Main objective: effect of oral sildenafil at 50 and 100 mg on SNP iontophoresis-induced hyperaemia
The higher dose of sildenafil (100 mg), but not the lower dose (50 mg), increased the overall response to SNP iontophoresis. The corresponding AUC values were 101 100 ± 70 415% CVCmax·s without sildenafil, 95 212 ± 62 119% CVCmax·s after 50 mg sildenafil and 146 285 ± 63 315% CVCmax·s after 100 mg sildenafil (P= 0.03 for 100 mg vs. control; Figure 2). Similarly, 100 mg, but not 50 mg, sildenafil increased the peak response to SNP iontophoresis (peak CVC 49.5 ± 18.9% CVCmax without sildenafil, 46.6 ± 20.3% CVCmax after 50 mg sildenafil and 63.9 ± 24.4% CVCmax after 100 mg sildenafil; P= 0.05 for 100 mg vs. without control). Time to peak of SNP iontophoresis was unchanged after sildenafil (17.2 ± 8.1 min without sildenafil, 20.3 ± 9.1 min after 50 mg sildenafil and 17.3 ± 7.4 min after 100 mg sildenafil). Finally, residual flux at 60 min remained higher under sildenafil at 100 mg (13.2 ± 12.7% CVCmax without sildenafil, 7.6 ± 20.2% CVCmax after 50 mg sildenafil and 25.5 ± 10.9% CVCmax after 100 mg sildenafil; P= 0.05 vs. without sildenafil).
Figure 2. Effect of 50 or 100 mg oral sildenafil combined with forearm SNP iontophoresis (20 µA during 20 min) on forearm CVC (mV·mm Hg−1). Oral sildenafil was taken 1 h before the iontophoresis onset. CVC was expressed as a percentage of maximal CVC at 44°C. Data from an adjacent control region of interest were subtracted from the iontophoresis region of interest to take into account any potential basal CVC variation induced by sildenafil alone. The increased CVC following SNP iontophoresis was significantly higher after 100 mg oral sildenafil than control (P= 0.03), as measured by the AUC over 60 min.
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No serious side effects occurred. We were able to give 100 mg sildenafil to all subjects. While sildenafil decreased MAP 1 h after oral intake, no individual variation in MAP was observed during the 20 min of SNP cutaneous iontophoresis, with or without sildenafil (Figure 4). At V4 (sildenafil 50 mg plus SNP iontophoresis), 10 min after the end of SNP iontophoresis, one woman exhibited pallor and dizziness associated with a 10 mm Hg drop in MAP from baseline (systolic/diastolic arterial pressure at 100/52 mm Hg at baseline, at 85/48 mm Hg during the symptoms). Her glycaemia was normal. She recovered after infusion of 100 mL NaCl 0.9%, exhibited no further side effect during the next 2 h and was able to leave the Clinical Research Center. The safety committee considered this event as not serious, but possibly related to the combination of sildenafil plus SNP iontophoresis. With her approval, she returned for the V5 and V6 (sildenafil 100 mg without and with SNP iontophoresis, respectively), where she exhibited no side effects.
Figure 4. Effect of 50 or 100 mg oral sildenafil with or without forearm SNP iontophoresis on MAP. Oral sildenafil was taken 1 h before the iontophoresis onset. Baseline refers to MAP before sildenafil was taken. Data from baseline to SNP iontophoresis are not shown for clarity.
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No headaches occurred during V2 (SNP iontophoresis alone). In contrast, four, three, three and three volunteers reported headaches at V3–V6, respectively (NS between sildenafil groups). Paracetamol (1 g orally) was taken by one, three, two and two volunteers, respectively (NS). No ophthalmic symptoms were reported.
Discussion and conclusions
We have shown that 100 mg oral sildenafil potentiated the increase in cutaneous blood conductance induced by local SNP iontophoresis, in the forearm of healthy volunteers. This pharmacodynamic potentiation occurred without an increased frequency of headaches compared with sildenafil alone. Symptomatic arterial hypotension occurred in one subject receiving 50 mg sildenafil 30 min after skin SNP iontophoresis.
Microvascular function can routinely be studied in human skin using non-invasive laser Doppler imaging (Turner et al., 2008), and thermal hyperaemia is commonly used as a reference of maximal vasodilatation (Cracowski et al., 2006), such as in the present study. In our study, 50 and 100 mg of sildenafil did not alter maximal CVC 2 h after drug intake. Indeed, such results compare with those of Gooding et al. (2006) who showed that maximal thermal hyperaemia was unchanged following 100 mg sildenafil taken orally. However, this treatment did increase cutaneous flux 30 min after the end of the local heating, that is, sildenafil increased the duration of the hyperaemia due to local heating, but not its amplitude. We chose to provide data as CVC rather than flux to allow for potential variations in arterial blood pressure, and we further normalized conductance over the maximal hyperaemia, as this is less variable than expressing data as a percentage of baseline (Cracowski et al., 2006).
Topical delivery of SNP via iontophoresis, in combination with oral sildenafil, could be considered as an alternative method of administration to increase SNP concentration locally, thus avoiding the major systemic adverse effects of both drugs. In this study, SNP iontophoresis was performed 1 h after oral intake of sildenafil, in order to match the maximal sildenafil plasma concentration, which is close to 1 h (Wright, 2006). Sildenafil absorption decreases during meals, and for this reason we chose to study fasting volunteers, in order to decrease the variability. Iontophoresis of SNP is a commonly used technique to assess non-endothelium-dependent skin microvascular function. SNP is a pro-drug, releasing NO through both enzymatic and non-enzymatic pathways, which leads to an activation of guanylate cyclase and an increased production of cGMP in smooth muscle cells. When infused intravenously, SNP has an onset of action within 30 s, with a peak effect occurring within 2 min. In our study, the effect of SNP, using iontophoresis, was delayed compared to that observed during intravenous infusion, with a peak close to 10 min. The more plausible reason for the observed potentiation of SNP increase in CVC by sildenafil is their synergistic pharmacodynamic action. However, we cannot rule out the possibility that sildenafil decreased skin resistance. Such a phenomenon would enable an increased iontophoretic SNP flux through a clearly different mechanism. As shown by Ramsay et al. (2002), variability in skin resistance was shown to increase the variability of iontophoresis, with high skin resistance being associated with lower vasodilation. However, in terms of therapy, the result would be similar, as it would tend to increase the vasodilator response to SNP, although through another mechanism.
Iontophoresis can cause non-specific axon reflex vasodilation. Lidocaine/prilocaine cream, applied for 1 h prior to iontophoresis, was therefore used in order to avoid such an axon reflex response during SNP iontophoresis (Cracowski et al., 2007). Indeed, using lidocaine/prilocaine cream as an anaesthetic inhibits the trigger for axon reflex (C fibre action potentials). As a consequence, the effect observed is not associated with a non-specific axon reflex, but with the effect of the vasodilating substances themselves. In addition, SNP iontophoresis data were expressed as values from the iontophoresis region of interest minus the value from the adjacent region of interest to correct for any effect of oral sildenafil. The adjacent region of interest was within the anaesthetized zone. Local thermal equilibrium might have been slightly different from that within the chamber, and thus this ‘adjacent area’ was not perfect. However, the effects of sildenafil alone, as shown in Figure 3, were minimal in comparison with those of SNP iontophoresis. This means that the potentiation observed after 100 mg sildenafil was not related to the baseline effect of sildenafil.
Oral sildenafil at 100 mg, but not at 50 mg, increased baseline CVC in the forearm. However, with a sample size of 10 volunteers, the study may have been under-powered to detect any effect with 50 mg. Indeed, such a short-term microvascular effect on skin vasculature is well known, as 50 mg oral sildenafil increased resting and post-ischaemic cutaneous capillary circulation in patients with coronary artery disease 1 h later (by 47%) (Park et al., 2004). In addition, mean nail-fold capillary flow velocity was increased by more than 400% in patients treated with 50 mg sildenafil twice daily for 4 weeks (Fries et al., 2005). Sildenafil improved the clinical condition and the anemometric flow velocity in nail-fold capillaries of patients with non-scleroderma Raynaud's phenomenon (Fries et al., 2005), and was associated with thermographic and symptomatic improvement in patients with systemic sclerosis and Raynaud's phenomenon (Kumar et al., 2006) or in severe digital ischaemia (Kumana et al., 2004). In some series, the effects of sildenafil on Raynaud's phenomenon or digital ulcers in systemic sclerosis were spectacular and appeared within a few weeks (Gore and Silver, 2005). Regarding the side effects, headache, facial flushing and nasal congestion are the most common adverse events observed following oral administration. Headaches after sildenafil occurred as expected, but were not increased by SNP iontophoresis. Decreased MAP was observed, as expected, 1 h after oral administration of sildenafil (Figure 4), without any significant additional effect of SNP iontophoresis. However, the MAP drop associated with dizziness and pallor that occurred 10 min after the end of SNP iontophoresis (i.e. 30 min after the start of SNP iontophoresis), in a young fasting woman, was possibly related to a pharmacodynamic interaction, but the cause was uncertain given the discrepancy between the 30 min delay and the short half-life of SNP when given intravenously (a few minutes). Re-challenge (100 mg sildenafil plus SNP iontophoresis) on a different day was negative. However, such a side effect should be considered as being potentially related to the association, and merits further safety studies.
In conclusion, 100 mg oral sildenafil potentiated local skin hyperaemia induced by iontophoresis of SNP, with no increased incidence of headaches. The combination of oral specific PDE5A inhibitors and nitrates administered through skin iontophoresis should be further investigated in diseases such as severe Raynaud's phenomenon. However, further studies are required to determine the incidence of arterial hypotension induced by this combination.