Drug Delivery Systems:

Figure 1: In vitro release of ciprofloxacin from HAP implants containing; () 2% (w/w), () 1% (w/w), and (•) 0.5% (w/w) of theoretical drug load.

Other examples of drug delivery systems:

• Inhibitions of tumor growth and metastasis:
  Protein: Cis-diamminedichloroplatinum (CDDP)
  Carrier: HAP or TCP
  CDDP-HAP or CDDP-TCP can be directly implanted into tumors or sub-cutaneously.

• Vitamin D supplements for neonatal vitamin D deficiency:
  Proteins: Serum 25-hydroxyvitamin D (calcidiol) and parathyroid hormone (PTH)
  Carrier: HAP or TCP with sodium hydroxide (NaOH).
  

Other examples of proteins and HAP:

• Lysine
  
• Antibiotic gentamicin sulfate (GS)
  
• Carboplatin (CBDCA)
  
• Glutaraldehyde treated bovine pericardium
  
• Bovine serum albumin (BSA)
  
• Native bone morphogenetic proteins (BMP)
  
• 6-mercaptopurine (6-MP)
  Multi-protein combinations can be loaded to the HAP micro-spheres; their release rates can be tailored using the binding strength and the morphology of the spheres. This constitutes one of the important aspects in being able to control drug release rates as accurately as possible.

Calcium phosphate apatite (CPA) is known to be one of the most important implantable materials due to its biocompatibility. Natural bone is approximately 70% CPA including hydroxyapatite (HAP) by weight and 50% by volume. Organic substances (such as co llagen) and water are the other constituents of bones. The CPA mineral is nanocrystalline with grains less than 50 nm. These nanocrystals are connected to each other to form the connective hard tissue, (i.e. the bone skeleton).

Bone is a complex structure with macro- and micro-pores. The pores are mostly interconnected to allow body fluid to carry nutrients and provide a medium where interfacial reactions between hard tissue and soft tissue can occur. Two types of bone struct ures have been described: cancellous and cortical structures. Cancellous bone differs from cortical bone in being open-spaced and trabecular. Since osteons average between 180 and 250 µm in diameter and intercommunicate through Volksmann canals, the size of the interconnected pores have similar dimensions.

Implant materials must provide properties that are similar to that of bone. Currently, only natural materials have been selected as bone substitute for bone grafting surgery. These include auto- or allograft, bovine, and coral blocks. Although these ma terials can closely replicate the structure of human bone, they present a number of disadvantages. They are typically very brittle and experience significant loss of strength during sterilization. They also can cause inflammation problems and carry the po tential for disease transmission.

To address these issues, our research is focussing in developing high strength synthetic biomaterials such as CPA and HAP ceramics. The ceramics are either sintered porous, or dense calcium-based mixtures (including HAP and TCP). Our research team rece ntly completed an extensive characterization of the sintering behavior of our narrow size distribution powder. We also have completed a study of the phase transformations that can occur in our sintered micro-porous HAP. Our research team is now designing high strength three-dimensional cellular biomaterials.

References:

Ping Luo, Nan Liu, and Michael Thelen, "Protein binding potential of biocompatible ceramic hydroxyapatite micro-spheres", 24th Annual Meeting of the Society for Biomaterials, Vol. 21, 278 (1998).

Hai H. Pham, Ping Luo, François Génin, and Alekha K. Dash, " Preparation and characterization of hydroxyapatite microspheres containing ciprofloxacin", AAPS Annual Meeting, New Orleans, November 14-18 1999.

P. Luo, F. Y. Génin, and T. G. Nieh, Synthetic calcium phsophate-based skeletons, Proceedings of the Fifth World Biomaterials Congress, Toronto, 627 (1996).

Ping Luo, "Methods of Treating Nuclear Hydroxyapatite Materials", US Patent 5994609, Nov. 1999