ratios of the derived constants were consistent. The kinetic constants obtained with the cytoplasmic enzyme from primary and secondary plots were consistent. 3. Deoxyglucose, as alternative substrate, inhibited cytoplasmic hexokinase by competition with glucose, but did not compete when MgATP(2-) was the substrate varied. The K(i) for deoxyglucose when glucose concentrations were varied was 0.25mm. 4. A range of ATP analogues was tested as potential substrates and inhibitors of hexokinase activity. GTP, ITP, CTP, UTP and betagamma-methylene-ATP did not act as substrates, nor did they cause significant inhibition. Deoxy-ATP proved to be almost as effective a substrate as ATP. AMP inhibited but did not act as substrate. 5. N-Acetyl-glucosamine inhibited all preparations competitively when glucose was varied and non-competitively when MgATP(2-) was varied. AMP inhibition was competitive when MgATP(2-) was the substrate varied and non-competitive when glucose was varied. 6. The results are interpreted as providing evidence for a random reaction mechanism in all preparations of brain hexokinase, cytoplasmic and mitochondrial. The kinetic properties and reaction mechanism do not change on extraction and purification of the particulate enzyme. 7. The results are discussed in terms of the participation of hexokinase in regulation of cerebral glycolysis.

Cerebral-cortex hexokinase. Comparison of properties of solubilized mitochondrial and cytoplasmic activities

1. Cerebral-cortex mitochondria, after purification by using high-density sucrose solutions, were extracted with Triton X-100. The total hexokinase activity of the intact mitochondria was increased by 50-80% in the Triton extracts. 2. Triton X-100 was removed from mitochondrial extracts by a combination of ammonium sulphate fractionation and DEAE-cellulose chromatography. Mitochondrial hexokinase remained soluble after removal of extractant. 3. The behaviour of solubilized mitochondrial hexokinase was compared with soluble cytoplasmic hexokinase from the same samples of cerebral cortex on identical columns of DEAE-cellulose. Two peaks were eluted from each source of hexokinase. The distribution between hexokinase peaks was similar for the two sources. Peak I (approx. 80% of the total hexokinase) from each was eluted at identical concentrations of potassium chloride and slight differences were observed in the elution profiles for peak II. 4. The purified mitochondrial hexokinase showed the following kinetic properties: peak I, K(m)(ATP) 0.60mm, K(m)(glucose) 0.042mm; peak II, K(m)(ATP) 0.66mm, K(m)(glucose) 0.043mm. The purified cytoplasmic hexokinase Michaelis constants were: peak I, K(m)(ATP) 0.56mm, K(m)(glucose) 0.048mm; peak II, K(m)(ATP) 0.68mm, K(m)(glucose) 0.062mm. 5. Although no significant differences between mitochondrial and cytoplasmic hexokinases were noted in chromatographic behaviour or in the kinetic properties studied, the purified mitochondrial enzyme was activated slightly (approx. 20%) by Triton X-100, in contrast with the cytoplasmic enzyme, which was not affected. 6. The results, taken to indicate basic similarity between mitochondrial and cytoplasmic hexokinases, are discussed in relation to the role of the two sources of enzyme in the metabolism of the tissue.

"Gulliver", an experiment for extraterrestrial life detection and analysis

Based on the probability that extraterrestrial life is biochemically somewhat similar to life on Earth, a life detection experiment is being prepared to explore Mars. The experiment will be performed by an automated device which will carry a microbiological medium being developed to support a wide range of earth microorganisms. Selected ingredients of the medium will be labeled with radioactive isotopes. A sticky string, shot out from and reeled back into the device, will gather a sample of the Martian soil. It is hoped the radioactive atoms in the compounds will be metabolized by the unknown organisms in the soil and evolved in a labelled gas. The gas will be collected by a chemical "getter" and the radioactivity measured for transmission to Earth. A positive response from the test unit and a negative, or lesser, response from a poisoned control unit would constitute evidence of life. The device can also differentiate between photosynthetic and nonphotosynthetic metabolic activity. Data from field tests on Earth are presented.