Biomedical Engineering Reference
In-Depth Information
We then implanted them into preformed tumors in nude mice for localized activation of the
prodrug ifosfamide in or close to the tumors [48]. Ifosfamide is a prodrug that is metabolized
into acrolein by CYP2B1 in the liver [49]. Due to the very short half-life of the activated drug
in plasma [50], the prodrug has to be given in a relatively high dose despite severe side
effects, such as leucopenia with granulocytopenia. Establishment of a second site of enzyme
conversion in or near the tumor using the cells expressing CYP2B1 is recognized as an
effective approach to reduce side effects without lowering the response rates to the drug [51-
54]. We injected the cell-enclosing microcapsules into tumors using a 26-gauge syringe after
suspending them in saline. The effectiveness was demonstrated by the significant regression
of tumors in the recipients implanted with the cell-enclosing microcapsules compared with
those implanted with the empty agarose microcapsules (Figure 7) [48].
Alginate-agarose Composite and Agarose-Gelatin Conjugate Microcapsules
Two other types of microcapsules were prepared, alginate-agarose composite
microcapsules [38] and agarose-gelatin conjugate microcapsules [37], using the thermal
gelation process which was previously shown to be harmless towards mammalian cells. The
former ones are for subsequent modification with a polyelectrolyte complex membrane.
Alginate has been one of the more successful components for bioencapsulation for purposes
of cell-encapsulation [13,55-57]. An attractive feature of alginate compared with agarose is
that it has anionic charged carboxyl groups in its structure [14]. Thus it is easy to modify the
resultant gel using cationic polymers such as poly-L-lysine [13,58-60] and chitosan [7,61] via
the formation of electrostatic bonds for controlling biocompatibility and molecular exclusion
properties. As described in the Introduction, alginate has a different gelation mechanism from
agarose. One well-known method for obtaining alginate microcapsules in a water-immiscible
liquid involves the coalescence between droplet(s) of aqueous sodium-alginate solution and
droplet(s) of an aqueous divalent cation solution such as calcium ions. Sugiura
et al.
[62] has
reported the preparation of cell-enclosing alginate microcapsules of about 150 μm in diameter
using the coalescence of droplets prepared with an array of micronozzles of 30 μm × 30 μm
square. The prepared droplets were highly spherical before gelation, but the resultant alginate
microcapsules were far from a spherical shape. Such an awkward shape results in problems
with mechanical stability. In contrast, our method for obtaining the alginate-agarose
composite microcapsules involves two steps: First, the droplets are obtained via extrusion of
the mixture solution into a co-flowing liquid paraffin flow which is cooled for the gelation of
agarose. After formation of spherical microcapsules via gelation of agarose, a 100 mM CaCl
2
solution is added to the suspension for the gelation of alginate. Without the gelation of
alginate, the alginate molecules are released from the microcapsules into the surrounding
aqueous solution. There was no apparent difference between agarose microcapsules and
alginate-agarose composite microcapsules. The effectiveness of the incorporation of alginate
molecules into microcapsules for the subsequent modification with a polyelectrolyte complex
membrane was demonstrated from the obvious suppression of the diffusion of bovine serum
albumin into the microcapsules after soaking in aqueous chitosan solution (Figure 8) [38].
