Lecture 6: Programmed/Pulsed Drug Delivery and Drug Delivery in Tissue Engineering

Transcription

1 Lecture 6: Programmed/Pulsed Drug Delivery and Drug Delivery in Tissue Engineering Last time: principles of controlled release from solid polymers Today: Reading: Pulsatile/regulated/multifactor controlled release: 3 case studies of controlled release Polymeric system for dual growth factor delivery, T.P. Richardson et al., Nat. Biotech. 19, (2001) Microchips as controlled drug-delivery devices, J.T. Santini et al., Andegwandte Chemie Intl. Ed. 39, (2000) Regulated controlled release Applications of regulated and pulsatile release Definition: release of cargo in bursts followed by periods of little/no release in a defined temporal pattern 1 Many applications would be best-served by non-monotonic and multi-cargo release profiles o Motivation: Single injection delivery of booster for vaccination Mimic natural secretion patterns of hormones Provide optimal therapy for tolerance-inducing drugs Constant drug levels cause receptor down-regulation Vaccine boosting hormone release patterns in vivo Lecture 5 Programmed Drug Delivery 1 of 12

2 Example: HIV-1 DNA vaccine delivered with boosters to elevate Ab titers 2 : o Mechanical and electrical devices that can provide digitized release typically require larger devices and surgical implantation (e.g. Pharm. Res. 1, 237 (1984)); also have high cost Show an example o Degradable polymers allow submicron, injectable devices Two types o Pre-programmed Release profile is encoded in structure and composition of device o Triggered External signal drives release Multilayer surface-eroding delivery devices Case study: multilayered delivery devices 3 polyphosphazene slow-degrading polylactide block Fast-degrading Polyanhydride block Hydrophilic PEG block Polyphosphazene: o Base-catalyzed degradation, acid-inhibited degradation PEG-b-Polyanhydride: o Rapid bulk erosion- use hydrophilic block to make hrs-long degradation time for mm-thick caps (very fast) o becomes porous during erosion, so need a means to prevent next layer from starting to degrade as water reaches drug-containing layer o creates acidic byproducts as it degrades Function of complete device: o First polyphosphazene layer: degrades quickly (first burst release) o Polyanhydride layer: degrades quickly, acidifies internal environment Even though water penetrates the polyanhydride, no degradation of polyphosphazene begins and no drug is released from the polyphosphazene layer until the polyanhydride has completely eroded and acidic products are removed from microenvironment Lecture 5 Programmed Drug Delivery 2 of 12

3 Polyanhydride layer acidifies environment as it degrades: Model drug release profiles: Water penetrates into device CO O- COO- C O O- COO- COO- Polyphosphazene only degrades quickly at neutral/basic ph : Low ph COO- refs for theory: J Contr Rel 20, 201 (1992); J Cont Rel 18, 159 (1992) Regulated release devices: case example- drug delivery microchips work from M. Cima and R. Langer labs: 4 Lecture 5 Programmed Drug Delivery 3 of 12

4 Principle of a gold electrochemical cell in the presence of aqueous chloride solution: o ON BOARD: e- Au cathode reduction Au anode oxidation 3 e- Au + 4 Cl - 4 Cl - + Au [AuCl4] - [AuCl4] - H 2 O, Na + Cl - [AuCl4] - 3 e- In reality, multiple reactions occur simultaneously at the anode under an applied voltage in the passive and transpassive regime 5 : o Au + 4Cl- -> [AuCl4]- + 3 e- o Au + mh 2 O -> Au(H2O) 3+ m + 3 e- o 2 Au + 3H 2 O -> Au 2 O 3 + 6H + + 6eo 2Cl - -> Cl 2 + 2eo Au 2 O 3 + 8Cl - + 6H + -> 2[AuCl 4 ] - + 3H 2 O Design of anode: o Need a material that is: stable in the presence of chloride ions in the absence of a potential many metals corrode with 0 applied potential in vivo many metals spontaneously form an oxide layer by reaction with water/o 2 in physiological conditions in presence of potential, reacts to form a biocompatible soluble compound Pourbaix diagram: shows thermodynamically favored species under applied potential at varying ph Evans diagram: shows current produced due to electrochemical dissolution of the anode; the current is a measure of the rate of electrons being produced and thus measures the kinetics of the reaction Lecture 5 Programmed Drug Delivery 4 of 12

5 o Shows that gold membranes corrode quickly Structure of the controlled release microchip: o anode is a gold membrane 0.3 µm thick o current limitation in design is size of battery needed to operate the device: ~ 1 cm 2 microchip itself could be reduced to ~ 2 mmx 2mm Lecture 5 Programmed Drug Delivery 5 of 12

6 Pourbaix diagram: thermodynamic stability [AuCl 4 ] Au(OH) 3 E/V Au ph WHY DOES GOLD DISSOLVE AT PH 7? POURBAIX DIAGRAM SHOWS OXIDE IS STABLE FORM (Cima work) 6 Release properties: Lecture 5 Programmed Drug Delivery 6 of 12

7 Controlled Release in Tissue Engineering Tissue Engineering/Regenerative Medicine 2 major approaches for regenerative medicine o In vitro tissue genesis in vivo application o In vivo tissue genesis in vivo application Schematic comparison of in vitro and in vivo tissue engineering approaches 7 : Skin: bone: Role of scaffold: o Provide functions of native ECM o Create a space for new tissue development o Deliver cells to site o Direct macroscopic size/shape of new tissue roles for soluble factor delivery in TE: o chemoattractant gradients used to draw desired cell types into structure o growth factors provided to induce cell proliferation to regenerate tissue o cytokines to induce tissue-specific cell functions Cytokine delivery from scaffolds Case Study: Induction of vascularization in TE scaffolds Challenge of providing nutrients and oxygen to large tissue constructs o Constructs ~500 µm thick or greater cannot be supported by diffusive transport- need vascularization Lecture 5 Programmed Drug Delivery 7 of 12

8 Structure of vasculature O 2 blood vessel structure: O 2 Endothelial cell lining O 2 Smooth muscle cells po 2 Angiogenesis 8 z Extensive cell death in center of construct Blood flow Intima (supportive ECM layer) Steps in angiogenesis: 1. VEGF (vascular endothelial growth factor) -attracts endothelial cells, induces proliferation -induces tube formation 2. PDGF (platelet-derived growth factor) -attracts smooth muscle cells, stabilizes new vessels Lecture 5 Programmed Drug Delivery 8 of 12

9 Dual growth factor delivery from degradable scaffolds for de novo blood vessel synthesis 9 : PDGF-containing microspheres PLGA particles Lyophilized VEGF NaCl particles Compression mold Fabrication process: 10 o PDGF encapsulated in PLGA microspheres by double emulsion approach o Microspheres (5-50 µm) mixed with PLGA particles ( µm), NaCl particles ( µm), and lyophilized VEGF particles (5-50 µm) in mold and compression molded to form a solid disk o Disk equilibrated with CO 2 at 800 psi 48hrs o Pressure rapidly dropped to ambient (14 psi) o Salt leached by soaking in distilled water 48 hrs 1. Fill with high-pressure CO 2 2. Rapidly depressurize Soak with water to leach out NaCl Lecture 5 Programmed Drug Delivery 9 of 12

10 In vivo experiments: o Scaffolds implanted subcutaneously in Lewis rats, examined histologically at 2 weeks and 4 weeks o Comparisons: Free cytokine injections with scaffolds vs. controlled release scaffolds Dual vs. single factor controlled release scaffolds Lecture 5 Programmed Drug Delivery 10 of 12

11 o Bolus injection of free cytokines is ineffective: 2 weeks 4 weeks blank VEGF PDGF VEGF + PDGF o controlled release scaffolds induce formation of more blood vessels with larger diameters: 2 weeks 4 weeks VEGF PDGF VEGF + PDGF Lecture 5 Programmed Drug Delivery 11 of 12

12 o vessels formed in controlled release systems show smooth muscle actin staining indicative of mature vessel formation: 400X 1,000X blank VEGF PDGF VEGF + PDGF References 1. Medlicott, N. J. & Tucker, I. G. Pulsatile release from subcutaneous implants. Adv Drug Deliv Rev 38, (1999). 2. Robinson, H. L. et al. Neutralizing antibody-independent containment of immunodeficiency virus challenges by DNA priming and recombinant pox virus booster immunizations. Nat Med 5, (1999). 3. Qiu, L. Y. & Zhu, K. J. Design of a core-shelled polymer cylinder for potential programmable drug delivery. Int J Pharm 219, (2001). 4. Santini, J. T., Jr., Cima, M. J. & Langer, R. A controlled-release microchip. Nature 397, (1999). 5. Frankenthal, R. P. & Siconolfi, D. J. The anodic corrosion of gold in concentrated chloride solutions. J. Electrochem. Soc. 129, (1982). 6. Santini Jr, J. T., Richards, A. C., Scheidt, R., Cima, M. J. & Langer, R. Microchips as Controlled Drug-Delivery Devices. Angew Chem Int Ed Engl 39, (2000). 7. Yannas, I. V. Tissue and Organ Regeneration in Adults (Springer, New York, 2001). 8. Darland, D. C. & D'Amore, P. A. Blood vessel maturation: vascular development comes of age. J Clin Invest 103, (1999). 9. Richardson, T. P., Peters, M. C., Ennett, A. B. & Mooney, D. J. Polymeric system for dual growth factor delivery. Nat Biotechnol 19, (2001). 10. Harris, L. D., Kim, B. S. & Mooney, D. J. Open pore biodegradable matrices formed with gas foaming. J Biomed Mater Res 42, (1998). Lecture 5 Programmed Drug Delivery 12 of 12