The main advantage of these micelles is that encapsulated drugs are burst release in the acidic intracellular compartments such as endosomes or lysosomes. These formulations improved anti-tumor activity through intracellular pH-sensitive drug delivery [
67]. Additionally, their folate conjugation was proven to enhance
in vivo anti-tumor efficacy at lower effective doses [
68]. Furthermore, pH-sensitive micelles poly(L-hystidine)-
b-PEG and PLA-
b-PEG-
l-FOL (PHSM-f) were superior compared to free and conventional polymer micelles [
69]. The
in vivo experiments using a sensitive micelle system also demonstrate accumulation of particles at the tumor site and tumor regression was 4-5 fold greater than free DOX after 27 days from the first i.v. injection. One study noted that the half-life of DOX in the pH sensitive micelles increased about 6-fold from free DOX in PBS and plasma media. Their uptake at pH 6.8 was 5 times more than at pH 7.4, indicating that the drug release triggered by the reduced tumor pH was effective after the micelles were accumulated by the EPR effect. Another novel tetra-block copolymer [poly(ethylene glycol)-
b-poly(L-histidine)-
b-poly(L-lactic acid)-
b-poly(ethylene glycol)] is capable of triggering release of DOX at pH 6.8 (i.e., tumor acidic pH) or pH 6.4 (i.e., endosomal pH) compared to normal pH 7.4 [
70]. This triggering or burst release effect is dependent upon the molecular weight of the PLA block existing in the tetra polymer which could be a successful therapy for treating solid cancers or delivering cytoplasmic cargo
in vivo. A new formulation composed of DOX in PDLLA-
b-PEG-b-poly(L-histidine)-TAT (transactivator of transcription) micelle was able to expose TAT only at a slightly acidic tumor extracellular pH to facilitate the internalization process [
71]. These micelles were tested with the xenograft models of human ovarian tumor drug-resistant A2780/AD, human breast tumor drug-sensitive MCF-7 and human lung tumor A549 in a nude mice model, and all tumors considerably regressed in size after three bolus injections at a dose of 10 mg DOX per kg body weight, at three day intervals, while minimum weight loss was observed. The conjugation of drugs to the acid cleavable micelle polymers facilitated prolonged release of drugs [
72]. Doxorubicin-conjugated PLLA-mPEG micelles were more potent because they were taken up within cells with simultaneous rapid release of cleaved doxorubicin into the cytoplasm from acidic endosomes [
72]. A Triblock copolymer conjugated with DOX through the end OH groups of copolymers, indicated that hydrazone linkage was cleaved under acidic conditions [
73]. This behavior was confirmed by flow cytometry and confocal microscopy which demonstrated the extent of cellular uptake of micelle conjugated DOX and distribution in the cytoplasm, endosomal/liposomal vesicles, and nucleus, while the free drug was localized within the nucleus.