Efficacy and Safety of Rilzabrutinib in Pemphigus: PEGASUS Phase 3 Randomized Study.

TRIAL DESIGN: Pemphigus is a rare but life-threatening autoimmune disease requiring long-term treatment that minimizes corticosteroid (CS) exposure while providing consistent disease control. The phase 2 pemphigus study of oral, reversible, covalent Bruton tyrosine kinase inhibitor rilzabrutinib demonstrated rapid and sustained efficacy with well-tolerated safety. METHODS: Adults (aged 18-80 years) were randomized 1:1 to 400 mg rilzabrutinib (n = 65) or placebo (n = 66) twice daily (with CS ≤ 0.5 mg/kg/d) for 37 weeks in the phase 3 PEGASUS study in moderate-to-severe pemphigus vulgaris/pemphigus foliaceus. RESULTS: The primary endpoint of complete remission from week 29 to week 37 with the amended endpoint CS dose ≤10 mg/d was not significant for 13 of 54 (24%) rilzabrutinib versus 10 of 55 (18%) placebo patients with PV (P = .45). Secondary endpoints showed numerical but nonsignificant improvements with rilzabrutinib (vs placebo) in reduced CS use, prolonged complete remission duration, and faster time to first complete remission. CONCLUSIONS: Overall, rilzabrutinib was well-tolerated, with similar adverse events reported in both groups. Using minimal CS dose ≤10 mg/d and excluding remote observations, the primary efficacy endpoint was not met. However, results from a prespecified sensitivity analysis using CS dose ≤5 mg/d, considering all observations, and including all patients support Bruton tyrosine kinase inhibition as a viable therapeutic approach for pemphigus.

Phase 1 clinical trial evaluating safety, exposure and pharmacodynamics of BTK inhibitor tolebrutinib (PRN2246, SAR442168).

Bruton's tyrosine kinase (BTK), expressed in B cells and cells of innate immunity, including microglia, is an essential signaling element downstream of the B-cell receptor and Fc-receptors. Tolebrutinib (PRN2246, SAR442168) is a potent BTK inhibitor that covalently binds the kinase, resulting in durable inhibition with the potential to target inflammation in the periphery and central nervous system (CNS). Tolebrutinib crosses the blood-brain barrier and potently inhibits BTK in microglial cells isolated from the CNS. A first-in-human randomized, double-blind, placebo-controlled study of tolebrutinib was conducted. The trial design consisted of five single ascending dose arms with oral administration of a single dose of 5, 15, 30, 60, and 120 mg (n = 6 per arm, n = 2 placebo), five multiple ascending dose arms with oral administration of 7.5, 15, 30, 60, and 90 mg (n = 8 per arm, n = 2 placebo) over 10 days, and one arm (n = 4) in which cerebral spinal fluid (CSF) exposure was measured 2 h after a single 120 mg dose. Tolebrutinib was well-tolerated in the study and all treatment-related treatment emergent adverse events were mild. Tolebrutinib was rapidly absorbed following oral administration with a rapid half-life of ~ 2 h. Peripheral BTK occupancy was assessed at various timepoints by an enzyme-linked immunosorbent assay-based readout using an irreversible probe. Assessments demonstrated extensive and prolonged peripheral BTK occupancy at steady-state with once daily doses as low as 7.5 mg. Further, CSF exposure was demonstrated 2 h after administration at 120 mg.

Open trial of Bruton's tyrosine kinase inhibitor (PRN1008) in the treatment of canine pemphigus foliaceus.

BACKGROUND: Bruton's tyrosine kinase (BTK) is important in B-cell signalling. Efficacy has been reported for BTK inhibitors (BTKi) in human autoimmune diseases. Canine pemphigus foliaceus (cPF) is one of the most common canine autoimmune skin diseases. OBJECTIVES: To determine the safety and efficacy of the BTKi PRN1008 in the treatment of cPF. ANIMALS: Four privately owned dogs. METHODS AND MATERIALS: Four dogs diagnosed with PF were administered BTKi PRN1008. Initial dosages approximated to 15 mg/kg once daily, increased to twice daily if inadequate response was seen. Treatment continued for 20 weeks, attempting to decrease to every other day. Dogs were monitored with complete blood counts, serum biochemistry panels and urinalyses, and evaluated with a modified version of a validated human Pemphigus Disease Activity Index (cPDAI). Serum anti-desmocollin-1 (DSC-1) and desmoglein-1 (DSG-1) immunoglobulin (Ig)G titres were performed before and after the treatment period. Drug bound to target was measured in peripheral blood mononuclear cells (PBMC). RESULTS: All four dogs showed reduction in lesions and cPDAI score during the first two weeks of treatment. Three dogs continued to improve and sustained near complete remission by 20 weeks, at which point three responses were considered "good" and one "fair". Final daily dosages were in the range 17-33 mg/kg. Anti-DSC-1 IgG titre decreased dramatically in one dog, was undetectable in two and was uninterpretable in one dog. No dogs had detectable IgG to DSG1. A possible adverse event occurred in one dog. CONCLUSIONS AND CLINICAL IMPORTANCE: BTKi PRN1008 monotherapy may have some beneficial effects in some cases of cPF.

Efficacy of a Bruton's Tyrosine Kinase Inhibitor (PRN-473) in the treatment of canine pemphigus foliaceus.

BACKGROUND: Bruton's tyrosine kinase (BTK) is important in B-cell signalling. Efficacy has been reported for BTK inhibitors (BTKi) in human autoimmune diseases. Canine pemphigus foliaceus (cPF) is the most common canine autoimmune skin disease. OBJECTIVES: To determine the safety and efficacy of a BTKi in cPF treatment. ANIMALS: Nine privately owned dogs. METHODS AND MATERIALS: Nine dogs diagnosed with PF were administered BTKi PRN473. Initial dosages were ≈15 mg/kg once daily, increased to twice daily if inadequate response was seen. Treatment continued for a maximum of 20 weeks, attempting decrease to every other day. Dogs were monitored with complete blood counts, serum biochemistry panels, urinalyses and evaluated with a modified version of a validated human Pemphigus Disease Activity Index (cPDAI). Anti-desmocollin-1 (DSC-1) and desmoglein-1 (DSG-1) immunoglobulin G (IgG) titres were performed before and after the treatment period. Drug bound to target was measured in peripheral blood mononuclear cells. RESULTS: All nine dogs showed reduction in lesions and cPDAI score during the first two weeks of treatment. At the end of the study, four responses were considered "good", two "fair", two "poor" and one dog withdrawn due to recurrence of a previously excised mast cell tumour. Four dogs continued to improve by Week 4; three sustained near complete remission by study's end. The anti-DSC-1 IgG titre decreased in three dogs, increased in two, was undetected in three and was not performed in the withdrawn dog. No dogs had detectable IgG to DSG1. Possible adverse effects occurred in three dogs. CONCLUSIONS AND CLINICAL IMPORTANCE: Bruton's tyrosine kinase inhibitor monotherapy may have beneficial effects in some cases of cPF.

Influence of menstrual cycle on cytochrome P450 2A6 activity and cardiovascular effects of nicotine.

BACKGROUND AND OBJECTIVES: The phase of the menstrual cycle has been reported to affect frequency of smoking, withdrawal symptoms, and the likelihood of smoking cessation in women. Cytochrome P450 (CYP) 2A6 is primarily responsible for the metabolism of nicotine. Our objective was to evaluate the effect of the phase of the menstrual cycle on the activity of CYP2A6 and the cardiovascular effects of nicotine. METHOD: Eleven healthy, nonsmoking women received a 30-minute combined infusion of deuterium-labeled nicotine and cotinine (0.5 microg . kg(-1) . min(-1) of each compound) during the midfollicular and midluteal phases of the menstrual cycle. Nicotine and cotinine pharmacokinetic parameters and plasma adrenocorticotropic hormone (ACTH), epinephrine, and norepinephrine responses were measured over time. RESULTS: There were no biologically or statistically significant differences in the comparison of menstrual cycle phases with regard to the pharmacokinetics of nicotine and cotinine. Nicotine clearance was 1000 +/- 315 mL/min and 1047 +/- 271 mL/min in the follicular and luteal phases, respectively (geometric mean ratio, 1.06; 90% confidence interval, 0.87-1.29). Cotinine clearance was 44 +/- 20 mL/min and 55 +/- 42 mL/min in the follicular and luteal phases, respectively (geometric mean ratio, 1.13; 90% confidence interval, 0.90-1.41). Nicotine infusion increased blood pressure, heart rate, and epinephrine concentrations. There were no differences in catecholamine, ACTH, or hemodynamic responses to nicotine infusion between menstrual cycle phases, although norepinephrine concentrations were constantly higher in the luteal phase compared with the follicular phase. CONCLUSIONS: CYP2A6 activity is not affected by menstrual cycle phase, and it is unlikely that menstrual cycle-related smoking habits of women are determined by changes in nicotine pharmacokinetics. The effects of nicotine on plasma ACTH and catecholamine levels and hemodynamic parameters are not altered by menstrual cycle phase in healthy, nonsmoking women.

Angiographic perfusion score: an angiographic variable that integrates both epicardial and tissue level perfusion before and after facilitated percutaneous coronary intervention in acute myocardial infarction.

BACKGROUND: Both epicardial and myocardial perfusion have been associated with clinical outcomes in the setting of ST elevation myocardial infarction (STEMI), and the performance of adjunctive/rescue percutaneous coronary intervention (PCI) may further improve clinical outcomes after fibrinolytic administration. METHODS: The goal was to develop a simple, broadly applicable angiographic metric that takes into account indices of epicardial and myocardial perfusion both before and after PCI to arrive at a single perfusion grade in patients undergoing cardiac catheterization after fibrinolysis. The angiographic perfusion score (APS) is the sum of the Thrombolysis in Myocardial Infarction (TIMI) flow grade (TFG; 0-3) added to the TIMI myocardial perfusion grade (TMPG; 0-3) before and after PCI (total possible grade, 0-12). Failed perfusion was defined as an APS of 0 to 3, partial perfusion was defined as an APS of 4 to 9, and full perfusion was defined as an APS of 10 to 12. The APS was evaluated in patients from the Double-blind, Placebo-contolled, Multicenter Angiographic Trial of Rhumab CD18 in Acute Myocardial Infarction (LIMIT-AMI; n = 394) and Enoxaparin as Adjunctive Antithrombin Therapy for ST-Elevation Myocardial Infarction-Thrombolysis In Myocardial Infarction (ENTIRE-TIMI) 23 trials (n = 483), and infarct size (120-216 hours after AMI SPECT Technetium-99m Sestamibi data) was assessed in the LIMIT-AMI trial. RESULTS: The APS was associated with the incidence of death or myocardial infarction (failed, 16.7% [n = 18]; partial, 2.5% [n = 155]; full, 2.4% [n = 82]; P =.039 for trend) and larger SPECT infarct sizes (failed, median 39% [n = 10]; partial, 12% [n = 79]; and full, 8% [n = 35]; P =.002). No patient with full APS died, whereas the mortality rate was 11.1% in patients with a failed APS (P =.03). CONCLUSIONS: The APS combines grades of epicardial and tissue level perfusion before and after PCI or at the end of diagnostic cardiac catheterization to arrive at a single angiographic variable that is associated with infarct size and the rates of 30-day death or MI. Partial or full angiographic perfusion scores are associated with a halving of infarct size, and no patients with full angiographic perfusion died.

Single lead ST-segment recovery: a simple, reliable measure of successful fibrinolysis after acute myocardial infarction.

BACKGROUND: Successful reperfusion after acute ST-elevation myocardial infarction improves prognosis. Among the different electrocardiographic markers of reperfusion, sum ST resolution is considered the hallmark of reperfusion, but is cumbersome to use. METHODS: To assess the usefulness of a single lead ST resolution at 90 minutes after fibrinolysis compared with the sum ST resolution in predicting Thrombolysis in Myocardial Infarction (TIMI) grade 3 flow, we used prospectively collected data from the Limitation of Myocardial Injury Following Thrombolysis in Acute Myocardial Infarction (LIMIT-AMI) study. All patients had electrocardiograms recorded at presentation and 90 minutes and a coronary angiogram 90 minutes after fibrinolysis. RESULTS: Infarction artery patency was assessed in 238 patients with 4 different ST resolution criteria: single lead ST resolution > or =50% and > or =70% and sum ST resolution > or =50% and > or =70%. The most sensitive criteria for TIMI grade 3 flow was single lead ST resolution > or =50% (sensitivity rate, 70%; specificity rate, 54%), whereas sum ST resolution > or =70% was most the specific criteria (sensitivity rate, 45%; specificity rate, 79%). The proportion of patients with TIMI grade 3 flow was similar in all 4 ST resolution groups (P =.84). Pre-discharge infarction size and ejection fraction were also similar. No single lead or sum lead measure of ST resolution was significantly associated with an increased risk of death, heart failure, or reinfarction. CONCLUSION: We propose that single lead ST-resolution > or =50% as an optimal electrocardiographic indicator for successful reperfusion 90 minutes after fibrinolysis. This simple electrocardiographic measure should be combined with bedside clinical and hemodynamic assessment to optimize decision making after fibrinolysis.

Time for contrast material to traverse the epicardial artery and the myocardium in ST-segment elevation acute myocardial infarction versus unstable angina pectoris/non-ST-elevation acute myocardial infarction.

Although the time for contrast material to fill the epicardial artery in the setting of acute coronary syndromes has been studied extensively, the time for contrast material to fill the myocardium has not been evaluated. We compared differences in myocardial contrast material transit among patients with unstable angina pectoris/non-ST-elevation acute myocardial infarction (UAP/NSTEAMI) with patients with ST-elevation acute myocardial infarction (STEAMI). The time it took for contrast material to first appear and to arrive at peak intensity in the myocardium was compared in 224 patients with STEAMI enrolled in the LIMIT-AMI study versus 430 patients with UAP/NSTEAMI enrolled in the TACTICS-TIMI 18 trial. In patients with STEAMI, there was a delay in both the time for contrast material to first enter the myocardium (5,619 +/- 1,789 vs 4,663 +/- 1,626 ms, p <0.0001) and the time from entrance to peak blush intensity (2,387 +/- 1,359 vs 1,959 +/- 1,244 ms, p = 0.003) compared with patients with UAP/NSTEAMI. STEAMI remained significantly associated with impaired entrance of contrast material into the myocardium (p <0.0001) in a multivariate model controlling for known correlates of impaired epicardial flow (presence of thrombus, percent diameter stenosis, left anterior descending artery location, and contrast material inflow in the epicardial artery [corrected TIMI frame count]). The time for contrast material to enter the myocardium is impaired to a greater degree in STEAMI compared with UAP/NSTEAMI, even after adjusting for other variables known to delay flow in the epicardial artery. These data provide insight into potential mechanistic differences between these 2 clinical syndromes.

Influence of arterial vs. venous sampling site on nicotine tolerance model selection and parameter estimation.

In this modeling study we utilize previously published nicotine pharmacokinetic (PK) and pharmacodynamic (PD, heart rate) data to investigate the influence of PK sampling site (venous vs. arterial) on the selection of a specific PD tolerance model and estimation of its parameters. We describe a general model for tolerance which includes as special cases feedback (TF), and kinetic based tolerance (TK) models. A TK model has arterial plasma drug concentrations (Ca) driving (hypothetical) effect (Ce) and antagonist (Cm) site concentrations, which drive a non-feedback effect (Enf): tolerance depends on the relative rate of equilibration of Ce and Cm with Ca. The TF model adds feedback which makes tolerance depend on Enf, not just on drug kinetics for nicotine. The arterial-sampling-analysis (PKPDa) has Ca driving Ce and Cm. The venous-sampling-analysis (PKPDv) does the same but estimates Ca from venous data by means of deconvolution. A TF model (with Cm = Ce) was always selected in the PKPDa. According to this model tolerance developed rapidly with a median half-life of 6.6 min, and median decrease of effect due to tolerance of 31%. Different variants of the TF or TK models were selected in the PKPDv. Parameter estimates for PKPDv show higher variability, and, for the TF model, lower rate and extent of tolerance development and threefold increase in EC50. The study shows that (i) TF models are more appropriate than TK models to describe nicotine effect data, (ii) venous sampling may lead to incorrect model selection and inaccurate and imprecise parameter estimation in respect to arterial sampling, and (iii) arterial sampling should be preferred for accurate (non-steady-state) PD modeling.

The relationship of intracoronary stent placement following thrombolytic therapy to tissue level perfusion.

BACKGROUND: Stenting has been shown to improve lumen diameters and thereby improve epicardial blood flow, but the impact of stent placement on tissue level perfusion has not been well characterized. METHODS: Data were drawn from the LIMIT trial of rhuMAb CD18 (anti WBC antibody) in acute myocardial infarction (AMI). Adjunctive/rescue stenting was performed at the discretion of the investigator. The TIMI Myocardial Perfusion Grade (TMPG) was assessed and digital subtraction angiography (DSA) was used to quantify brightness of the myocardial blush. RESULTS: TIMI 3 flow was 54.2% (64/118) before stent placement, and improved to 87.2% (102/117, p < 0.001) following stent placement. Likewise, Corrected TIMI Frame Counts (CTFCs) improved from medians of 37.6 to 21 (p < 0.001). By DSA, the rate of growth in brightness also tended to be greater after stenting (2.3 +/- 0.4 Gray/sec, n = 54 vs 3.1 +/- 0.3, n = 54, p = 0.07). The incidence of TMPG 0 decreased following stent placement (25.2% (29/118) vs 14.3% (16/118), p = 0.03) and the incidence of a stain in the myocardium (TMPG 1) increased (13.5% (16/118) vs 28.6% (34/118), p = 0.004). CONCLUSION: Adjunctive stenting following thrombolytic administration in AMI improves epicardial TIMI 3 flow and TIMI frame counts as well as dye inflow into the myocardium: TMPG 0 is reduced and myocardial blush measured quantitatively by DSA tends to be brighter. However, more TMPG 1 or dye staining was present on next injection, suggesting dye outflow may be impaired.

Pharmacokinetics of rhuMAb CD18, a recombinant humanised monoclonal antibody fragment to CD18, in normal healthy human volunteers.

BACKGROUND AND OBJECTIVES: Leucocyte 2 integrin adhesion receptors are hypothesised as a therapeutic target to modify immune responses to ischaemia-reperfusion injury that may be detrimental to recovery in a variety of disease states. Two phase I studies were designed to evaluate the pharmacokinetics, immunogenicity and safety of rhuMAb CD18, a humanised monoclonal antibody F(ab')(2) fragment to the CD18 receptor, in normal healthy human volunteers. STUDY DESIGN AND METHODS: The first study evaluated six escalating doses of rhuMAb CD18 (0.06, 0.12, 0.25, 0.5, 1.0, 2.0 mg/kg) in 36 subjects given two intravenous (IV) bolus injections 12 hours apart. In the second study, 16 subjects received IV doses of 1.0 and 2.0 mg/kg as a single dose or as two doses given 12 hours apart. Study endpoints were rhuMAb CD18 serum pharmacokinetics, change in white blood cell (WBC) count, and safety and tolerability. The two studies enrolled a total of 53 subjects. RESULTS: Serum concentration-time profiles demonstrated a monophasic decline and were best characterised by a one-compartment pharmacokinetic model. At the doses administered, the volume of distribution approximated the serum volume (range of means: 42 to 58 ml/kg). The serum clearance decreased with increasing dose until becoming consistent at doses of 0.5 to 2.0 mg/kg (range of means: 3.1 to 5.0 ml/h/kg). At doses of 0.5 to 2.0 mg/kg, the mean elimination half-life ranged from 7.0 to 9.6 hours. WBC counts increased at doses of above 0.06 mg/kg, returning to within 20% of predose values by day 7. Antibodies to rhuMAb CD18 were not detected at day 28. Mild-to-moderate adverse events were observed in both the placebo and treated groups, and were limited to flu-like symptoms. One subject experienced a serious adverse event (febrile reaction) and recovered with minimal intervention. There was no evidence of an increase in infection in subjects who received rhuMAb CD18. CONCLUSIONS: Upon IV bolus administration, rhuMAb CD18 serum concentration-time data fit a one-compartment pharmacokinetic model. At doses of 0.5 to 2.0 mg/kg, the pharmacokinetics were linear and the half-life ranged from 7.0 to 9.6 hours with a volume of distribution that approximated the serum volume. No antibodies to rhuMAb CD18 were detected. A transient, dose-dependent increase in the WBC count was observed, consistent with the expected effect of rhuMAb CD18 on leucocyte demargination. No increase in infection was observed. rhuMAb CD18 administered by IV bolus was well tolerated, with the exception of one febrile reaction.

Smoking and mortality following acute myocardial infarction: results from the National Registry of Myocardial Infarction 2 (NRMI 2).

BACKGROUND: Current smokers have lower mortality following acute myocardial infarction (AMI) than non-smokers. This is often referred to as the "smokers' paradox". Our study explored possible explanations of this phenomenon. METHODS: From the 510,044 cases of AMI in the NRMI 2 from 1 June 1994, through 30 April 1997, 297,458 cases without hospital transfer were analyzed. Characteristics and treatments of tobacco smokers and non-smokers were compared before and after age-standardization. Multivariate logistic models investigated possible associations with in-hospital mortality using clinically relevant variables and interaction terms. RESULTS: Twenty-four per cent of AMI cases were current smokers. Smokers were 14 years younger than non-smokers (mean age 58 vs. 72 years, p<0.001) and had lower in-hospital mortality (8.0% vs. 16.4%, p<0.001). After age-standardization, smokers were more likely than non-smokers to suffer a Q-wave type of infarction, and were less likely to have a prior history of diabetes, hypertension, AMI, angina, cardiac failure, and coronary interventions. The unadjusted odds ratio (OR) for smoking and mortality was 0.44 (95% confidence interval, CI 0.43-0.45). After adjustment for age the OR was 0.81 (95% CI 0.78-0.83). Additional adjustment for previous medical history/cardiovascular risk factors changed the OR to 0.86 (95% CI 0.83-0.89). Adjustment for additional covariates and interaction terms had little effect. CONCLUSIONS: Smokers with AMI were on average 14 years younger than non-smokers, explaining most of the apparent association of smoking with differences in presentation and treatment, and lower in-hospital mortality. The residual association of smoking and better prognosis, the "smoker's paradox", was not fully explained by measured covariates.

TIMI myocardial perfusion grade and ST segment resolution: association with infarct size as assessed by single photon emission computed tomography imaging.

BACKGROUND: The TIMI myocardial perfusion grade (TMPG) and ST-segment resolution both reflect perfusion and are associated with mortality after thrombolysis for acute myocardial infarction. We hypothesized that these measures would also be associated with infarct size by single photon emission computed tomography (SPECT). Methods and Results- In the LIMIT AMI trial (Limitation of Myocardial Injury following Thrombolysis in Acute Myocardial Infarction) of lytic monotherapy versus lytic plus rhuMAb CD18, early 90-minute TMPG (n=221) and ST segment resolution (n=242) were compared with subsequent SPECT Technetium-99 m Sestamibi, measuring the percentage of the left ventricle with no Sestamibi uptake. Infarct sizes were larger with TMPG 0 or 1 (a closed or stained myocardium) than with TMPG 2 or 3 (open myocardium, median 13% versus 7%, P=0.004). Infarcts were also larger in patients with no ST segment resolution (median 15%) or incomplete resolution (11%) than in those with complete resolution (6%, overall P=0.0001). The difference in infarct size by TMPG persisted when stratified by category of ST resolution. CONCLUSIONS: There may be a pathophysiological link between early restoration of tissue-level perfusion and reduced subsequent infarct size that may partially explain why these early angiographic and electrocardiographic measures are associated with long-term survival.

Modeling nicotine arterial-venous differences to predict arterial concentrations and input based on venous measurements: application to smokeless tobacco and nicotine gum.

Significant arterio-venous differences in nicotine concentrations have been observed during and after cigarette smoking, nicotine nasal spray, and intravenous nicotine administration. In this paper we describe a novel mathematical method for estimating arterial blood levels from venous blood level data. The model allows to quantify: (i) the influence of the microcirculation in the hands and forearm on the distribution of nicotine, and (ii) the influence of disregarding the venous to arterial circulation in the estimate of systemic inputs. We also (iii) propose a general method to predict arterial concentrations and inputs given venous data. The basic model we adopt is based on the relationship Cv = T * Ca, where Cv and Ca are the concentration in the venous and arterial site, respectively, T is the arterio-venous transfer function and * indicates convolution. We use empirical data to estimate T. We then compare estimates of systemic inputs to the venous site obtained taking into account the transfer function or, as usually done, disregarding it. The relationship we use to compare estimated inputs are: Cv = T * ka * A (where Ka is the arterial disposition function and A the systemic input), and Cv = Kv * A (where Kv is the venous disposition function), respectively. Finally, the estimated transfer function allows to estimate (average) Ca or A given arbitrary venous data. (i) Our analysis suggests that a bi-exponential T is needed to describe observed arterial-venous differences. The estimated transfer function indicates that no elimination of nicotine is involved in the forearm. (ii) Disregarding T, as usually done, erroneously obtains too complex venous input functions (because these input functions incorporate T). (iii) Disregarding T erroneously estimates significantly higher total inputs. (iv) Using the proposed model and previously published venous nicotine level data we predict substantial arterial-venous differences in blood nicotine levels for smokeless tobacco and nicotine gum. The use of disposition functions obtained from venous data may lead to erroneous estimation of the rates of entry into the circulation and systemic bioavailability for many drugs.