One hundred and one patients with stable angina awaiting elective angiography were screened and thirty-one patients were found to be eligible and gave informed written consent. Out of these, nine patients had non-obstructive coronary arteries and therefore, did not require stenting. Two further patients had complex coronary anatomy: one requiring left main bifurcation stenting and the other had surgical revascularization, and therefore were excluded from our study.

Inclusion criteria included patients undergoing elective PCI for stable angina; age above 18 years; and able to give informed consent for the study. Exclusion criteria included any severe co-morbidity with expected life expectancy < 6 months; use of warfarin, nicorandil, glibenclamide, sitagliptin, vildagliptin, saxagliptin, linagliptin, alogliptin, exenatide, liraglutide, lixisenatide and insulin use; women of child-bearing age; breast-feeding women; myocardial infarction within the previous 3 months in a remote territory; heart failure with ejection fraction < 50%; deranged renal function with eGFR < 60 ml/min/1.73 m² by Modification of Diet in Renal Disease (MDRD); deranged liver function with alanine transaminase (ALT) > 3 times upper limit of normal; active peptic ulcer disease confirmed on endoscopy; history of seizures; history of tachyarrhythmias; patients already taking oral theophylline; allergy to theophylline or caffeine.

Twenty patients having percutaneous coronary intervention (PCI) were studied in two groups: those receiving post-PCI infusions of GLP-1 + Theophyline (Group GT) and Theophylline (Group T) respectively. Data from these two groups were compared with historically-recruited patients who received GLP-1 infusion (Group G) and placebo (normal saline infusion) (Group P) [15]. Theophylline infusion was used with and without GLP-1 as an adenosine receptor antagonist to determine any adenosine mediated effect of GLP-1.

All patients received aspirin, 300 mg and clopidogrel, 300 mg preloading, unless they were already established on these antiplatelets. Patients were anticoagulated with a heparin bolus (70–100 U/kg) after arterial sheath insertion (radial or femoral) to achieve an activated clotting time > 250 s. Iopromide (Ultravist; Bayer HealthCare Pharmaceuticals, Leverkusen, Germany) was used as the contrast agent for all cases. The choice of stent and implantation technique was left to operator discretion. Following successful stent implantation baseline bloods were taken to measure serum adenosine levels using a Stop solution.

A Pressure wire X (Abbott Vascular, Santa Clara), connected wirelessly to Coroflow (Coroventis, Uppsala), was positioned and maintained in the distal third of the stented coronary artery. A 0.2 mg bolus of intracoronary glyceryl trinitrate (GTN) was administered, and once steady state coronary haemodynamics were achieved, the baseline coronary pressures [aortic pressure (Pa) and distal wire pressures (Pd)] and flow velocity measurements were measured. The latter was derived from the reciprocal of mean transit time (Tmn) of an intracoronary injectate of room temperature saline (thermodilution technique) measured in triplicate [17, 18]. These measurements were repeated following intravenous administration of adenosine at 140 mcg/kg/min. Coronary wedge pressure (Pw) was measured separately as Pd during the occlusive coronary balloon inflation.

An intravenous infusion of GLP-1 (1.2 pmol/kg/min)(7–36) amide (Bachem AG, Switzerland) and an adenosine receptor inhibitor, theophylline (5 mg/kg in 100 ml 0.9% NaCl over 20 min) or GLP-1 or Theophylline (5 mg/kg in 100 ml 0.9% NaCl over 20 min) (Hameln pharma Ltd; UK) or Placebo (100 ml 0.9% NaCl over 20 min) was infused depending upon patient’s group. At the end of infusion, a repeat blood sample was taken from the coronary catheter to measure theophylline and adenosine levels. All the haemodynamic measurements were repeated at rest and hyperaemia after completion of the infusion, usually within 30-min of baseline. At the end of the procedure, the pressure wire was withdrawn to the coronary ostium to enable pressure-drift correction of Pd, if necessary. Pv was assumed to be 5 mmHg in all the patients in this study.

These measurements enabled offline calculation of, basal microvascular resistance (BMR = Pa × Tmn × ((Pd − Pw)/(Pa − Pw))_baseline) and index of microvascular resistance (IMR = Pa × Tmn × ((Pd − Pw)/(Pa − Pw)) _hyperaemia), both corrected for collaterals, fractional flow reserve, (FFR = (Pd)/(Pa)_hyperaemia), coronary flow reserve (CFR = (Tmn) _baseline/(Tmn)_hyperaemia) and collateral flow index by pressure (CFI_P = (Pw − Pv)/(Pa − Pv)_baseline) and coronary resistive reserve ratio (RRR = BMR/IMR), as previously described and validated [19, 20] (Fig. 2).Fig. 2

Example of Coroventis screen during invasive haemodynamic assessment of coronary artery. Blue curves show resting Tmn, orange showing hyperaemic Tmn. Resting as well as hyperaemic pressure and flow indices including Pa, Pd, FFR, IMR and CFR are displayed in the upper right-hand corner

Adenosine has a very short half-life and therefore we used a previously published composition of Stop solution to prevent enzymatic breakdown of the extracted serum adenosine samples [21, 22] Blood samples to determine adenosine concentration were collected from the guide catheter, at the completion of PCI and after 20 min of study drug infusion, and placed directly into vacutainers containing Stop solution. This comprised dipyridamole 0.2 mmol/L, 4.2 mmol/L ethylene-diamine-tetraacetic acid disodium (Na2 EDTA), erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA) 5 mmol/L, α,β-methyleneadenosine-5′-diphosphate (AMPCP) 79 mmol/L, heparin sulfate 1 IU/mL, deoxycoformycin 1 μg/mL in 0.9% NaCl, all sourced from Merck, UK. After centrifugation, supernatants were deproteinized, and serum adenosine concentration measured by high-performance liquid chromatography. Similarly, blood for theophylline levels was collected from coronary arteries just before the end of infusion and analyzed to confirm therapeutic levels.

On the basis of previous data, we calculated that 10-paired data sets would provide 80% power to detect a clinically significant difference (ΔBMR, 20 mmHg s; SD, 15 mmHg s) after administration of GLP‐1.

Data are given as mean (SD) or median (Q1, Q3) as appropriate unless otherwise stated. Comparisons were made for any significant differences by unpaired T test, one-way ANOVA or Kruskal–Wallis test, where appropriate using GraphPad Prism version 8.1.2 (227) (GraphPad Software, La Jolla California USA). Similarly, a simple linear regression was performed between resting adenosine levels and basal coronary flow velocity before and after the study infusion to explore any correlation. A two-sided value of p < 0.05 was deemed significant. Authors had full access to the data and take full responsibility its integrity.

Baseline characteristics are summarized in Table 1. All four groups (G, P, GT and T) were well matched although the GT group had more female patients and fewer patients receiving GLP-1 were taking an ACE-inhibitor or angiotensin receptor blocker. Of note, the majority of patients recruited into this study were not diabetic.Table 1

Baseline characteristics

Haemodynamic data are summarized in Table 2 and illustrated in Fig. 2. There were no differences in the baseline or post-infusion heart rate and blood pressure between all the four groups. Similarly, there were no differences in CFR and FFR immediately after stenting and following infusion. The basal microvascular resistance (BMR) before infusion was similar in all groups. BMR was significantly lower in the groups receiving GLP-1 (G and GT) after infusion compared to baseline: delta BMR − 20.83 (24.8) and − 21.20 (30.1), p = 0.97 respectively, confirming that theophylline did not attenuate the GLP-1 vasodilatory effect in group GT (Fig. 2). The lower BMR was attributed to a significantly lower (faster) resting Tmn after infusion in the G and GT groups: − 0.23 (0.27) and − 0.18 (0.37) respectively, whereas it was essentially unaltered in group T, 0.04 (0.15), p < 0.001. GLP-1 exhausted the vasodilatory capacity of the microvasculature, with delta median resistive reserve ratio (RRR) in group G: − 1.00 (− 2.95, − 0.01) and in group GT: − 0.88 (− 3.79, − 0.27) compared to 2.01 (0.23, 3.57) in group S and − 0.16 (− 1.06, 0.61) in group T, p = 0.03, (Fig. 3). Similarly, delta CFR was lower in group G: − 0.89 (− 2.01, 0.97) versus the other three groups. There was no significant difference in IMR or delta IMR across all the four groups.Table 2

Haemodynamic data of study patients

The mean theophylline levels measured at the end of infusion were in the therapeutic range (10–20 mcg/ml) and similar between GT and T groups: 13.58 mcg/ml (4.48) versus 15.05 mcg/ml (2.15), p = 0.40. There was no difference in delta median adenosine levels after infusion between the two groups: GT, − 2.00 ng/ml (− 117.1, 14.8) versus T, − 0.50 ng/ml (− 19.6, 9.4), p = 0.60.

There was no correlation found in the measured adenosine levels and basal coronary flow velocity, before or after the study infusion (Additional file 1).