BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 6: Programmed/Pulsed Drug Delivery and Drug Delivery in Tissue Engineering Last time: principles of controlled release from solid polymers Today: Pulsatile/regulated/multifactor controlled release: 3 case studies of controlled release Reading: ‘Polymeric system for dual growth factor delivery,’ T.P. Richardson et al., Nat. Biotech. 19, 1029-1034 (2001) ‘Microchips as controlled drug-delivery devices,’ J.T. Santini et al., Andegwandte Chemie Intl. Ed. 39, 2396-2047 (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: null Single injection delivery of ‘booster’ for vaccination null Mimic natural secretion patterns of hormones null 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 null 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 null Show an example o Degradable polymers allow submicron, injectable devices ? Two types o Pre-programmed null Release profile is encoded in structure and composition of device o Triggered null External signal drives release Multilayer surface-eroding delivery devices Case study: multilayered delivery devices 3 Fast-degrading Polyanhydride block Hydrophilic PEG block polyphosphazene slow-degrading polylactide 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 null 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Polyanhydride layer acidifies Water penetrates into device Model drug release profiles: environment as it degrades: COO- C O O - COO- C O O- COO- C O O - Low pH Polyphosphazene only degrades quickly at neutral/basic pH : ? 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 ? Principle of a gold electrochemical cell in the presence of aqueous chloride solution: o ON BOARD: H 2 O, Na + Cl - Au cathode Au anode 4 Cl - + Au [AuCl4] - 3 e- [AuCl4] - Au + 4 Cl - [AuCl4] - e- 3 e- reduction oxidation ? 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) m 3+ + 3 e- o 2 Au + 3H 2 O -> Au 2 O 3 + 6H + + 6e- o 2Cl - -> Cl 2 + 2e- o Au 2 O 3 + 8Cl - + 6H + -> 2[AuCl 4 ] - + 3H 2 O ? Design of anode: o Need a material that is: null 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 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 null microchip itself could be reduced to ~ 2 mmx 2mm Lecture 5 – Programmed Drug Delivery 5 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Pourbaix diagram: thermodynamic stability 1.6 1.2 0.8 E/V0.4 0.0 -0.4 -0.8 -1.2 [AuCl 4 ] Au(OH) 3 2 4 6 8 10 12 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 5 – Programmed Drug Delivery 8 of 12 ? Structure of vasculature O 2 O 2 O 2 z pO 2 Blood flow blood vessel structure: Intima (supportive ECM layer) Endothelial cell lining Smooth muscle cells Extensive cell death in center of construct ? Angiogenesis 8 Steps in angiogenesis: 1. -attracts endothelial cells, induces proliferation -induces tube formation 2. -attracts smooth muscle cells, stabilizes new vessels VEGF (vascular endothelial growth factor) PDGF (platelet-derived growth factor) BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 5 – Programmed Drug Delivery 9 of 12 ? Dual growth factor delivery from degradable scaffolds for de novo blood vessel synthesis 9 : Compression mold PLGA particles Lyophilized VEGF PDGF-containing microspheres NaCl particles ? Fabrication process: 10 o PDGF encapsulated in PLGA microspheres by double emulsion approach o Microspheres (5-50 μm) mixed with PLGA particles (150-250 μm), NaCl particles (250-500 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 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 ? In vivo experiments: o Scaffolds implanted subcutaneously in Lewis rats, examined histologically at 2 weeks and 4 weeks o Comparisons: null Free cytokine injections with scaffolds vs. controlled release scaffolds null Dual vs. single factor controlled release scaffolds Lecture 5 – Programmed Drug Delivery 10 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Bolus injection of free cytokines is ineffective: blank VEGF VEGF + PDGF PDGF 2 weeks 4 weeks o controlled release scaffolds induce formation of more blood vessels with larger diameters: VEGF VEGF + PDGF PDGF 2 weeks 4 weeks Lecture 5 – Programmed Drug Delivery 11 of 12 BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o vessels formed in controlled release systems show smooth muscle actin staining indicative of mature vessel formation: 400X 1,000X blank VEGF VEGF + PDGF PDGF References 1. Medlicott, N. J. & Tucker, I. G. Pulsatile release from subcutaneous implants. Adv Drug Deliv Rev 38, 139-149 (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, 526-34 (1999). 3. Qiu, L. Y. & Zhu, K. J. Design of a core-shelled polymer cylinder for potential programmable drug delivery. Int J Pharm 219, 151-60 (2001). 4. Santini, J. T., Jr., Cima, M. J. & Langer, R. A controlled-release microchip. Nature 397, 335-8 (1999). 5. Frankenthal, R. P. & Siconolfi, D. J. The anodic corrosion of gold in concentrated chloride solutions. J. Electrochem. Soc. 129, 1192-1196 (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, 2396-2407 (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, 157-8 (1999). 9. Richardson, T. P., Peters, M. C., Ennett, A. B. & Mooney, D. J. Polymeric system for dual growth factor delivery. Nat Biotechnol 19, 1029-34 (2001). 10. Harris, L. D., Kim, B. S. & Mooney, D. J. Open pore biodegradable matrices formed with gas foaming. J Biomed Mater Res 42, 396-402 (1998). Lecture 5 – Programmed Drug Delivery 12 of 12