Organic Pharmaceutical Chemistry IV
1st Semester, Year 5 (2016-2017)
Lecture 5
Chemical Delivery Systems
(Polymeric Prodrugs)
Types and structure of polymers; cross-linking agents and polymeric
chemical delivery systems.
Dr Asim A. Balakit
Pharmaceutical Chemistry Dept.,
College of Pharmacy,
Babylon University
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Properties of Polymeric Prodrugs 1- Prolongation of Action of the Drug The profile of plasma concentration of drugs is an important determinant of their quantitative access to peripheral targets. The plasma profile is usually measured as the area under the curve (AUC). In general, slow renal elimination and metabolic inactivation promote better access of drugs to remote targets, although this can also cause elevated toxicity. Many drugs in routine use are membrane permeable because their sites of action are intracellular and such drugs typically exhibit high volumes of distribution and rapid plasma clearance. By linking a drug onto a polymer, a conjugate is obtained with a higher hydrodynamic volume. This results in a slower renal excretion, longer blood circulation and an endocytotic cell uptake.
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To select polymers as candidate drug carriers a number of requirements should be fulfilled:
1. Availability of suitable functional groups for covalent coupling with drugs.
2. Biocompatibility: preferably nontoxic, nonimmunogenic.
3. Biodegradability or a molecular weight below the renal excretion limit.
4. Availability.
Along with the polymer, the physico-chemical properties of the drug or biomolecule to be conjugated are equally important. The following properties of the drug molecules make it suitable as an ideal candidate to form the polymeric conjugate:
1. Lower aqueous solubility.
2. Instability at varied physiological pHs.
3. Higher systemic toxicity.
4. Reduced cellular entry. 3
A number of reviews cover what has been done over the past 20 years in the field of soluble polymers as potential drug carriers . The polymers selected for preparing macromolecular prodrugs can be categorised according to:
1. The chemical nature (vinylic or acrylic polymers, polysaccharides, poly(
amino acids).
2. The back bone stability (biodegradable polymers, stable polymers).
3. The origin (natural polymers, synthetic polymers).
4. The molecular weight (oligomers, polymers).
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Vinyl polymers
Vinyl polymers can be easily prepared by radical polymerisation of the corresponding vinyl monomer. They are interesting drug carrier candidates. Since copolymerisation of selected monomers results in polymers with a variable composition, different polymer properties can be achieved. In a way, the candidate carriers can be tailor-made to fulfil the requirements for the design of the polymer–drug conjugate. However, vinyl polymers are not biodegradable. Hence, in order to avoid undesirable storage, the molecular weight should at least be below the renal filtration limit (40–50 kDa).
C C
H
H R2
R1H2C C
R2
R1
n
Free radical vinyl polymerization
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The field of polymeric vectors for drug delivery has been dominated by work of Duncan, Kopecek and Seymour. The most well known of their polymers—PK1, an untargeted doxorubicin conjugate, has been evaluated in Phase II clinical trials for breast, lung and colorectal cancers.
Typically, they synthesised copolymers of N-(2-hydroxypropyl) methacrylamide (HPMA) and a second monomer featuring an active ester, as a pendant group; the active ester was linked to the polymerisable moiety by a peptide linker, Gly-Gly or Gly-Phe-Leu-Gly for example.
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These polymers were then reacted with the ‘drug’ to be delivered and a glycosamine in order to provide targeting. HPMA copolymer–Gly-Phe-Leu-Gly-doxorubicin containing galactosamine (PK2) was the only polymer–drug conjugate bearing a targeting ligand to be tested clinically. It was designed as an asialoglycoprotein (ASGP) biomimetic with the aim of targeting the hepatocyte asialoglycoprotein receptor for the treatment of liver cancer.
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a N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer–doxorubicin (PK1). b HPMA copolymer–doxorubicin containing galactosamine (PK2) to promote liver targeting via the asialoglycoprotein receptor. These conjugates are composed of an HPMA copolymer backbone (green), a tetrapeptide Gly-Phe-Leu-Gly linker (yellow) and doxorubicin (red).
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Polymer-Neocarzinostatin Conjugate (SMA-NCS) NCS is the first enediyne antitumor antibiotic that is in clinical use for the treatments of leukemia, gastric carcinoma and pancreatic adenocarcinoma. In clinical studies NCS is shown to be active against acute leukemia. Studies revealed that NCS give remedy of 9 out of 51 patients completely and of 9 a partial remedy. NCS also is found to effective against hepatoma and hematologic malignancies.
Both continuous and intermittent intravenous infusion was the process of NCS administration to patients with a variety of malignant diseases. Leukemic patients on intermittent therapy showed greater changes in bone marrow cellularity than those treated by continuous infusion. 9
Side Effects: Anorexia, nausea, and vomiting were the most frequent side effects of NCS administration. In phase I and phase II evaluations, dose limiting toxicity was late myelosuppression. Gastrointestinal side effects were mild. Three patients had a severe acute reaction resembling anaphylaxis. Allergic reactions were more frequent with intermittent than with continuous infusions. Therefore, the clinical trials of NCS were obstructed by anaphylactic responses. NCS is actually a prodrug that requires sulfhydryl activation for the activity , which results in lower selectivity and cytotoxic activity. However, initial clinical trials were hindered by anaphylactic (serious allergic reaction) responses due to its non-covalently bound protein component. It was observed that if NCS could be made immunologically inert then this allergic action can be removed.
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Poly(styrene-co-maleic acid/anhydride) (SMA) is a vinyl polymer introduced by Maeda and co-workers. It was used to synthesise the prodrug, SMANCS, a conjugate of a low-molecular-weight styrene maleic anhydride copolymer (SMA, 1.6 kDa) and the antitumor protein neocarzinostatin (NCS).
A principal advantage in the use of SMANCS is the tumor targeting mechanism through an enhanced permeability and retention effect. Use of SMANCS has resulted in potential reduction or elimination of toxicity as is generally accounted in bone marrow toxicity associated with the use of NCS. SMANCS was used for the treatment of hepatoma, gastric carcinoma and lung cancer.
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Poly(ethylene glycol) (PEG) has been used to modify a number of therapeutically interesting proteins. PEG is a polymer with many useful properties, it is soluble in water and in organic solvents, it is not toxic and not immunogenic. It is approved by the FDA to be used in nose sprays, food and cosmetics. Grafting of PEG onto proteins reduces their immunogenicity, improves their resistance to proteolytic degradation and improves their thermostability.
PEG-Peptide-Dox Conjugates (a typical conjugate structure)
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Adriamycin-conjugated poly(ethylene glycol)–poly(aspartic acid) block copolymer.
Micelle-Forming Block Copolymers Conjugates of adriamycin with poly(ethylene glycol)–poly(aspartamide) block copolymers tend to form micelles. The hydrophilic PEG chains form the outer shell and the hydrophobic poly(aspartic acid)–doxorubicin components form the inner core. It was demonstrated that these systems have a very high invivo antitumor activity and show a reduced non-specific accumulation in heart, lung and liver.
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Dextrans are a family of polysaccharides mainly composed of 1,6-linked -d-glucose units. Dextran possesses multiple primary and secondary hydroxyl groups and therefore can be easily conjugated with drugs and proteins with reactive groups either by direct conjugation or by incorporation of a spacer arm.
The conjugate of dextran and MP with succinic acid as a linker is a good example:
Some other polymers are also used for the synthesis of different conjugates such as: • Synthetic poly(α-amino acids) • Polysaccharides • Proteins
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2. Controlled Drug Release After administration, it is necessary that the macromolecular prodrug is stable during circulation in the bloodstream but the cytotoxic drug should be released from the macromolecular drug conjugate intracellulary in the lysosomes (lysosomotropic drug delivery) and/or intratumorally (tumoritropic drug delivery). This controlled release from polymeric drug conjugates by enzymatic or hydrolytic cleavage can only be achieved by proper selection of linkage between drug and polymeric carrier. In the development of spacers, the most interest has been focussed on pH-sensitive spacers (pH-controlled drug release) and oligopeptide spacers (enzyme-assisted drug release).
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pH Controlled Drug Release: When the macromolecular drug conjugate is taken up by the cell through endocytosis, the conjugate is predestined to be exposed to the acidic pH of the lysosome. Also in or near the tumor tissue the pH is slightly acidic in comparison with healthy tissue. This relatively low pH can be exploited to design pH-sensitive spacers. The hydrazon linkage and the N-cis-aconityl spacer are examples for such controlled release:
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Structures of Acid Sensitive Linkers
Drug Release by Lysosomal and/or Tumor-Associated Enzymes: After the cell uptake of the polymeric prodrug through endocytosis and after fusion of the endosome with the lysosome, the drug conjugate is not only exposed to the acid environment but also to the degrading nature of the lysosomal enzymes. When the lysosomal hydrolases degrade the spacer—most likely an oligopeptide spacer—the drug is released inside the cell. The lysozymes are not only present in normal cells but are often overexpressed in tumor tissues. If the substrate is a specific oligopeptide for lysosomal proteases, the cytostatic drug can be released by these enzymes in or near the tumor tissue. Subsequently, the tumor cells can be selectively destroyed. For the design of a specific polymer drug conjugate, the site and the rate of the cleavage will then depend on the amino acid composition of the oligopeptide.
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3. The “Enhanced Permeability and Retention Effect” In 1980’s, Maeda and his colleagues found that macromolecules such as polymers and proteins with molecular weight larger than 40-50 kDa showed selective accumulation in tumor tissues, far more than that observed in normal tissues, moreover, they retained in tumor tissues for long periods, i.e., > 24 h. They coined this unique phenomenon enhanced permeability and retention (EPR) effect. Accordingly, an EPR based tumor targeting strategy (macromolecular therapy) was developed by using polymer modification, nanoparticles, micelles, liposome and so on, all of which exhibited more than 10-200 times higher concentrations in tumor than that in normal tissues, such as skin, muscle, heart, and kidney, after systemic administration. These findings led to generalization of the concept of the EPR effect.
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Biological rationale for the design of polymeric anticancer therapeutics. Tumour targeting of long-circulating polymer therapeutics occurs passively by the ‘enhanced permeability and retention’ (EPR) effect. Hyperpermeable angiogenic tumour vasculature allows preferential extravasation of circulating macromolecules and polymeric micelles. Once present in the tumour interstitium, polymer therapeutics act either after endocytic internalization or extracellularly. 20
Polymer–drug conjugates designed for lysosomotropic delivery of small-molecule drugs. Also shown is the use of bioresponsive, endosomolytic polymers to facilitate cytosolic access of genes and proteins from the endosome.
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Use of polymer-based systems to deliver drug within the tumour interstitium, or to destroy tumour cells following interaction with the cell membrane. Polymer-directed enzyme prodrug therapy (PDEPT) is a two-step approach that relies on activation of a polymer–drug conjugate by a complementary polymer–enzyme conjugate. Polymer–enzyme liposome therapy (PELT) relies on the liberation of drug from liposomes by the action of a polymer–phospholipase conjugate. Polymers that are conjugated to membrane active peptides or drugs that are known to activate the apoptosis pathway also have the potential to act at the level of the plasma membrane.
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4. Alteration of the body distribution and the cell uptake by active targeting: Antibody conjugates The use of monoclonal antibodies to direct drug conjugates is based on the fact that surfaces of tumors contain a wide variety of proteins, some of which are specific to the tumor type. The monoclonal antibodies used as targeting group selectively seek out the tumor cells by binding to such tumor-specific antigens. As a result, the drug conjugate should bind very specifically these tumor cells. Frequently, however, the quantity of drug that can be selectively targeted is limited by the number of antigens available. Hence, in cancer therapy the targeted-drug approach has been most successful for extremely potent agents such as the plant toxins, which in conjugation with antibodies have been termed the ‘immunotoxins’. Further problems associated with the use of monoclonal antibodies as targeting moiety are lack of tumor selectivity, tumor access and immunogenecity.
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One antibody-based targeting strategy is antibody directed enzyme prodrug therapy (ADEPT). An enzyme, capable of converting a non-toxic prodrug into a potent cytotoxic drug, is covalently attached to a tumor selective monoclonal antibody. Following localisation of the antibody enzyme conjugate at the tumor site and clearance of residual conjugate from the bloodstream, the prodrug is administered. This prodrug can be converted by the enzyme into a potent cytotoxic drug at the tumor site, so minimising non-specific toxicity.
One major advantage over conventional antibodytargeting is the inherent amplification stage, meaning that for every successful enzyme-targeting event a very large number of prodrug molecules can be activated.
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References: The material of this lecture is collected the following references: 1. K. Hoste, K. De Winne, E. Schacht, Polymeric prodrugs, International Journal of
Pharmaceutics 277 (2004) 119–131. 2. Jayant Khandare, Tamara Minko, Polymer–drug conjugates: Progress in polymeric
prodrugs, Prog. Polym. Sci. 31 (2006) 359–397. 3. Alexander T Florence, David Attwood, Physicochemical Principles of Pharmacy
FOURTH EDITION, Pharmaceutical Press 2006 (USA). 4. Haesun Park, Kinam Park, Waleed S.W. Shalaby, Biodegradable Hydrogels for Drug
Delivery, Technomic Publications, 1993 (USA). 5. Rohini, Neeraj Agrawal, Anupam Joseph and Alok Mukerji, Polymeric Prodrugs:
Recent Achievements and General Strategies, J Antivir Antiretrovir 2013, S15 6. Heidi L. Perez et. al. Antibody–Drug Conjugates: Current Status and Future Directions,
Drug Discovery Today _ Volume 00, Number 00 _ December 2013. 7. Ruth Duncan the Dawning Era of Polymer Therapeutics, Nature Reviews | Drug
Discovery, Volume 2 | May 2003 | 347.
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