**3.1.3 Drug loading and controlled release**

An essential step in the use of MFMNPs for drug delivery is the controlled release of the therapeutic payload in the desired tumor cells or tissues. The drug-loading capacity and release rate are correlated with the binding affinity of the drug. Strong carrier–drug interactions may enhance the loading capacity and decrease the release rate of the drug from the carrier; therefore, the choice of proper carrier–drug interactions is critical in the design and preparation of nanocarriers for drug delivery.

Successful MNP delivery devices with a prolonged circulation time can carry a chemotherapeutic payload and can be engineered to release its drugs after cell internalization. To successfully integrate a drug into a NP system, several design strategies can be explored, including physical complexation with hydrophobic drugs, or covalent bonding with cleavable linkages for intracellular release. Currently, several chemical drug formulations have been combined with MNPs, including paclitaxel, doxorubicin, and methotrexate, all specifically developed for cancer therapy. For example, methotrexate, an anticancer drug, has an affinity to the target cells, and after grafting the drug to the surface,

Fig. 4. Confocal microscopic images of HeLa cells incubated with (a, b) doxorubicin-loaded MFMNPs without folate conjugation or (c, d) folate–conjugated and doxorubicin-loaded

An essential step in the use of MFMNPs for drug delivery is the controlled release of the therapeutic payload in the desired tumor cells or tissues. The drug-loading capacity and release rate are correlated with the binding affinity of the drug. Strong carrier–drug interactions may enhance the loading capacity and decrease the release rate of the drug from the carrier; therefore, the choice of proper carrier–drug interactions is critical in the design

Successful MNP delivery devices with a prolonged circulation time can carry a chemotherapeutic payload and can be engineered to release its drugs after cell internalization. To successfully integrate a drug into a NP system, several design strategies can be explored, including physical complexation with hydrophobic drugs, or covalent bonding with cleavable linkages for intracellular release. Currently, several chemical drug formulations have been combined with MNPs, including paclitaxel, doxorubicin, and methotrexate, all specifically developed for cancer therapy. For example, methotrexate, an anticancer drug, has an affinity to the target cells, and after grafting the drug to the surface,

MFMNPs in (a, c) folate-containing or (b, d) folate-free media (Guo et al., 2011).

**3.1.3 Drug loading and controlled release** 

and preparation of nanocarriers for drug delivery.

MNPs can be internalized more rapidly. Kohler et al. (2006) first demonstrated this utility in a study where methotrexate was covalently attached to the surface of PEG-coated MNPs via a cleavable amide linkage. Recently, Sun et al. (2008b) further modified the same MFMNP system with chlorotoxin to enhance the NP's targeting abilities against brain tumor cells.

Ideal drug delivery systems should be stable with a long circulation time, and should keep the loaded drugs unreleased during circulation in the bloodstream or in normal tissues. Upon reaching the tumor tissues and being taken up by cancer cells, the systems should release the drugs rapidly to kill cancer cells.

To achieve this purpose, stimuli-triggered drug delivery systems have been used that respond to characteristics of the local microenvironment, such as pH, temperature, redox potential, etc. [reviewed by Danhier et al. (2010) and Muthu et al. (2009)]. In particular, pH gradients have been used widely to design responsive nanoparticle delivery systems. Various nanocarriers with pH-responsive delivery behaviors have been developed on the basis of the differential pH values of blood plasma (pH 7.4), extracellular tumor matrix (pH 5.8–7.2), and endocytic compartments such as endosomes (pH 5–6) and lysosomes (pH 4–5). Drugs have been loaded into polymeric nanocarriers by acidic pH-induced cleavable covalent bonds, creating smart drug delivery systems that respond to the endosomal/lysosomal pH (Yoo et al., 2002; Bae et al., 2003; Gillies et al., 2004; Hruby et al., 2005). The pH-induced cleavage of such bonds can accelerate drug release from the nanocarriers.

Drugs are otherwise loaded into the core of polymeric micelles by noncovalent (e.g., hydrophobic) interactions. Compared to chemical attachment, noncovalent entrapment is convenient and easy to achieve. Nasongkla et al. (2006) described the preparation of micelles of PEG-b-poly(D, L-lactide) that encapsulated doxorubicin and a cluster of SPIO nanoparticles by noncovalent hydrophobic interactions. The protonation of doxorubicin under acidic conditions increased its water-solubility and induced its release.

Recently, our group reported a MFMNP platform that can load drugs with ionizable groups and hydrophobic moieties by the combined action of ionic bonding and hydrophobic interactions (Guo et al., 2008, 2010) (Fig. 5). The use of double noncovalent interactions resulted in a high loading affinity at a neutral pH (7.4), preventing premature release into the bloodstream. At an endosomal/lysosomal pH (<5.5), protonation of polycarboxylate anions in the polymer chains led to ionic bond breakage and drug release. The release process was controlled, responded well to pH, and displayed good kinetics.

Fig. 5. Schematic illustration of the MFMNP structure, and the load and release of model drug adriamycin (ADR) (Guo et al., 2008).
