Receptor-mediated activation of a proinsulin-transferrin fusion protein in hepatoma cells

Research Advances in Fusion Protein-Based Drugs for Diabetes Treatment

Diabetes mellitus (DM) is a chronic metabolic disease characterized by elevated blood glucose levels, resulting in multi-organ dysfunction and various complications. Fusion proteins can form multifunctional complexes by combining the target proteins with partner proteins. It has significant advantages in improving the performance of the target proteins, extending their biological half-life, and enhancing patient drug compliance. Fusion protein-based drugs have emerged as promising new drugs in diabetes therapeutics. However, there has not been a systematic review of fusion protein-based drugs for diabetes therapeutics. Hence, we conducted a comprehensive review of published literature on diabetic fusion protein-based drugs for diabetes, with a primary focus on immunoglobulin G (IgG) fragment crystallizable (Fc) region, albumin, and transferrin (TF). This review aims to provide a reference for the subsequent development and clinical application of fusion protein-based drugs in diabetes therapeutics.

Seed- and leaf-based expression of FGF21-transferrin fusion proteins for oral delivery and treatment of non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is a global disease with no effective medication. The fibroblast growth factor 21 (FGF21) can reverse this liver dysfunction, but requires targeted delivery to the liver, which can be achieved via oral administration. Therefore, we fused FGF21 to transferrin (Tf) via a furin cleavage site (F), to promote uptake from the intestine into the portal vein, yielding FGF21-F-Tf, and established its production in both seeds and leaves of commercial Nicotiana tabacum cultivars, compared their expression profile and tested the bioavailability and bioactivity in feeding studies. Since biopharmaceuticals need to be produced in a contained environment, e.g., greenhouses in case of plants, the seed production was increased in this setting from 239 to 380 g m^-2 a^-1 seed mass with costs of 1.64 € g^-1 by side branch induction, whereas leaves yielded 8,193 g m^-2 a^-1 leave mass at 0.19 € g^-1. FGF21-F-Tf expression in transgenic seeds and leaves yielded 6.7 and 5.6 mg kg^-1 intact fusion protein, but also 4.5 and 2.3 mg kg^-1 additional Tf degradation products. Removing the furin site and introducing the liver-targeting peptide PLUS doubled accumulation of intact FGF21-transferrin fusion protein when transiently expressed in Nicotiana benthamiana from 0.8 to 1.6 mg kg^-1, whereas truncation of transferrin (nTf338) and reversing the order of FGF21 and nTf338 increased the accumulation to 2.1 mg kg^-1 and decreased the degradation products to 7% for nTf338-FGF21-PLUS. Application of partially purified nTf338-FGF21-PLUS to FGF21^-/- mice by oral gavage proved its transfer from the intestine into the blood circulation and acutely affected hepatic mRNA expression. Hence, the medication of NASH via oral delivery of nTf338-FGF21-PLUS containing plants seems possible.

Enhanced insulin receptor interaction by a bifunctional insulin-transferrin fusion protein: an approach to overcome insulin resistance

Bifunctional fusion protein design has been widely utilized as a strategy to increase the efficacy of protein therapeutics. Previously, we proposed a novel application of the bifunctional fusion protein design through the introduction of proinsulin-transferrin (ProINS-Tf) fusion protein as a liver-specific protein prodrug to achieve a glucose-lowering effect in type 1 diabetic mice. In this report, we studied the binding characteristics of this activated fusion protein to the insulin receptor to elucidate its mechanism in eliciting insulin receptor-mediated signaling. We found that, with the assistance of the transferrin moiety binding to the transferrin receptor, the activated ProINS-Tf exhibited significantly higher binding affinity to the insulin receptor compared with the native insulin, resulting in a prolonged and stronger Akt phosphorylation. This enhanced induction by activated ProINS-Tf overcame insulin resistance in palmitate-treated HepG2 cells. ProINS-Tf also demonstrated a better glucose-lowering effect than native insulin, even with a much lower dose and less frequent injections, in non-obese diabetic mice with insulin resistance symptoms. The activated ProINS-Tf, serving as a bivalent protein molecule, could be a new insulin analog to overcome insulin resistance, which is associated with several diseases, including type 2 diabetes and non-alcoholic fatty liver disease.

Discovery of An Orally Effective Factor IX-Transferrin Fusion Protein for Hemophilia B

Hemophilia B is a severe blood clotting disorder caused by the deficiency of factor IX (FIX). FIX is not bioavailable when given orally due to poor stability and permeability in the gastrointestinal tract. The feasibility of fusing FIX with transferrin (Tf) to enhance the oral bioavailability of FIX is explored. Seven recombinant fusion proteins (rFIX-Tf) with different linkers were constructed and expressed in HEK293 cells and characterized by in vitro transcytosis and transferrin receptor (TfR) binding assay in Caco-2 cells and a one-stage clotting assay. The in vivo efficacy study was performed using a tail-bleeding model in hemophilia B mice. Fusion proteins rFIX-Tf/G₂ and rFIX-Tf/SVSQ were most permeable and showed a specific binding ability to TfR in Caco-2 cells. Both proteins retained FIX activity in clotting generation. The in vivo efficacy study showed that both proteins by intravenous injection significantly reduced blood loss. Most significantly, rFIX-Tf/G₂ demonstrated anti-bleeding activity when administered orally. Our results showed that the fusion protein technique with Tf could be potentially used for oral delivery of FIX and the linker between FIX and Tf in the fusion protein is crucial. rFIX-Tf/G₂ appears to be the most promising fusion protein as potential oral therapeutics for hemophilia B.

Recent Advances in Half-life Extension Strategies for Therapeutic Peptides and Proteins

Peptides and proteins are two classes of molecules with attractive possibilities for therapeutic applications. However, the bottleneck for the therapeutic application of many peptides and proteins is their short halflives in vivo, typically just a few minutes to hours. Half-life extension strategies have been extensively studied and many of them have been proven to be effective in the generation of long-acting therapeutics with improved pharmacokinetic and pharmacodynamic properties. In this review, we summarize the recent advances in half-life extension strategies, illustrate their potential applications and give some examples, highlighting the strategies that have been used in approved drugs and for drugs in clinical trials. Meanwhile, several novel strategies that are still in the process of discovery or at a preclinical stage are also introduced. In these strategies, the two most frequently used half-life extension methods are the reduction in the rate of renal clearance or the exploitation of the recycling mechanism of FcRn by binding to the albumin or IgG-Fc. Here, we discuss half-life extension strategies of recombinant therapeutic protein via genetic fusion, rather than chemical conjugation such as PEGylation. With the rapid development of genetic engineering and protein engineering, novel strategies for half-life extension have been emerged consistently. Some of these will be evaluated in clinical trials and may become viable alternatives to current strategies for making next-generation biodrugs.

Characterization and Oral Delivery of Proinsulin-Transferrin Fusion Protein Expressed Using ExpressTec

Proinsulin-transferrin fusion protein (ProINS-Tf) has been designed and successfully expressed from the mammalian HEK293 cells (HEK-ProINS-Tf). It was found that HEK-ProINS-Tf could be converted into an activated form in the liver. Furthermore, HEK-ProINS-Tf was demonstrated as an extra-long acting insulin analogue with liver-specific insulin action in streptozotocin (STZ)-induced type 1 diabetic mice. However, due to the low production yield from transfected HEK293 cells, there are other interesting features, including the oral bioavailability, which have not been fully explored and characterized. To improve the protein production yield, an alternative protein expression system, ExpressTec using transgenic rice (Oryza sativa L.), was used. The intact and active rice-derived ProINS-Tf (ExpressTec-ProINS-Tf) was successfully expressed from the transgenic rice expression system. Our results suggested that, although the insulin-like bioactivity of ExpressTec-ProINS-Tf was slightly lower in vitro, its potency of in vivo blood glucose control was considerably stronger than that of HEK-ProINS-Tf. The oral delivery studies in type 1 diabetic mice demonstrated a prolonged control of blood glucose to near-normal levels after oral administration of ExpressTec-ProINS-Tf. Results in this report suggest that ExpressTec-ProINS-Tf is a promising insulin analog with advantages including low cost, prolonged and liver targeting effects, and most importantly, oral bioactivity.

DOPA-based paclitaxel-loaded liposomes with modifications of transferrin and alendronate for bone and myeloma targeting

Treatment for multiple myeloma (MM) with a combined strategy of bone and tumor targeting remains a crucial technical challenge due to the incorporation of various functional components into one single system. Here, we developed dioleoyl phosphatidic acid (DOPA)-based paclitaxel (PTX)-loaded liposomes with modifications of alendronate and transferrin (Ald-/Tf-modified PTX-L), which were capable of bone affinity mediated by phosphate groups in DOPA and alendronate, and tumor targeting offered by transferrin. Ald-/Tf-modified PTX-L had clear and well-defined spherical shape with an intermediated size of 118.8 ± 4.8 nm, a highly negative surface charge of -46.9 ± 6.8 mV and a drug entrapment efficiency (DEE) of approximately 80%. When the pH was changed from pH 7.4 to pH 6.5, the accumulative release of PTX from Ald-/Tf-modified PTX-L significantly increased from 26.7 ± 3.7% to 41.7 ± 4.9%. Importantly, liposomes based on DOPA displayed an obviously stronger affinity with hydroxyapatite (HAp) than 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-based liposomes. Compared to PTX-L, Ald-/Tf-modified PTX-L exhibited obvious improvement of cytotoxicity (IC₅₀=1.25 ± 0.09 μg/mL), significant enhancement on PTX intracellular accumulation (16.58 ± 0.62 μg/mg) and notable promotion to apoptosis induction (45.21 ± 3.10%) toward myeloma (MM1s) cells. In this study of antitumor efficacy, Ald-/Tf-modified PTX-L with bone-specific targeting showed a significant effect on extending the median survival time (48 days) and terminal survival time (> 58 days) against the MM1S-injected nude mice among all formulations. The results suggested that Ald-/Tf-modified PTX-L had potential as an efficient anticancer drug delivery system for MM therapy.

Proinsulin-Transferrin Fusion Protein Exhibits a Prolonged and Selective Effect on the Control of Hepatic Glucose Production in an Experimental Model of Type 1 Diabetes

An ideal basal insulin (INS) replacement therapy requires the distribution or action of exogenous INS to more closely mimic physiological INS in terms of its preferential hepatic action. In this paper, we introduce a novel strategy to exert liver-specific INS action by hepatic activation of INS's precursor, proinsulin (ProINS). We demonstrated the conversion of human ProINS-transferrin (Tf) fusion protein, ProINS-Tf, into an active and immuno-reactive form of INS-Tf in the liver via the slow Tf receptor mediated recycling pathway. ProINS-Tf displayed prolonged basal blood glucose lowering effects for up to 40 h in streptozotocin-induced type 1 diabetic mice following a single subcutaneous injection. The effect of ProINS-Tf on blood glucose levels was observed predominantly under fasting conditions, with little effect under free-feeding conditions. In addition, both the pyruvate tolerance assay in normal mice and the Akt-phosphorylation assay in H-4-II-E hepatoma cells indicated that the hepatic-activated ProINS-Tf possessed a much longer effect on the control of hepatic glucose production than INS. These results indicated that ProINS-Tf may serve as an effective and safe hepatoselective INS analog to reduce the frequency of INS injections as well as avert severe hypoglycemia episodes and other side effects frequently encountered with long-acting INS therapeutics due to their peripheral action.

Tissue barriers and novel approaches to achieve hepatoselectivity of subcutaneously-injected insulin therapeutics

Current subcutaneously (s.c.)-injected insulin (INS) products result in a hyperinsulin exposure to peripheral tissues (skeletal muscle and adipose) while INS hardly accesses to liver after injection. This unphysiological distribution raises risks of hypoglycemia episode and causes weight gain after long term treatment. An ideal INS replacement therapy requires the distribution or action of exogenous INS to more closely mimic physiological INS in terms of its preferential hepatic action. However, there are 2 factors that limit the ability of s.c. injected INS to restore the liver: peripheral gradient in INS deficient diabetes patients: (1) the transport of INS in capillary endothelium and peripheral tissues from the injection site; and (2) peripheral INS receptor (IR) mediated INS degradation. In this review, the tissue barriers against efficient liver targeting of s.c. injected INS are discussed and current advances in developing hepatoselective insulin therapeutics are introduced.

Fusion Proteins for Half-Life Extension of Biologics as a Strategy to Make Biobetters

The purpose of making a "biobetter" biologic is to improve on the salient characteristics of a known biologic for which there is, minimally, clinical proof of concept or, maximally, marketed product data. There already are several examples in which second-generation or biobetter biologics have been generated by improving the pharmacokinetic properties of an innovative drug, including Neulasta(®) [a PEGylated, longer-half-life version of Neupogen(®) (filgrastim)] and Aranesp(®) [a longer-half-life version of Epogen(®) (epoetin-α)]. This review describes the use of protein fusion technologies such as Fc fusion proteins, fusion to human serum albumin, fusion to carboxy-terminal peptide, and other polypeptide fusion approaches to make biobetter drugs with more desirable pharmacokinetic profiles.

Proinsulin-transferrin fusion protein as a novel long-acting insulin analog for the inhibition of hepatic glucose production

Proinsulin-transferrin (ProINS-Tf) fusion protein was evaluated for its in vivo pharmacokinetics, efficacy, and mechanism. Our previous studies have shown that ProINS-Tf was converted to active insulin-transferrin (INS-Tf) via the transferrin (Tf)-receptor-mediated pathway in hepatoma cells. We hypothesized that this fusion protein can be administered as a prodrug and be converted to a biologically active protein with specificity for the liver versus other insulin (INS)-sensitive tissues (muscle and adipose). Administration as an inactive prodrug with liver-specific action compared with other INS-sensitive tissues conceivably reduces negative side effects seen with other INS analogs. In this report, the data show that ProINS-Tf exhibited a slow, but sustained, in vivo hypoglycemic efficacy and long plasma half-life. The fusion protein showed activity in the liver, as evidenced by decreased expression of two key hepatic glucose production (HGP) enzymes, PEPCK and glucose-6-phosphatase, and increased glycogen levels under feeding conditions. Furthermore, the INS receptor (IR) phosphorylation (activation) in liver and muscle tissues was compared with postinjection of INS or ProINS-Tf. While INS activated IR in both the liver and muscle, ProINS-Tf only showed activation in the liver. Thus, ProINS-Tf fusion protein can potentially be administered as a prodrug with sustained Tf-mediated activation and selectivity in inhibiting HGP.

Enhanced antitumor efficacy of cisplatin by tirapazamine-transferrin conjugate

Combination of tirapazamine (TPZ) with cisplatin has been studied extensively in clinical trial for tumor therapy. However, in phase III clinical trial, the combination therapy did not show overall survival improvement in patients. To decrease the side effects and increase the efficacy of the combination therapy, TPZ was conjugated with transferrin (Tf-G-TPZ) for targeted delivery and co-administered with cisplatin. In vitro toxicity study showed that the combination of Tf-G-TPZ with cisplatin induced substantially higher cytotoxicity of tumor cells than the combination of TPZ and cisplatin. After Tf-G-TPZ was intravenously injected into tumor-bearing mice, its total accumulation in tumor was 2.3 fold higher than that of the unmodified TPZ, suggesting transferrin-mediated target delivery of TPZ into the tumor tissue. With the increased accumulation of Tf-G-TPZ in tumor, the synergistic anti-tumor effects of Tf-G-TPZ and cisplatin were also enhanced as showed by the 53% tumor inhibition rate. Meanwhile, the side effects such as body weight lost were not significantly increased. Therefore, Tf-G-TPZ holds great promise to a better substitute for TPZ in the combination therapy with cisplatin.