ATTEMPTS: a heparin/protamine-based prodrug approach for delivery of thrombolytic drugs

Targeted thrombolysis by magnetoacoustic particles in photothrombotic stroke model

BACKGROUND

Recombinant tissue plasminogen activator (rtPA) has a short half-life, and additional hemorrhagic transformation (HT) can occur when treatment is delayed. Here, we report the design and thrombolytic performance of 3 [Formula: see text]m discoidal polymeric particles loaded with rtPA and superparamagnetic iron oxide nanoparticles (SPIONs), referred to as rmDPPs, to address the HT issues of rtPA.

METHODS

The rmDPPs consisted of a biodegradable polymeric matrix, rtPA, and SPIONs and were synthesized via a top-down fabrication.

RESULTS

The rmDPPs could be concentrated at the target site with magnetic attraction, and then the rtPA could be released under acoustic stimulus. Therefore, we named that the particles had magnetoacoustic properties. For the in vitro blood clot lysis, the rmDPPs with magnetoacoustic stimuli could not enhance the lytic potential compared to the rmDPPs without stimulation. Furthermore, although the reduction of the infarcts in vivo was observed along with the magnetoacoustic stimuli in the rmDPPs, more enhancement was not achieved in comparison with the rtPA. A notable advantage of rmDPPs was shown in delayed administration of rmDPPs at poststroke. The late treatment of rmDPPs with magnetoacoustic stimuli could reduce the infarcts and lead to no additional HT issues, while rtPA alone could not show any favorable prognosis.

CONCLUSION

The rmDPPs may be advantageous in delayed treatment of thrombotic patients.

Targeted delivery of thrombolytic enzymes

Although thrombolytic agents have been used for several decades in the treatment of thromboembolic conditions, there is an unmet need for the development of safer thrombolytic agents. The development of new molecules themselves may not be sufficient. This has sparked a growing interest in the design of novel nanoscale drug carrier systems for the delivery of thrombolytic enzymes in an effort to address its fatal side effects. There are numerous proof-of-concept reports on such nanoscale systems that seek to capitalize on the pathophysiologic signatures of thrombosis as well as external biochemical/physical triggers. Although there may be a long road ahead before we have such new nanoscale therapeutics on the bedside, hopes remain high.

Improved Protein Toxin Delivery Based on ATTEMPTS Systems

BACKGROUND

Ribosome-inactivating proteins (RIPs) are wildly found in multiple species of plants, bacteria and fungi. As a special family of protein toxins, RIPs can inhibit protein synthesis and induce cell death via inactivating ribosome in eukaryotic cells. Thus, RIPs have been applied for anti-tumor therapy in the past two decades. However, because of poor cell permeability, nonselective mode of action for tumor cells, poor pharmacokinetic profiles and immunogenicity, their clinical application has been severely constrained. As an effort to overcome these obstacles, tumor-specific monoclonal antibodies (mAb) have been conjugated to RIPs (forming so called "immunotoxins") specifically to increase their cytotoxicity and provide tumor targeting. Nevertheless, immunotoxins yet have not fully resolved all the issues and critical challenges still remain, such as immunogenicity and inability to penetrate into the deep site of tumor.

OBJECTIVE

To overcome the constrain of immunotoxins, the novel cell-penetrating peptide (CPP)- modified ATTEMPTS systems based on combination of CPP-mediated penetration and antibodymediated tumor targeting, with triggerable drug release function, were developed to achieve effective and safe delivery of protein toxin.

RESULTS

The CPP-modified ATTEMPTS systems showed effective protamine-triggered CPP-toxin release and thus enhanced CPP-mediated cellular uptake and cytotoxicity. It also showed antibodymediated in vivo tumor targeting and significantly increased in vivo tumor growth suppression with limited systematic toxicity.

CONCLUSION

The CPP-modified ATTEMPTS systems were developed and demonstrated as a proof-ofconcept for CPP-based protein toxin delivery with triggerable antibody targeting to improve the druggability of protein toxin drugs. The systems showed the potential application of protein toxin clinical translation in anticancer treatment.

Trends and Prospects of Plant Proteases in Therapeutics

The main function of proteases in any living organism is the cleavage of proteins resulting in the degradation of damaged, misfolded and potentially harmful proteins and therefore providing the cell with amino acids essential for the synthesis of new proteins. Besides this main function, proteases may play an important role as signal molecules and participate in numerous protein cascades to maintain the vital processes of an organism. Plant proteases are no exception to this rule. Moreover, in contrast to humanencoded enzymes, many plant proteases possess exceptional features such as higher stability, unique substrate specificity and a wide pH range for enzymatic activity. These valuable features make plant-derived proteolytic enzymes suitable for many biomedical applications, and furthermore, the plants can serve as factories for protein production. Plant proteases are already applied in the treatment of several pathological conditions in the human organism. Some of the enzymes possess antitumour, antibacterial and antifungal activity. The collagenolytic activity of plant proteases determines important medical applications such as the healing of wounds and burn debridement. Plant proteases may affect blood coagulation processes and can be applied in the treatment of digestive disorders. The present review summarizes recent advances and possible applications for plant proteases in biomedicine, and proposes further development of plant-derived proteolytic enzymes in the biotechnology and pharmaceutical industries.

Recent advances in targeted delivery of tissue plasminogen activator for enhanced thrombolysis in ischaemic stroke

Tissue plasminogen activator (tPA) is the only FDA approved medical treatment for the ischaemic stroke. However, it associates with some inevitable limitations, including: short therapeutic window, extremely short half-life and low penetration in large clots. Systemic administration may lead to complications such as haemorrhagic conversion in the brain and relapse in the form of re-occlusion. Furthermore, ultrasound has been utilised in combination with contrast agents, echogenic liposome, microspheres or nanoparticles (NPs) carrying tPA for improving thrombolysis - an approach that has resulted in slight improvement of tPA delivery and facilitated thrombolysis. Most of these delivery systems are able to extend the circulating half-life and clot penetration of tPA. Various technologies employed for ameliorated thrombolytic therapy are in different phases, some are in final steps for clinical applications while some others are under investigations for their safety and efficacy in human cases. Here, recent progresses on the thrombolytic therapy using novel nano- and micro-systems incorporating tPA are articulated. Of these, liposomes and microspheres, polymeric NPs and magnetic nanoparticles (MNPs) are discussed. Key technologies implemented for efficient delivery of tPA and advanced thrombolytic therapy and their advantages/disadvantages are further expressed.

Heparin-Regulated Prodrug-Type Macromolecular Theranostic Systems for Cancer Therapy

Heparin is a kind of naturally occurring polymer with excellent biocompatibility and solubility. It is characterized by dense of negative charge, higher than any endogenous components. Heparin can bind with various cationic peptides and proteins, thereby providing a useful noncovalent linkage for building a drug delivery system. As a case in point, heparin/cell-penetrating peptides (CPP) interaction is strong, and remains stable in vivo. They can be used to modify different proteins, respectively, and subsequently, by simply mixing the modified proteins, a protein-protein conjugate can be form via the stable heparin/CPP linkage. This linkage could not be broken unless addition of protamine that bears higher cationic charge density than CPP, and CPP thus can be substituted and released. Of note, heparin is a potent antagonist of CPP, and their binding naturally inhibits CPP-mediated drug cell penetration. Based on this method, we developed a heparin-regulated macromolecular prodrug-type system, termed ATTEMPTS, for drug targeting delivery. In this review article, we mainly summary the application of ATTEMPTS in delivery of various macromolecular drugs for cancer therapy, and also introduce the heparin-regulated nanoprobes for tumor imaging.

A t-PA/nanoparticle conjugate with fully retained enzymatic activity and prolonged circulation time

A major issue in the therapeutic use of tissue plasminogen activator (t-PA) for the treatment of thrombotic diseases is its very short half-life in the circulation due to the effects of inhibitors. The present study aims to resolve the issue using a t-PA/gold nanoparticle (t-PA/AuNP) conjugate prepared via bio-affinity ligation under physiological conditions. The ligation is based on the specific interactions between t-PA and ε-lysine (a ligand that has affinity to a specific domain in t-PA) immobilized on the AuNP surface through polyvinyl pyrrolidone (PVP) as a spacer. The conjugate can not only retain almost full enzymatic activity and clot dissolving efficiency, but also protect t-PA from inhibition by PAI-1 to some extent as compared with free t-PA in vitro. Moreover, the conjugate showed prolonged circulation time in vivo.

Conjugation of polymers to proteins through an inhibitor-derived peptide: taking up the inhibitor “berth”

A hexapeptide derived from an enzyme inhibitor was used to conjugate a hydrophilic polymer to the inhibitor “berth” in the enzyme, affording the enzyme resistance to the inhibitor.

15 years of ATTEMPTS: a macromolecular drug delivery system based on the CPP-mediated intracellular drug delivery and antibody targeting

Traditionally, any drug intended for combating the tumor would distribute profoundly to other organs and tissues as lack of targeting specificity, thus resulting in limited therapeutic effects toward the tumor but severe drug-induced toxic side effects. To prevail over this obstacle of drug-induced systemic toxicity, a novel approach termed "ATTEMPTS" (antibody targeted triggered electrically modified prodrug type strategy) was designed, which directly introduces both of the targeting and prodrug features onto the protein drugs. The ATTEMPTS system is composed of the antibody targeting component consisting of antibodies linked with heparin, and the cell penetrating peptide (CPP) modified drug component. The two components mentioned above self-assembled into a tight complex via the charge to charge interaction between the anionic heparin and cationic CPP. Once accumulated at the targeting site, the CPP modified drug is released from the blockage by a second triggering agent, while remaining inactive in the circulation during tumor targeting thus aborting its effect on normal tissues. We utilized the heparin-induced inhibition on the cell-penetrating activity of CPP to create the prodrug feature, and subsequently the protamine-induced reversal of heparin inhibition to resume cell transduction of the protein drug via the CPP function. Our approach is the first known system to overcome this selectivity issue, enabling CPP-mediated cellular drug delivery to be practically applicable clinically. In this review, we thoroughly discussed the historical and novel progress of the "ATTEMPTS" system.

Heparin-triggered release of camouflaged tissue plasminogen activator for targeted thrombolysis

A targetable, heparin-triggered release system for tissue plasminogen activator (tPA) was designed to prevent the excessive 'lytic' state associated with the current tPA therapy for acute thrombotic conditions, such as myocardial infarction (MI). The strategy is, upon target accumulation, to trigger tPA release from a prodrug construct by a usual heparin dose. A relatively inactive form of tPA was constructed by conjugating tPA with low-molecular weight heparin followed by complexation with albumin-protamine conjugate, termed 'camouflage'. The modified tPA was ~97% as active as native tPA. The prodrug construct of tPA significantly masked the enzymatic activity, which was fully recovered upon heparin addition. The camouflaged tPA was stable in human blood for at least 30min and was able to trigger enzyme activation in vitro at heparin level of 0.4U/mL. In vivo studies on jugular vein rat thrombosis model showed that the clot lysis of the heparin-triggered camouflaged tPA group was equivalent to the tPA+heparin group without prolongation of activated partial thromboplastin time (aPTT) before and after the treatment. This proof-of-principle study suggests that the activity of the tPA prodrug construct can be triggered at the thrombus site at therapeutic heparin concentration conjunctively used for MI with reduced bleeding risk.

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

A proinsulin-transferrin (ProINS-Tf) recombinant fusion protein was designed and characterized for the sustained release of an active form of insulin (INS) by hepatoma cells. During incubation with H4IIE hepatoma cells, a gradual decline of ProINS-Tf concentration, with a concomitant generation of the immuno-reactive insulin-transferrin (irINS-Tf), was detected in the culture medium by using INS- or proinsulin (ProINS)-specific radioimmunoassay (RIA) system. Further studies indicated that the conversion of ProINS-Tf to irINS-Tf was a transferrin receptor (TfR) mediated process that was pH-sensitive, and temperature- and microtubule-dependent. These results suggest that the conversion occurred during the slow recycling route of transferrin (Tf)-TfR pathway, possibly processed by proteases in the slow recycling compartments juxtaposed to the trans-Golgi network (TGN). ProINS-Tf exhibited little activity in the short-term promotion of glucose uptake in adipocytes, indicating that it was in an inactive form similar to ProINS. Stimulation of Akt phosphorylation by ProINS-Tf was detected only after prolonged incubation with H4IIE cells. On the other hand, ProINS-Tf pre-incubated with H4IIE cells for 24h acquired an immediate activity of stimulating Akt phosphorylation. Furthermore, ProINS-Tf elicited a strong activity in the inhibition of glucose production following 24h incubation with H4IIE cells. Based on these findings, we conclude that the Tf-TfR endocytosis and recycling pathway enables the conversion and release of ProINS-Tf in an active form of irINS-Tf. Results from this study suggest that the Tf-TfR pathway can be exploited for the design of prohormone-Tf fusion proteins as protein prodrugs for their sustained and targeted activation.

Prodrugs in cardiovascular therapy

Prodrugs are biologically inactive derivatives of an active drug intended to solve certain problems of the parent drug such as toxicity, instability, minimal solubility and non-targeting capabilities. The majority of drugs for cardiovascular diseases undergo first-pass metabolism, resulting in drug inactivation and generation of toxic metabolites, which makes them appealing targets for prodrug design. Since prodrugs undergo a chemical reaction to form the parent drug once inside the body, this makes them very effective in controlling the release of a variety of compounds to the targeted site. This review will provide the reader with an insight on the latest developments of prodrugs that are available for treating a variety of cardiovascular diseases. In addition, we will focus on several drug delivery methodologies that have merged with the prodrug approach to provide enhanced target specificity and controlled drug release with minimal side effects.

Conjugation of an antibody to cross-linked fibrin for targeted delivery of anti-restenotic drugs

There is an urgent need to treat restenosis, a major complication of the treatment of arteries blocked by atherosclerotic plaque, using local delivery techniques. We observed that cross-linked fibrin (XLF) is deposited at the site of surgical injury of arteries. An antibody to XLF, conjugated to anti-restenotic agents, should deliver the drugs directly and only to the site of injury. An anti-XLF antibody (H93.7C.1D2/48; 1D2) was conjugated to heparin (using N-succinimidyl 3-(2-pyridyldithio)propionate), low molecular weight heparin (LMWH) (adipic acid dihydrazide) and rapamycin (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysuccinimide), and the conjugates purified and tested for activity before use in vivo. Rabbits had their right carotid arteries de-endothelialised and then given a bolus of 1D2-heparin, 1D2-LMWH or 1D2-rapamycin conjugate or controls of saline, heparin, LMWH, rapamycin or 1D2 (+/-heparin bolus) and sacrificed after 2 or 4 weeks (12 groups, n=6/group). Rabbits given any of the conjugates had minimal neointimal development in injured arteries, with up to 59% fewer neointimal cells than those given control drugs. Rabbits given 1D2-heparin or 1D2-LMWH had an increased or insignificant reduction in luminal area, with positive remodelling, while the medial and total arterial areas of rabbits given 1D2-rapamycin were not affected by injury. Arteries exposed to 1D2-heparin or 1D2-rapamycin had more endothelial cells than rabbits given control drugs. Thus, XLF-antibodies can site-deliver anti-restenotic agents to injured areas of the artery wall, where the conjugates can influence remodelling, re-endothelialisation and neointimal cell density, with reduced neointimal formation.

ATTEMPTS: a heparin/protamine-based triggered release system for the delivery of enzyme drugs without associated side-effects

A prodrug type delivery system based on competitive ionic binding for the conversion of the prodrug to an active drug has been developed for delivery of enzyme drugs without their associated toxic side-effects. This approach, termed "ATTEMPTS" (antibody targeted, triggered, electrically modified prodrug-type strategy), would permit the administration of an inactive drug and then subsequently triggered release of the active drug at the target site. The underlying principle was to modify the enzyme with small cationic species so that it could bind a negatively charged heparin-linked antibody, and the latter would block the activity of the enzyme drug until it reached the target. To provide the enzyme drug with appropriate binding strength to heparin, a cationic poly(Arg)(7) peptide was incorporated onto the enzyme either by the chemical conjugation method using a bifunctional crosslinker or by the biological conjugation method using the recombinant methodology. Methods for drug modification, heparin-antibody conjugation, and the prodrug and triggered release features of the "ATTEMPTS" approach are described in detail in this review article.