Pluronic block copolymers: evolution of drug delivery concept from inert nanocarriers to biological response modifiers

Abstract

Polymer nanomaterials have sparked a considerable interest as vehicles used for diagnostic and therapeutic agents; research in nanomedicine has not only become a frontier movement but is also a revolutionizing drug delivery field. A common approach for building a drug delivery system is to incorporate the drug within the nanocarrier that results in increased solubility, metabolic stability, and improved circulation time. With this foundation, nanoparticles with stealth properties that can circumvent RES and other clearance and defense mechanisms are the most promising. However, recent developments indicate that select polymer nanomaterials can implement more than only inert carrier functions by being biological response modifiers. One representative of such materials is Pluronic block copolymers that cause various functional alterations in cells. The key attribute for the biological activity of Pluronics is their ability to incorporate into membranes followed by subsequent translocation into the cells and affecting various cellular functions, such as mitochondrial respiration, ATP synthesis, activity of drug efflux transporters, apoptotic signal transduction, and gene expression. As a result, Pluronics cause drastic sensitization of MDR tumors to various anticancer agents, enhance drug transport across the blood brain and intestinal barriers, and causes transcriptional activation of gene expression both in vitro and in vivo. Collectively, these studies suggest that Pluronics have a broad spectrum of biological response modifying activities which make it one of the most potent drug targeting systems available, resulting in a remarkable impact on the emergent field of nanomedicine.

Figure 1

Pluronic block copolymer molecule (A) and micelle with a solubilized drug (B).

Figure 2

Growth curves of s.c. A2780 ovarian carcinoma tumors inoculated in female nu/nu mice; DOX (3 mg/kg) was either dissolved in PBS (positive control) or encapsulated in mixed micelles; drug injections were combined or not combined with tumor sonication. Treatments were initiated when the tumor volume reached 75–125 mm³; three consecutive treatments were applied on days 1, 3, and 5. Ultrasound parameters: frequency-1 MHz; power density—3.4 W/cm², duty cycle 50%, duration 30 s, time of application—4 h after the i.v. injection of the drug. Based on Ref. [39].

Figure 3

Multiple effects of Pluronic block copolymers in MDR cells: 1) incorporation of Pluronic molecules into membranes and decrease of the membrane microviscosity; 2) induction of ATP depletion; 3) inhibition of drug efflux transporters; 4) release of cytochrome C from mitochondria and increase in ROS levels in cytoplasm; 5) increase of pro-apoptotic signaling and decrease of anti-apoptotic defense in MDR cells; 6) inhibition of the glutathione/glutathione S-transferase detoxification system; and 7) abolishment of drug sequestration within cytoplasmic vesicles.

Figure 4

Effects of Pluronic P85 on intracellular ATP levels in resistant MCF7/ADR (filled circles) and sensitive MCF7 (empty circles) cells. Cell monolayers were exposed to various concentrations of Pluronic P85 for two hours. Following treatment, the cells were washed with ice-cold PBS, solubilized in Triton X-100 (1%), and frozen immediately for subsequent ATP quantification by a luciferin/luciferase assay. Based on Ref. [57].

Figure 5

A) Schematic representation of the BBB; B) Effect of Pluronic P85 on the digoxin brain/plasma ratio in the wild-type mice at five hours postdose; digoxin brain/plasma ratio in mdr 1a/b (-/-) knockout mouse is shown for comparison. The brain/plasma ratio in the wild-type mouse group was significantly less than in Pluronic P85-treated wild-type or the knockout groups, p < 0.001; no statistical difference was seen between Pluronic P85-treated wild-type mouse and the knockout groups. Based on Ref. [47] with permission.

Figure 6

Effect of Pluronic P85 and BSO on [³H]-Vin accumulation in COR-L23/R cells. Cells were pre-treated for 24 hours with various concentrations of BSO or Pluronic P85 and then incubated for 90 min with [³H]-Vin in the presence of BSO or Pluronic P85. Control cells were incubated with [³H]-Vin in assay buffer.

Figure 7

Development of MDR in cancer cells. A: Time course of the development of resistance in MCF7 cell lines cultured with Dox either alone (filled diamonds) or in combination with 0.001% Pluronic P85 (open diamonds). [Dox] is the concentration of Dox in the growth medium. B: Western blot data for expression of Pgp in: MCF7 parental cells (lane 1), and selected MCF7 cells tolerating: 10ng/ml Dox with 0.001% Pluronic P85 (lane 2); 200ng/ml Dox (lane 3); 10,000ng/ml Dox (lane 4); and C: Rhodamine 123 (R123) accumulation in the developed cells tolerating various concentrations of Dox in the culture media (same lane assignments as designated above). Based on Ref. [83] with permission.

Figure 8

Promoter-dependent effects of Pluronic P85 on gene expression (A) and number of DNA copies following intramuscular injections of plasmid DNA in the muscle. (A) Groups of five Balb/c mice were injected i.m. with 0.3% Pluronic P85-formulated (black bars) or non-formulated (white bars) luciferase-encoding plasmids driven by CMV, SV40, CRE, AP1, NFκB or p53. Luciferase activity in the muscle was determined 24 h after injections. Data are the percentage of gene expression with Pluronic P85-formulated DNA over the naked DNA, mean ± s.e.m (n = 10). * p < 0.05, n.s. not significant (p = 0.06). (B) Balb/c mice were injected i.m. with luciferase-expressing vectors controlled by CMV or SV40 promoters with or without 0.3% Pluronic P85. The relative level of luciferase DNA in the muscles was measured 24 h after injection of the DNA by the real-time PCR. The bars show luciferase expression normalized for GAPDH (n = 6). Statistical comparisons were made for DNA/P85 versus DNA alone: * p < 0.05. Based on Ref. [86].

Figure 9

Pluronic block copolymers activate NF-κB signaling in cells: A) Pluronic affects NF-κB signaling presumably by interacting with cell plasma membrane; B) this leads to rapid phosphorylation of I-kB accompanied by the release of the NF-kB; c) the released NF-kB translocates to the nucleus and activates transcription of the corresponding genes.