Reproducibility of forearm vasodilator response to intra-arterial infusion of calcitonin gene-related peptide assessed by venous occlusion plethysmography

First-in-human development of a pharmacodynamic biomarker for PAC1 receptor antagonists using intradermal injections of maxadilan

Maxadilan, a potent vasodilator peptide, selectively activates the PAC₁ receptor, a promising target for migraine therapy. Therefore, maxadilan has been suggested as a tool to study the pharmacodynamics (PDs) of PAC₁ receptor antagonists. The objectives of this first-in-human study were to: (1) determine the safety, tolerability, dose response, and time course of the dermal blood flow (DBF) changes after intradermal (i.d.) injections of maxadilan in the human forearm, and (2) assess the inter-arm and inter-period reproducibility of this response. This was a single-center, open-label study in healthy subjects, comprising three parts: (1) dose-response (n = 25), (2) response duration (n = 10), and (3) reproducibility (n = 15). DBF measurements were performed using laser Doppler imaging (LDI) up to 60 min postinjection, or up to 5 days for the response duration assessments. To assess reproducibility, the intraclass correlation coefficient (ICC) and sample sizes were calculated. The i.d. maxadilan (0.001, 0.01, 0.1, 0.9, 3, and 10 ng) produced a well-tolerated, dose-dependent increase in DBF, with a half-maximal effective concentration fitted at 0.0098 ng. The DBF response to 0.9 ng maxadilan was quantifiable with LDI up to 72 h postinjection. The inter-period reproducibility of the DBF response was better upon 0.9 ng (ICC > 0.6) compared to 0.01 ng (ICC < 0.4) maxadilan. However, irrespective of the study design or maxadilan dose, a sample size of 11 subjects is sufficient to detect a 30% difference in DBF response with 80% power. In conclusion, intradermal maxadilan provides a safe, well-tolerated, and reproducible PD biomarker for PAC₁ receptor antagonists in vivo in humans.

Development of anti-migraine therapeutics using the capsaicin-induced dermal blood flow model

The efficacy of calcitonin gene-related peptide (receptor) (CGRP-(R)) blocking therapeutics in the treatment of acute migraine headache provided proof-of-concept for the involvement of CGRP in the pathophysiology of this disorder. One of the major hurdles for the development of any class of drugs, including CGRP blocking therapeutics, is the early clinical development process during which toxic and inefficacious compounds need to be eliminated as early as possible in order to focus on the most promising molecules. At this stage, human models providing proof of target engagement, combined with safety and tolerability studies, are extremely valuable in focusing on those therapeutics that have the highest engagement from the lowest exposure. They guide the go/no-go decision making, establish confidence in the candidate molecule by de-risking toxicity and safety issues and thereby speed up the early clinical development. In this review the focus is on the so called 'capsaicin model' as a typical example of a target engagement biomarker used as a human model for the development of CGRP blocking therapeutics. By applying capsaicin onto the skin, TRPV1 channels are activated and a CGRP-mediated increase in dermal blood flow can be quantified with laser Doppler perfusion imaging. Effective CGRP blocking therapeutics in turn, display blockade of this response. The translation of this biomarker model from animals to humans is discussed as well as the limitations of the assay in predicting the efficacy of anti-migraine drugs.

Endothelial function in migraine: a cross-sectional study

Background

Migraine has been associated with cardiovascular disorders. Endothelial dysfunction may be a mechanism underlying this association. The present study tested the hypothesis that endothelium-dependent vasodilation, basal endothelial nitric oxide release and endothelial fibrinolytic capacity are impaired in migraine patients.

Methods

Graded doses of sodium nitroprusside (SNP, 0.2 to 0.8 μg.min^-1.dL^-1 forearm), substance P (0.2 to 0.8 pmol.min^-1.dL^-1 forearm) and N^G-monomethyl-L-arginine (L-NMMA, 0.1 to 0.4 μmol.min^-1.dL^-1 forearm) were infused into the brachial artery of 16 migraine patients with or without aura during a headache-free interval and 16 age- and sex-matched subjects without a history of migraine. Forearm blood flow (FBF) was measured by strain-gauge venous occlusion plethysmography. Local forearm release of tissue plasminogen activator (t-PA) in response to substance P infusion was assessed using the arteriovenous plasma concentration gradient. Responses to infused drugs were compared between patients and matched controls by analysis of variance.

Results

In both migraine patients and control subjects, SNP and substance P caused a dose-dependent increase, and L-NMMA a dose-dependent decrease in FBF (P < 0.001 for all responses). In both groups, substance P caused an increase in t-PA release (P < 0.001). FBF responses and t-PA release were comparable between migraine patients and control subjects.

Conclusions

The absence of differences in endothelium-dependent vasodilation, basal endothelial nitric oxide production and stimulated t-PA release between migraine patients and healthy control subjects argues against the presence of endothelial dysfunction in forearm resistance vessels of migraine patients.

Intra- and inter-tester reproducibility of venous occlusion plethysmography: comparison between a manual and a semi-automatic method of blood flow analysis

Venous occlusion plethysmography (VOP) is a valid non-invasive method to assess peripheral blood flow (BF) in humans. Our aim was to determine intra- and inter-tester reproducibility of BF analysis using a traditional manual method and a novel system, based on a semi-automatic approach. Ten healthy subjects and ten subjects with chronic heart failure (CHF) were evaluated. Blood flow was measured on the forearm at baseline and after 5 min of circulatory occlusion (reactive hyperemia (RH)). Two testers independently and blindly analyzed each VOP recording. Both methods were highly reproducible intra- and inter-testers. In addition, there was a high association between the methods, since the intraclass correlation coefficients (ICCs) for healthy subjects were 0.99, 0.99 and 0.99, and the coefficients of variation (CVs) were 1.8, 2.4 and 1.6% for baseline, RH peak and RH area under the curve, respectively. For CHF subjects, the ICCs were 0.99, 0.98 and 0.99, and the CVs were 2.9, 3.6 and 2.0%. In addition, the time spent on the semi-automatic analyses was shorter (p < 0.05). In conclusion, both methods demonstrated high intra- and inter-tester reproducibility for baseline and RH BF analysis. However, since the semi-automatic method was faster to generate the results, the present study supports its usage for the analysis of BF measured by VOP.

Analysis of short-term reproducibility of arterial vasoreactivity by pulse-wave analysis after pharmacological challenge

1. Pulse-wave analysis (PWA) is an established method to assess arterial wave reflections and arterial vasoreactivity in humans. A high short-term reproducibility of baseline augmentation index (AIx) has been reported. However, the short-term reproducibility of AIx changes following pharmacological challenge with either inhaled salbutamol (endothelium-dependent vasodilatation) or sublingual glyceryl trinitrate (GTN; endothelium-independent vasodilatation), using appropriate statistical methods, is largely unknown. 2. Baseline AIx and GTN- and salbutamol-mediated changes in AIx (all corrected for a heart rate of 75 b.p.m.) were measured on two separate occasions, 1 h apart, in 22 healthy controls (mean (+/-SD) age 52.0 +/- 13.4 years) and 11 elderly patients with chronic heart failure (CHF; 73.1 +/- 8.7 years). Reproducibility was assessed by measuring intraclass correlation coefficients (ICC), coefficients of variation (CV) and Bland-Altman plots. 3. Baseline AIx showed good short-term reproducibility with high ICC in both the control and CHF groups (0.90 and 0.87, respectively). In contrast, in the control and CHF groups, the ICC of GTN- (0.58 and 0.17, respectively) and salbutamol-mediated (0.18 and 0.04, respectively) changes in AIx were substantially low. The CV was relatively low for baseline AIx in control and CHF groups (25.0 and 22.5%, respectively), but not for GTN- (22.3 and 59.8%, respectively) or salbutamol-mediated (45.1 and 184.0%, respectively) changes in AIx. Bland-Altman analysis revealed poor reproducibility, with limits of agreement beyond either +15% or -15% for changes in AIx after GTN and salbutamol for both control and CHF groups. The changes in blood pressure and heart rate following pharmacological challenge were similar between the two measurements. 4. The poor reproducibility of changes in AIx following pharmacological challenge questions the use of this method in acute studies.

Calcitonin gene-related peptide8-37 antagonizes capsaicin-induced vasodilation in the skin: evaluation of a human in vivo pharmacodynamic model

The purpose of this study was to identify the mediators involved in capsaicin-induced vasodilation in the human skin and to evaluate a pharmacodynamic model for the early clinical evaluation of calcitonin gene-related peptide (CGRP) receptor antagonists. Dermal blood flow (DBF) response of the forearm skin to topically applied capsaicin was measured using laser Doppler perfusion imaging in 22 subjects. The effect of intra-arterially administered CGRP(8-37) (1200 ng . min(-1) . dl(-1) forearm), indomethacin (5 mug . min(-1) . dl(-1) forearm), and N(G)-monomethyl-l-arginine (l-NMMA; 0.2 mg . min(-1) dl(-1) forearm), and orally administered aprepitant (375 mg) on capsaicin-induced dermal vasodilation was assessed. Furthermore, the diurnal variation of the DBF response to capsaicin was studied. CGRP(8-37) inhibited the capsaicin-induced DBF increase: 217(145, 290)% in infused versus 370 (254, 486)% in the noninfused arm [mean (95% CI); p = 0.004]. In contrast, indomethacin, l-NMMA, aprepitant, and the time of assessment did not affect the DBF response to capsaicin. Thus, capsaicin-induced vasodilation in the human forearm skin is largely mediated by CGRP, but not by vasodilating prostaglandins, nitric oxide, or substance P. The response to capsaicin does not display a circadian rhythm. A pharmacodynamic model is proposed to evaluate CGRP receptor antagonists in humans in vivo.

Reproducibility of the capsaicin-induced dermal blood flow response as assessed by laser Doppler perfusion imaging

AIMS

Part I: to establish the dose and appropriate application site of capsaicin on the human forearm in order to produce a robust and reproducible dermal blood flow (DBF) response. Part II: to evaluate the within-subject arm-to-arm and period-to-period reproducibility.

METHODS

Both parts consisted of two study visits. In part I, placebo and 100, 300 and 1000 microg capsaicin were applied at four predefined sites on the volar surface of both forearms. Placebo and capsaicin doses were randomized and balanced by site between subjects. Changes in DBF were assessed by laser Doppler perfusion imaging up to 60 min after capsaicin application. In part II, only 1000 microg capsaicin was applied on the proximal forearm and changes in DBF assessed up to 30 min (t(30)). DBF response was expressed as percent change from baseline +/- SD and the corresponding AUC(0-30). Reproducibility assessment included calculation of the concordance correlation coefficient (CCC).

RESULTS

Part I (n = 12 subjects): compared with placebo, 300 and 1000 microg capsaicin increased DBF (P < 0.05) at all time points except at 10 min. This increase was reproducible at the two most proximal sites from the 30-min time point onwards when compared between arms (CCC >or= 0.8, i.e. substantial to almost perfect reproducibility). In part II (n = 11), t(30) averaged 390 +/- 120% and arm-to-arm reproducibility was almost perfect (CCC = 0.91) for AUC(0-30).

CONCLUSIONS

Capsaicin induces a reproducible within-subject arm-to-arm increase in DBF. We provide a non-invasive pharmacodynamic model in humans to test antagonists of mediators involved in capsaicin-induced dermal vasodilation, including calcitonin gene-related peptide antagonists.