Rethinking CRITID Procedure of Brain Targeting Drug Delivery: Circulation, Blood Brain Barrier Recognition, Intracellular Transport, Diseased Cell Targeting, Internalization, and Drug Release

Abstract

The past decades have witnessed great progress in nanoparticle (NP)-based brain-targeting drug delivery systems, while their therapeutic potentials are yet to be fully exploited given that the majority of them are lost during the delivery process. Rational design of brain-targeting drug delivery systems requires a deep understanding of the entire delivery process along with the issues that they may encounter. Herein, this review first analyzes the typical delivery process of a systemically administrated NPs-based brain-targeting drug delivery system and proposes a six-step CRITID delivery cascade: circulation in systemic blood, recognizing receptor on blood-brain barrier (BBB), intracellular transport, diseased cell targeting after entering into parenchyma, internalization by diseased cells, and finally intracellular drug release. By dissecting the entire delivery process into six steps, this review seeks to provide a deep understanding of the issues that may restrict the delivery efficiency of brain-targeting drug delivery systems as well as the specific requirements that may guarantee minimal loss at each step. Currently developed strategies used for troubleshooting these issues are reviewed and some state-of-the-art design features meeting these requirements are highlighted. The CRITID delivery cascade can serve as a guideline for designing more efficient and specific brain-targeting drug delivery systems.

Keywords: brain‐targeting; drug delivery; dual‐targeting; intracellular trafficking; stimulus‐responsive.

Figure 1

Schematic diagram of the proposed CRITID delivery cascade of brain‐targeting NPs delivery system: circulation in systemic blood, recognizing receptor on BBB, intracellular transport with BECs, diseased cell targeting after entering into parenchyma, internalization by diseased cells and finally intracellular drug release. The overall efficiency (E) is the product of six step's efficiencies.

Figure 2

Brain‐targeting NPs with optimized ligand density. A) Schematic diagram of structure and composition of ANG‐PS‐SAP as well as proposed mechanism of glioma cell targeting after intravenous injection. B) In vivo fluorescence imaging of U87 MG‐luc glioma‐bearing nude mice at 24 h post‐injection of DiR‐loaded PS (I) and ANG‐PS with varying ANG densities (II: 10%, III: 20%, IV: 30%). C) Ex vivo fluorescence imaging of excised brains. D) Semiquantitative analysis of the fluorescence intensity of DiR‐loaded PS and ANG‐PS. Reproduced with permission.^{[ 131 ]} Copyright 2018, Wiley‐VCH.

Figure 3

Brain‐targeting NPs with pH‐cleavable linker for accelerating intracellular transport. A) Schematic diagram of preparation of DD‐MCT and proposed mechanism of intracellular pH‐responsive detachment and glioma‐targeting delivery. B) Confocal images of C6 glioma cells in the acceptor chamber of transwell model after incubation with different formulations at donor chamber for 4 h. C) Fluorescence imaging of glioma‐bearing mice injected with Cy7‐labeled DD‐MCT at different time intervals. D) Fluorescence distribution of DC‐MCT at glioma site after intravenous administration for 24 h, glioma slices were immune‐stained with anti‐TfR antibody and anti‐GLUT1 antibody. Reproduced with permission.^{[ 96 ]} Copyright 2018, Wiley‐VCH. E) Schematic diagram of the acid‐responsive programmed AD‐targeted delivery depot for D‐DTCT7/siRNA. F) The representative swimming paths of mice in MWM test, numbers in the lower right indicate the time spent to reach the platform. G) Nissl staining in the hippocampus of mice in different treatment groups, the red box shows the hippocampal CA1 area. Reproduced with permission.^{[ 133 ]} Copyright 2020, Wiley‐VCH.

Figure 4

Illustration of dual‐targeting strategies based on single ligand or dual ligands for targeted treatment of brain diseases.

Figure 5

Single ligand‐based dual‐targeting delivery. A) Schematic mechanism of ANG/PEG‐UCNPs for dual‐targeting delivery to glioma cells. Reproduced with permission.^{[ 161 ]} Copyright 2014, American Chemistry Society. B) Schematic diagram of the preparation and mechanism of DGL‐PEG‐RVG29‐D‐peptide/DNA NPs for dual‐targeting delivery to neurons from AD brain. Reproduced with permission.^{[ 146 ]} Copyright 2016, Elsevier. C) Schematic diagram of the synthesis and mechanism of RNP‐DFO for dual‐targeting delivery to dopaminergic neurons from PD brain. Reproduced with permission.^{[ 164 ]} Copyright 2018, American Chemistry Society. D) Schematic diagram of preparation and mechanism of MNET for dual‐targeting delivery to neurons from ischemic stroke brain. Reproduced with permission.^{[ 150 ]} Copyright 2020, American Chemical Society.

Figure 6

Dual‐targeting strategy based on one target on BBB and one target on diseased cell. A) Schematic mechanism of FeGd‐NH@Pt@LF/RGD2 for dual‐targeting delivery to glioma cells. Reproduced with permission.^{[ 152 ]} Copyright 2018, American Chemical Society. B) Schematic diagram of preparation and mechanism of circular Tau‐TfR aptamer for dual‐targeting delivery to p‐tau protein from AD brain. Reproduced with permission.^{[ 153 ]} Copyright 2020, American Chemical Society. C) Schematic mechanism of T7/SHp‐P‐LPs/ZL006 for dual‐targeting delivery ischemic neuron. Reproduced with permission.^{[ 154 ]} Copyright 2016, Elsevier.

Figure 7

Dual‐targeting strategy based on one target on BBB and two targets on diseased cell. A) Schematic illustration of the functional moieties of dual‐targeting nanoconjugates and proposed mechanism of dual‐targeting delivery to BM after intravenous injection. Survival curve of HER2⁺ BT‐474 BM‐bearing mice treated with B) P/trastuzumab/MsTfR‐mAb/HER2‐AON or C) P/Hu/MsTfR‐mAb/EGFR‐AON. D,E) Western analysis of tumors excised from mouse brains. Reproduced with permission.^{[ 155 ]} Copyright 2015, American Chemical Society.

Figure 8

Dual‐targeting strategy based on two targets on BBB and two on different diseased cells. A) Schematic diagram of functional moieties of iRGD‐DMTPLN and proposed mechanism of dual‐targeting delivery to TNBC and TAMs, respectively. B) In vivo fluorescence images of BM‐bearing mice post‐injection with iRGD‐DMTPLN and DMTPLN at different time intervals. C) In vivo bioluminescence images of representative BM over 28 days. D) Survival plot of BM‐bearing mice treated with different formulations. Reproduced with permission.^{[ 160 ]} Copyright 2019, Wiley‐VCH.

Figure 9

Biomimetic brain‐targeting siRNA delivery system with pH‐responsive release capacity. A) Schematic diagram of fabrication and proposed mechanism of Ang‐RBCm‐CA/siRNA. B) TEM images of Ang‐RBCm‐CA/siRNA and Ang‐RBCm‐SA/siRNA incubated at pH 7.4 or 5.0 for 1, 2, or 3 h. Scale bar: 200 nm. Illustration of the mechanism pH‐triggered Ang‐RBCm‐CA/siRNA membrane disruption. C,D) In vivo and ex vivo fluorescence of orthotopic U87MG‐luc human glioma‐bearing nude mice post‐injection with different formulations at different time intervals. E) Luciferase expression of the brain in mice before or post‐injection of different formulations. F) Semi‐quantitative analysis of the bioluminescence intensity. G) Luminescence images of orthotopic U87MG‐luc human glioma‐bearing nude mice treated with different siPLK1‐loading formulations. Reproduced with permission.^{[ 179 ]} Copyright 2020, American Chemical Society.

Figure 10

Brain‐targeting delivery system with GSH‐responsive release capacity. A) Schematic mechanism of ApoE‐CP for BBB transcytosis, glioma targeting, and GSH‐triggered SAP release. B) In vitro BBB model transport ratio of Cy5‐labeled CP, ANG‐CP, and ApoE‐CP following 24 h of incubation. Blockade experiments were conducted by pretreating U87 MG cells with free ApoE (100 µg mL⁻¹, 30 min). C) In vivo luminescence images of human U87‐MG‐luc glioma‐bearing mice model treated with different formulations (n = 7). D) Survival plot of U87‐MG glioma‐bearing mice treated with different formulations. Reproduced with permission.^{[ 140 ]} Copyright 2018, American Chemical Society.

Figure 11

Brain‐targeting delivery system with ROS‐responsive release capacity. A) Schematic diagram of fabrication and proposed mechanism of Ang‐3I‐NM@siRNA for glioma‐targeting delivery and ROS‐triggered siRNA release. B) Schematic illustration of siRNA released from 3I‐NM@siRNA under H₂O₂ triggering and gel electrophoresis analysis of siRNA release from 3I‐NM@siRNA following H₂O₂ treatment. C) Size change of 3I‐NM@siRNA following H₂O₂ treatment determined by DLS. D) Fluorescence imaging of orthotopic U87MG‐luc human glioma‐bearing nude mice at different time points post‐injection of Ang‐3I‐NM@siRNA and 3I‐NM@siRNA. E) Survival plot of glioma‐bearing mice treated with different formulations. Reproduced with permission.^{[ 186 ]} Copyright 2018, Wiley‐VCH.

Figure 12

Brain‐targeting delivery system with dual stimuli‐responsive release capacity. A) Schematic diagram of fabrication and proposed mechanism of ALBTA for glioma‐targeting delivery and pH and ROS‐triggered drug release. B) The cumulative release ratio of TMZ from LGTA (blank line) and LBTA (blue line) incubated at pH 7.4 and 5.5 at 37 °C. C) Survival plot of glioma‐bearing mice treated with various formulations. D) Representative in vivo T2‐weighted MRI images of brain in intracranial glioma‐bearing mice before and after injection of LBTA (upper) or ALBTA (lower). Reproduced with permission.^{[ 189 ]} Copyright 2018, Wiley‐VCH.