Chronic myocardial ischaemia and ventricular remodelling after MI are important causes of HF. In recent years, gene therapy based on angiogenic factors has become a potential method to promote angiogenesis, inhibit ventricular remodelling, and improve cardiac function [22, 23]. The cytokines that have been studied in preclinical and clinical research include VEGF, HGF, fibroblast growth factor, and insulin-like growth factor, and some studies have achieved encouraging results [24, 25]. Recently, Wang et al. [26] injected adenovirus-mediated VEGF165 into the peri-infarct myocardium in an MI rat heart, and their results suggested that VEGF165 gene therapy could improve cardiac function by inducing angiogenesis and inhibiting cardiomyocyte apoptosis.
HGF and its potential angiogenic, anti-apoptotic, and anti-fibrotic effects [27, 28] have been extensively studied in various models of ischaemic disease, such as MI [29] and peripheral artery occlusion disease [30]. A Phase III clinical trial of intramuscular injection of plasmid HGF for the treatment of severe limb ischaemia has been successful [31], suggesting that HGF may also be used for the treatment of IHD. The biological function of HGF is mediated by its unique tyrosine kinase receptor c-Met [32]. Activation of the c-Met receptor further activates many intracellular signalling pathways, including RAS-mitogen activated protein kinase, signal transducer and activator of transcription, phosphatidylinositol-3 kinase, protein kinase B, mammalian target of rapamycin, and β-catenin pathway [33, 34]. Our group has previously shown that HGF therapy could promote cardiac repair and improve cardiac function in MI rats through the above mechanisms [35].
Although HGF has been widely studied in some large animal MI models, the clinical trials related to HGF therapy have just started [36], which prompted us to conduct a meta-analysis on the preclinical data. Our study showed that the cardiac pump function of the HGF group was significantly better than that of the control group within 1-2 months after MI, with an increase in LVEF of about 9.73%. In addition to the improvement of LVEF, local application of HGF promoted angiogenesis and increased blood supply to the ischaemic area. Compared with the control group, the capillary density in the HGF treatment group was significantly increased (about 97.33 capillaries/mm2 difference). Our study fully demonstrates that HGF treatment after MI can promote angiogenesis and improve the cardiac pump function. Neovascularization can improve the blood supply in ischaemic areas, provide preconditions for the tissue repair of the heart and, to some extent, reverse the fibrosis induced by hypoxia [12].
In terms of vectors and routes of delivery, plasmids, adenoviruses and injectable hydrogels have been widely accepted for gene therapy [37]. As mature and traditional vectors, plasmid- and adenovirus-mediated HGF gene transfer can significantly improve HGF expression in myocardial tissue. Wang et al. [15] transferred Ad5-HGF (4×109 pfu) into the myocardium via the right coronary artery four weeks after left anterior descending coronary artery (LAD) ligation in pigs, and detected the expression level of HGF by ELISA three weeks later. The results showed that the expression level of HGF in the experimental group increased to nearly 18 times that of the control group (109.3±7.8 vs 6.2±2.6), and the LVEF value increased by nearly 45% (43.9±4.3 vs 30.4±2.8). Similarly, Funatsu et al. [38] injected 125 microg of plasmid encoding human HGF (hHGF) into the peri-infarct regions of canines four weeks after LAD ligation, and the expression of hHGF was specifically detected by ELISA after four weeks (endogenous HGF could not be detected). The results showed that hHGF expression in the HGF group was 4.7±1.7 ng/g, while no hHGF protein expression was found in the control group, and the number of capillaries in the ischaemic area of the HGF group also increased to 140% of that in the normal area. Therefore, both plasmids and adenoviruses as vectors can effectively improve the expression of HGF. In addition, studies have shown that local application of plasmid HGF or Ad5-HGF is reliable in terms of safety [39]. As illustrated by the above points, plasmids and adenoviruses are ideal vectors for gene therapy. Our analysis showed that intracoronary Ad5-HGF gene transfer and intramyocardial plasmid HGF injection had similar effects on cardiac pump function at conventional doses, and both treatments could bring significant improvement in LVEF (12.63±3.2 and 9.4±1.09; P=0.06). Injectable hydrogel is a promising synthetic biomaterial, with advantages such as mild gelation and cardiac-compatible properties [40, 41]. Recent studies have shown that the injection of hydrogel in ischaemic areas after MI could promote cardiac repair and improve cardiac function [42]. Hydrogels can also encapsulate therapeutic drugs and deliver them to target areas to achieve sustained therapeutic effects [43]. Steele et al. [44] encapsulated two cytokines, dimeric fragment of HGF and engineered stromal cell-derived factor 1α, in hydrogels and delivered them to the peri-infarct regions, and found that the release time of the cytokines was significantly prolonged and the therapeutic effects were enhanced when the cytokines were encapsulated in hydrogels. These unique advantages of hydrogels potentially make them a more attractive method of exogenous administration.
All of the studies included in this meta-analysis adopted local administration. The traditional route of local administration is either intracoronary delivery of therapeutic drugs through a catheter or intramyocardial injection through thoracotomy, which can be used locally in coronary artery bypass grafting [45]. Our analysis showed that intramyocardial injection of plasmid HGF had a stronger angiogenic capacity than intramyocardial injection of Ad5-HGF and HGF in hydrogel (P<0.00001). Recently, Shi et al. [46] designed phase-transition microneedles coated with adeno-associated virus (AAV), which achieved uniform distribution of AAV delivery and were superior to direct muscular injection. This new delivery system may further optimise the efficacy of intramuscular injection. The advantage of local administration is that it effectively avoids the accumulation of drugs in other tissues and makes the concentration of drugs in the heart reach its peak. The disadvantage is that it limits the convenience and repeatability of its application. In contrast, systemic administration is more convenient, but it is necessary to find and apply heart-specific vectors to reduce the distribution of drugs in other tissues and organs.
This meta-analysis had certain strengths and limitations. First of all, this is the first meta-analysis pooling data from preclinical large animal studies, which provides strong evidence for the feasibility of HGF therapy in the treatment of MI. A second strength was that we analysed the therapeutic effects and characteristics of different vectors and delivery routes, and introduced a new intramuscular delivery system, providing a methodological basis for HGF gene therapy and other drug gene therapy. In terms of limitations, some studies did not provide relevant statistical data (three without LVEF data, four without capillary density data), which may partially affect the accuracy of the results. Secondly, most studies only used western blot to qualitatively analyse the expression difference of HGF between the experimental group and the control group without quantifying HGF expression. Therefore, we were unable to compare the differences in the expression of HGF between studies. In addition, there was some methodological heterogeneity in the included studies; however, they all observed the effects of HGF on cardiac function and angiogenesis in the chronic phase of MI, so we relaxed this aspect of our inclusion criteria and conducted some valid subgroup analyses to address the issue.