A novel improved therapy strategy for diabetic nephropathy

Abstract

Diabetic nephropathy (DN), is a disorder that causes significant morbidity and mortality. Studies on the pathological mechanisms of DN reveal that advanced glycation end products (AGEs) play an important role in the pathogenesis of DN through interacting with receptors for advanced glycation end products (RAGE), which activate a series of intracellular signaling pathways. AGEs and RAGE have therefore been considered to be two potential key targets. Although multiple studies have been made for anti-DN therapy against AGEs or RAGE, the results have been disappointing due to poor effectiveness or to side effects in clinical practice. In this hypothesis article, we propose a novel treatment based on a dual-target approach. A kind of multi-functional intelligent nanoparticle is constructed, which has a core-shell nanoparticle structure to load the dual-target drugs (AGEs inhibitors and RAGE inhibitors), and has a functional “RAGE analog” to be used as “bait” to catch AGEs and target them to the kidney. Owing to its advantages of having a dual-target, synergistic effect and high efficiency, the proposition may have potential applications in DN therapy.

Background

Diabetic nephropathy (DN), a leading cause of end-stage renal disease,¹ is associated with increasingly significant morbidity and mortality. It is already known that several factors are incriminated in the genesis and progression of DN: these include advanced glycation end products (AGEs), protein kinase C, the renin-angiotensin system, transforming growth factor-β1, etc.^1-3 Among these, the AGEs play an especially important role in the pathogenesis of DN via multi-factorial mechanisms.^1,4,5

AGEs are formed through a series of reactions from Schiff bases and Amadori products to stable irreversible end products,⁶ which accumulate in glomerular basement membrane, mesangial cells, endothelial cells, and podocytes of the patients with DN and/or end-stage renal failure.⁷ Their receptors (RAGE) are expressed by mesangial cells, tubular cells, podocytes and endothelial cells.^8-11 Furthermore, the AGEs-RAGE interaction is considered as a causative factor for DN through activating a series of intracellular signal-cascade pathways^12-14 which might induce the generation of further signaling factors, such as vascular endothelial growth factor (VEGF), fibronectin, methyl-accepting chemotaxis protein-1 (MCP-1), transforming growth factor (TGFβ), nuclear factor-κβ (NFκβ), etc. Those signaling factors might cause mesangial expansion, glomerulosclerosis, glomerular hyperpermeability, and tubular inflammation, etc.⁷ AGEs have, therefore, been regarded as a focal target to inhibit the irreversible deterioration of DN.

In clinical management of DN, increasing attention has been paid to targeting AGEs.^1,15 There are two possible ways to reduce the deleterious effects of AGEs in DN clinical practice: (1) inhibiting the generation of intermediate products and the cross-linking of AGEs; (2) suppressing RAGE expression and inhibiting RAGE activity to inhibit AGEs-RAGE signaling.

Overwhelming research evidence suggests that AGEs inhibitors and cross-link breakers can ameliorate DN.^16,17 Although great efforts have been made to improve their applications in clinic, the therapeutic effect remains poor owing to the single target, and side effects such as pernicious anemia and hepatic dysfunction are serious challenges.¹⁷ Furthermore, clinical investigations show that an exogenous soluble form of RAGE (sRAGE), an anti-RAGE antibody and agents for suppressing RAGE expression can block the AGEs-RAGE signaling pathways.^16,18-20 However, a growing body of clinical evidence shows that the application of sRAGE in DN clinics may be less beneficial than supposed, because AGEs will renew in the circulation for they are not entirely cleared and the circulating level of sRAGE is elevated, which will worsen renal function further.^21,22

So, based on the etiological factors of DN, designing a new dual-target therapy against both AGEs formation and the AGEs-RAGE signaling pathway is necessary to improve the therapeutic effect and reduce the side effects.

Compared with conventional formulations, nanotechnology-based drug carriers have more medical benefits. Furthermore, a nanocarrier could exhibit good targeting through conjugation with various interesting active bio-ligands such as antibody,²³ sugar,²⁴ biotin,²⁵ etc. At the same time, core-shell and multilayered nanoparticles could be loaded with several drugs and control the drug-release profile.²⁶ The integration of nanocarriers into DN therapeutic may therefore be a very promising field.

Hypothesis

Using the aforementioned technology, a multimodal, intensified intervention might be designed for DN therapy. We propose a kind of multi-functional core-shell nanoparticle to release dual-target drugs to inhibit AGEs formation and RAGE expression, which could improve on the disappointing effects of single therapy. The functional “RAGE analog” is also used as “bait” for AGEs and by targeting to the kidney, it may attenuate the side effect of non-specific target (Fig. 1).

Figure 1. Schematic diagram of multi-functional core-shell nanoparticle. RAGE analog has the properties of targeting, recognizing and conjugating AGEs like RAGE, but without the ability of causing the signaling cascades.

In hypothesis: (1) the AGEs inhibitor and the RAGE inhibitor should be loaded in the shell and core regions, respectively; (2) conjugating the core-shell nanoparticle surface with the “RAGE analog.” When the AGEs are excess in kidney, the functional “RAGE analog” is used as “bait” for AGEs, which could lead the nanoparticle to target to the kidney and conjugate with AGEs specifically. Subsequently, the AGEs inhibitor and RAGE inhibitor are released from the core-shell nanoparticle slowly (Fig. 2).

Figure 2. The dual-target drug delivery system of the core-shell nanoparticle. (A) The morphology of core-shell nanoparticle when the AGEs in kidney are normal. (B) When the AGEs are excess in kidney, RAGE analog and endogenous RAGE will start a competitive conjugation with AGEs, which can block the AGE-RAGE signaling cascades indirectly, at the same time, the core-shell nanoparticle releases AGEs inhibitors to inhibit the intermediate product of AGEs. (C) The RAGE inhibitors release sequentially to suppress endogenous RAGE expression.

To achieve the “RAGE analog” conjugated onto the surface of the multi-functional core-shell nanoparticle, two problems should be solved: (1) to find some desirable chains to connect the RAGEs to the nanoparticle surface perfectly; (2) to keep the “RAGE analog” bioactivity when conjugated onto the core-shell nanoparticle. What is more, even though these two problems have been solved, there are further problems of how to encapsulate the two different drugs in the shell and core regions of the nanoparticle respectively and how to ensure the high loading capacity of the nanoparticle and the sustained release profile of drugs; these are something to be improved in the future.

Evaluation and Discussion of the Hypothesis

When preparing core-shell nanoparticles, three elements are absolutely necessary: model drugs, carriers and preparation methods. First, the choice of drugs is the easiest of all because there are several different AGEs inhibitors available, such as aminoguanidine, phridoxamine, aldose reductase enzyme inhibitor and so on.^27-30 It has also been demonstrated that irbesartan, nifedipine and olmesartan, etc.^31,32 can suppress the expression of RAGE. Second, the core-shell and multilayered nanoparticles formed by various materials have been investigated.^33,34 Finally, the preparation methods of core-shell nanoparticles have also been reported in the previous articles. The co-electrospraying, emulation and spray drying technologies could be used to prepare core-shell nanoparticles.^35,36 Modifying a nanoparticle's surface with some biologically active ligands is a very mature technology.^37,38 Therefore, we should be able to find a desirable technology to construct the core-shell nanoparticles conjugated with functional “RAGE analog” in our hypothesis.

We would like to propose the possible ways of testing our hypothesis by using the core-shell nanoparticle in animal models of DN. However, the following puzzles are still unknown to us: do the ‘RAGE analog’ have activities of targeting, recognizing and conjugating with the AGEs after being conjugated onto the surface of nanoparticle?

The advantages of this hypothesis are as follows: (1) targeting effect: “RAGE analog” works as a “bait” can lead the drugs to the targets exactly. (2) Dual-target: the novel core-shell nanoparticles target to AGEs information and AGEs-RAGE signaling pathway blocking. (3) Synergistic effect: releasing dual-target drugs to inhibit AGEs formation and suppress RAGE expression. Therefore, a multimodal therapeutic strategy based on the novel core-shell nanoparticle should improve the treatment of DN.