Poly(Glutamic Acid) as Versatile Biodegradable Drug Carrier

Polyglutamates are well known to be highly biocompatible, biodegradable and multifunctional polymers, which have already been used as building blocks in polymer drug conjugates and polymeric micelles. Those systems have been utilized for various medical applications ranging from therapy to molecular imaging. Furthermore, a PGA paclitaxel conjugate has already entered clinical studies: Opaxio™ PGA-paclitaxel (PTX) conjugate is currently in

phase III of clinical trials as maintenance therapy in ovarian cancer and has been granted orphan drug designation by the FDA for the treatment of malignant brain cancer. In this context, a synthetic pathway to a plethora of functional polyglutamates (homopolymers, block-co-polymers) with well-defined structure, adjustable molecular weight (MW) and low dispersity (D = Mw/Mn < 1.2) applying the ring opening polymerization (ROP) of N-carboxyanhydrides (NCA) are offered. Additionally, as the acid moieties of the polyglutamates can be activated, various functionalities were introduced by “post-polymerization modification” yielding a set of orthogonal reactive side chains. The reactive moieties, such as azides, maleimides, thiols, or alkynes offer the opportunity of specific conjugation of drugs, targeting moieties or markers.

Besides introducing reactive groups, the functionalization strategy has also been used for PEGylation of PGA. This modification could reduce charge induced interactions and therefore change pharmacological properties such as blood circulation.

In summary, a tool kit of various polyglutamates is offered enabling the synthesis of a variety of polymer drug conjugates or polymer based imaging agents. The functional polymeric precursors allow functionalizing and therefore adjusting the polymer properties to many desired applications.

Background information:

An ideal polymer to be used as carrier for drug delivery or molecular imaging should be characterized by

1. biodegradability or adequate molecular weight that allows elimination from the body to avoid progressive accumulation in vivo.
2. low polydispersity to ensure an acceptable homogeneity of the final system allowing to adjust pharmacokinetics.
3. long body residence time either to prolong the conjugate action or to allow distribution and accumulation in the desired body compartments (therefore high molecular weight is desired).
4. availability of many reactive groups especially for small drug conjugation in order to achieve a satisfactory drug loading or to allow polymerbased combination therapy (multivalent polymers).

Most polymer conjugates in the market and in the clinics use N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers, PEG or more recently polyglutamic acid (PGA) as carriers. Biopersistent carriers (PEG, HPMA) present disadvantages, if chronical parenteral administration and/or high doses are required as there is the potential to generate ‘lysosomal storage disease’ syndrome. Alternatively, polyglutamates are well known to be highly biocompatible, biodegradable (by thiol protease cathepsin B) and multifunctional polymers, which have been already applied to various applications that range from drug delivery systems, tissue engineering, sensing, and catalysis. PGA is considered a promising material for the design of novel nanomedicine due to its high biocompatibility, multivalency and in vivo degradability. As a prominent example for its use as nanopharmaceutical, one has to mention a conjugate of polyglutamic acid (PGA) and paclitaxel (OpaxioTM, formerly Xyotax, PPX, CT-2103) in phase III of clinical trials. Another clinical example is provided by several polymeric micelles – firstly developed by Kataoka – that were designed based on the block-copolymer PEGPGA, namely NK 105, NK-6004, Nanoplatin or NC-4016 in Phase I-III trials. Other recent examples of the use of this multifunctional, biodegradable polyanionic carrier can be found in many drug delivery applications not only in cancer, but also in other diseases including tissue regeneration. PGA has also been used due to its multivalency in the development of polymer-based combination therapy applications.

Polyglutamates are commonly obtained by ring-opening

polymerization (ROP) of amino acid-N-carboxyanhydrides (NCA). The polymerization method enables access to polypeptidic architectures which are beyond nature’s possibilities. Due to the variety of natural and non-natural amino acids and the versatility of the polymerization method, a plethora of polypeptides has been created and characterized, as reviewed in literature. So far, the most promising chemical approaches are based on initiation of purified NCAs with primary amines, amine hydrochloride salts, heavy metal catalysts or hexamethyldisilazanes. All those methods have certain limitations in the synthesis of well defined polypeptides of high and reproducable quality. The commercial offer so far of PGA was very limited. In order to overcome these limitations, a controlled and living polymerization methodology has been developed based on the modification of the initiators for the ROP of NCAs to produce polypeptides and polypeptidebased block copolymers on a multigram scale. With this controlled NCA methodology we managed to enhance the degree of polymerization (DP), structural versatility and decrease polydispersity index (D) of polypeptides obtained by NCA polymerization. The method employed effectively suppressed side reactions. Therefore, the control over polymer end groups has been also enhanced enabling the synthesis of well-defined homo or diblock polypeptides of a variety of molecular weight and sidechain and terminal chain functionalities.

PGA - A Modern Versatile Polymer as Drug Carrier for Drug Delivery, Tissue Engineering, Sensing, Catalysis, NanoMedicine

Poly(glutamic acid) is a biocompatible and biodegradable polymer which can be conjugated through side chain condensation with any suitable molecule. Due to a controlled proprietary and patented process with living polymerization technology a superior quality of PGA is achieved. Usual poly amino acids carry significant amounts of cyclic structures, carbamates or isocyanates. Through a very well controlled polymerization process, well defined terminal groups and polymeric structures are achieved in high purity and with superior polydispersity. Through “living polymer” technology also multifunctional PGA polymers can be produced through post polymerization modification. PGA can be used for polymer therapeutics application for large biopharmaceuticals and also for small molecule drugs. A controlled loading of small molecules onto PGA polymer can be achieved and brings the advantage of polymer therapy also to small molecules.

Published Applications:

In the following published application, PGA equips a hydrophobic small peptoidic drug molecule which has poor water solubility with superior pharmacokinetic properties, excellent water solubility, and increased membrane permeability.

Reference:

• Modulation of Cellular Apoptosis with Apoptotic Protease-Activating Factor 1 (Apaf-1) Inhibitors; L. Mondragón, M. Orzáez, G. Sanclimens, A. Moure, A. Armiñán, P. Sepúlveda, A. Messeguer, M. J. Vicent and E. Pérez-Payá; J Med Chem 2008; 51: 521-529. doi:10.1021/jm701195j

Drug Carrier and Release System for Multiple Drug Therapy

Through post polymerization modification, PGA can be equipped with additional functional groups, like alkyne or azides for click conjugation; however, also the base polymer can be loaded with different molecules as shown in the following example. Paclitaxel (PTX) is a widely-used potent cytotoxic drug that also exhibits anti-angiogenic effects at low doses. Its use at its full potential is limited by severe side effects. The PGA polymer PTX nano-scaled conjugate passively targets tumor tissue exploiting enhanced permeability and retention effect. The polymer is enzymatically-degradable, leading to PTX release under lysosomal acidic pH. The cyclic RGD peptide enhances the effect of PGA-PTX alone by targeting αvβ3 integrin, which is overexpressed on tumor endothelial and epithel cells.

Reference:

• Integrin-assisted drug delivery of nano-scaled polymer therapeutics bearing paclitaxel; A. Eldar-Boock, K. Miller, J. Sanchis, R. Lupu, M. J. Vicent and R. Satchi-Fainaro; Biomaterials 2011; 32: 3862-3874. doi:10.1016/j.biomaterials.2011.01.073

Bi-functional PGA Drug Carrier available for Sophisticated Applications: Combination Therapy – Personalized Medicine

PGA provides ideal possibilities for multi-drug therapies. Already the base PGA polymer can be utilized for these purposes, while multifunctional derivatives increase the number of options for the medicinal chemist.

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