Inhibitors targeting the N-terminal domain. Significant attention has been paid to discovering inhibitors targeting the NTD of HSP90 in the last few decades (20). At present, there are 18 inhibitors of HSP90 that have been designed and have entered clinical trials (18,19). These inhibitors can be divided into five categories based on their chemical structures: i) natural products and their derivatives; ii) purine-based; iii) benzamide; iv) resorcinol-containing; and v) miscellaneous compounds (Table I).

Geldanamycin (GDA) was discovered as an HSP90 inhibitor in the early 1990s; its binding to the NTD interrupts ATPase activity and blocks the binding of the N-domain to client proteins, resulting in the degradation of oncoproteins and thus, in the abrogation of several cellular processes (67). It is worth mentioning that there is a reactive quinone in GDA which can also induce cell death independently of HSP90 inhibition (68). The toxicity and instability of GDA in vivo indicate that it has limited use in clinical trials (69). It has been demonstrated that the C-17 position of GDA is crucial concerning its function and toxicity. Based on this discovery, derivatives of GDA, including tanespimycin (17-allylamino-17-demethoxygeldanamycin; 17-AAG), retaspimycin hydrochloride (17-allylamino-demethoxygeldanamycin; 17-AAGH2; IPI-504), 17-amino-17-demethoxygeldanamycin (17-AG; IPI-493) and 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) were designed to correct the deficiency in HSP90-targeted treatment (67). 17-AAG has entered and has been evaluated in several phase II trials, where it has been demonstrated to exhibit a short-lived anti-HSP90 biological activity, and a poor solubility in water, resulting in poor bioavailability; these factors have limited the performance of further trials and other structural modifications (70–72). 17-AAGH2 was synthesized to improve the metabolic profile by eliminating the requirement for reduction (73). It is administered intravenously and is prepared by reducing the quinone portion of 17-AAG to hydroquinone using sodium hydrosulfite, forming hydrochloride salt (74). 17-AAGH2 monotherapy has exhibited good tolerance and antitumor activity in patients with gastrointestinal stromal tumors and in non-small cell lung cancer (NSCLC) (74,75). 17-AG (IPI-493) is a semi-synthetic benzoquinone ansamycin derivative, developed into an oral formulation that shows increased efficacy regarding HSP90 inhibition (76). 17-DMAG is a semi-synthetic derivative of geldanamycin. Compared with 17-AAG, 17-DMAG exhibits improved water solubility, bioavailability, reduced metabolism and antitumor efficacy (77). However, the majority of trials on this agent were postponed or terminated, and none of those derivatives entered phase III trials.

Purine-based inhibitors have a purine (or purine-like) scaffold with amine and aryl substituents. PU-3 is the first reported fully synthetic purine-based inhibitor of HSP90 (69). It displays antitumor efficacy by interrupting the proliferation and differentiation of breast cancer cells. Researchers designed derivatives by enhancing the purine chemical scaffold; in doing so, they synthesized new purine or purine-like compounds: BIIB021, BIIB028, MPC-3100, PU-H71 and Debio093 (78). BIIB021 alters the metabolic activity of GIST and improves the response rate in gastrointestinal stromal tumors. Additionally, it exhibits oral bioavailability and is well-tolerated in patients with no substantial hepatotoxicity observed. BIIB028 is an optimization of BIIB021, as the antitumor efficacy of BIIB021 was achieved with substantially high doses and the chemicals required for synthesis are toxic. Researchers found that the N-7 position on the purine scaffold is an optimal site for modification, selecting BIIB028 as the optimal candidate for a series of alkynol analogs. A clinical trial confirmed HSP90 inhibition and objective responses of BIIB028 in refractory metastatic or locally advanced solid tumors (79–82). MCP-3100 and Debio0932 also entered three trials but failed to move beyond phase II (69). PU-H71 has been reported to downregulate the expression of the oncoproteins involved in cell proliferation and cell apoptosis, whilst promoting the degradation of oncoproteins without exhibiting hematological, renal, or hepatic toxicity. These anticancer effects were also observed in triple-negative breast cancer cells, B-cell lymphomas and Ewing sarcoma. PU-H71 is now listed in six clinical trials (NCT03166085; NCT03373877; NCT01581541), and three of these (NCT03935555; NCT01393509; NCT01269593) are still active or recruiting, indicating that PU-H71 is the most promising purine-based inhibitor of HSP90 (83–85).

The only benzamide inhibitor, SNX-5422, was discovered in the pyrazole and was separated and purified using an ATP-affinity column. It has been investigated in multiple clinical trials demonstrating that SNX-5422 exhibited antiproliferative potency at low nanomolar concentrations and high oral bioavailability (86,87).

A resorcinol-containing inhibitor was identified using high-throughput screening (88). To optimize the solubility of this compound, researchers added substituents to it and synthesized the novel compound AUY922 (luminespib) (89), which demonstrated biological efficacy against tumor proliferation, invasion, and metastasis in vivo (90,91). STA-9090 (ganetespib) was synthesized by modifying the resorcinol scaffold. STA-9090 displayed improved performance in downregulating the expression of oncoproteins and the associated pathways than AUY922. STA-9090, having high biological activity and more optimal binding efficacy to HSP90, was discovered as part of a fragment-based drug design approach, combining NMR and X-ray crystallography (92). Notably, STA-9090 has entered nearly 40 trials and has thus far progressed to phase III trials, and it has been revealed to exhibit superior clinical potential regarding HSP90 inhibition (93). KW-2478 was discovered by Nakashima et al (94). The anti-proliferative and cell apoptosis-inducing effects were observed in KW-2478-treated tumor cells. KW-2478 interfered with the interactions between HSP90 and client proteins, such as fibroblast growth factor receptor3, proto-oncogene c-Maf, and cyclin D1, and depleted the transcriptional kinase Cdk9 and the translational inhibitor phosphorylated 4E-BP1 (94–96). KW-2478 was found to be well tolerated and displayed no dose-limiting toxicity in clinical trials (97).

Other HSP90 inhibitors have been identified, targeting the NTD. However, they are not classified into any of the four categories discussed above or do not present with publicly available structures. These compounds include FW-04-806, CH5164840 and XL888. FW-04-806 interferes with the interaction between HSP90 and Cdc37, disassociating the HSP90-Cdc37 complex compound and promoting the degradation of the associated client proteins (98). A decrease in cell viability and proliferation and an increase in programmed cell death were observed in HER2-positive breast cancer cells treated with FW-04-806 (98,99). CH5164840 is a macrocyclic compound bearing a 2-amino-6-arylpyrimidine moiety; it has been shown to exhibit high oral bioavailability in mice (F=70.8%) and potential antitumor efficacy in colorectal cancer (CRC) (100). XL888 was designed by modifying 5-position amine substituent on the 4-carboxamido-2-methylbenzamide, resulting in the reduction of HSP90 client protein expression in vitro, and in presenting with antitumor effects in vivo (101).

There are three domains in HSP90, the C-terminus, N-terminus, and the middle domain. As discussed above, there is more research focus on the NTD domain of HSP90. Several compounds have been demonstrated as potential candidates for HSP90 inhibitors that target NTD. While HSF1 has been demonstrated to be a key factor of resistance to N-terminal inhibitors, it binds to HSP90 and forms homotrimers after N-terminal inhibition, thus inducing the pro-survival heat shock response (102). To overcome this ‘resistance machinery’, inhibitors targeting the CTD have been investigated. NTD inhibitors block HSP90 activity and several have entered clinical trials; however, there is also an ATP binding region in CTD (103–105). This indicates that both the C- and N-terminal regions function as co-chaperones and client proteins and subsequently regulate the associated biological activities; however, the mechanisms involved differ notably (106). N-terminus inhibitors competitively bind to ATP binding sites to abrogate ATPase activity, whereas C-terminus inhibitors disrupt the activities of co-chaperones containing TPR motifs, resulting in aberrant chaperone function. C-terminus inhibitors are emerging candidates for the development of novel cancer chemotherapeutics (27,107,108). Novobiocin is an inhibitor of the DNA gyrase ATP-binding site, and is able to interrupt HSP90 protein folding machinery, leading to the hydrolysis of client proteins and the induction of the apoptosis of cancer cells. Novobiocin can selectively restrain the open-conformation HSP90 and block the progression of the open and close conformation cycle through a cascade of cumulative dynamic changes (105,107,109,110). KU-32 and KU-569 are derivatives of novobiocin. They bind to the C-terminus and lead to a structural shift in the chaperone, which can simultaneously enhance ATP binding and promote ATPase activity (109,111,112). LB76 is the first C-terminus inhibitor designed de novo, by interrupting co-chaperone binding. LB76 is derived from an HSP90 co-chaperone and selectively pulls down HSP90 from cell lysates. Further investigation confirmed that the identity of the binding region was a MEEVD motif in the C-terminus. LB76 restrains the protein-folding function of HSP90, thus blocking protein-protein interactions between HSP90 and co-chaperones (113,114). A de novo delivery system for LB76 produced by polymer nanoparticles and functioning by delivering LB76 into cells and releasing them in a pH-responsive manner, displayed improved inhibitory activity (115). The non-toxic profile and significant HSP90 inhibitory activity make LB70 a promising cancer-targeting candidate therapeutic. KU711 and KU757 target the C-domain of HSP90 in tumor-initiating cells that have stem cell-like properties. They exhibit anti-malignant potential in breast cancer stem cells and head and neck squamous cell carcinoma cancer stem cells by inhibiting invasion, EMT, and self-renewal (116,117). KU711 has been demonstrated to selectively inhibit HSP90 function in thyroid cancer stem cells and induce a potent antitumor response to cell growth, invasion, and migration (118). KU135, compared with the N-terminal inhibitor 17-AAG, displayed more potent anti-proliferative effects in human leukemic cells and most melanoma cells. 17-AAG functions by inducing cell cycle arrest whereas KU135 induces cell cycle arrest and cell apoptosis. The dual effects on tumor cells are in line with the expected effects of antitumor agents (119,120). KU363 was synthesized at the University of Kansas-Lawrence, and KU135 exhibited antiproliferative effects in different cancer models of bronchioalveolar carcinoma and epithelial lung carcinoma (121). It was demonstrated to induce cell apoptosis, and limit cell viability and proliferation in head and neck squamous cell carcinoma cells in vitro and in vivo (122). NCT-50 is a C-terminal-targeting hybrid compound of novobiocin and deguelin. It demonstrated a significant effect on inhibiting the viability, colony-forming ability, and angiogenic ability of NSCLC cells. NCT-50 overcomes the neurotoxicity observed with deguelin and displays more significant pro-apoptotic efficacy on NSCLC cells than either deguelin or novobiocin (123).

As mentioned above, the mammalian HSP90 family is comprised of four isoforms, HSP90α, HSP90β, GRP94 and TRAP1. Among these, HSP90α and HSP90β are the most abundantly expressed isoforms and are primarily located in the cytoplasm (32). The extracellular form of HSP90α is key for cancer cell invasion and migration (124). GRP94 is present in the endoplasmic reticulum, and functions in ‘quality control’ of a small subset of proteins. It recognizes and binds to misfolded proteins, correcting, and refolding said proteins. GRP94 inhibitors block this process, resulting in the translocation of these misfolded proteins to the cytoplasm and hence their subsequent degradation (37). In addition, there are Ca²⁺ binding sites on GRP94 which play vital roles in tumor processes (125). TRAP1, located in the mitochondria, plays a crucial role in mitochondrial homeostasis. It is involved in the regulation of the organelle's redox state and in the disruption to the energy metabolism of cancer cells (1,38). Developing isoform-selective inhibitors allows for the alleviation of the challenges faced with the use of ‘pan-inhibitors’, permitting more targeted and personalized therapies.

HSP90α and HSP90β. For cancer cells, HSP90α- and HSP90β-selective inhibitors are optimal choices considering that they are the most abundantly expressed isoforms and they function by interacting with oncogenic client proteins (1,32). SNX-0723 is a promising candidate, due to its central nervous system permeability and selectivity to HSP90α and HSP90β both. The selectivity of SNX-0723 to cytosolic HSP90 isoforms is ~100-fold vs. GRP94 and ~300-fold vs. TRAP1. Additionally, its affinity to both HSP90α and HSP90β is similar (126,127). Previously, researchers identified a benzolactam-hydroindolone derivative of SNX-0723 that contains a cyclopentyl substituent, compound 31, that exhibited similar pharmacokinetics as SNX-0723, although with a reduced cellular toxicity and ~1,000-fold affinity for HSP90α and HSP90β vs. GRP94 and TRAP1 (128). TAS-116 was the first reported cytosolic-isoform selective inhibitor to enter clinical trials. In human xenograft mouse models, TAS-116 was found to induce HSP90 client protein degradation and reduce tumor burden (129). In another study, a total of 61 patients with advanced solid tumors participated in a clinical trial to identify the safety, maximum tolerated dose, and overall response rates of TAS-116 in monotherapy intervention (130). TAS-116 exhibited antitumor activity with acceptable adverse reactions was acceptable, suggesting further development of this HSP90 inhibitor.

TAS-116 had an acceptable safety profile with notable antitumor activity, supporting the further development of this HSP90 inhibitor. GRP94-selective inhibitors were the first isoform-selective inhibitors being of utmost scientific interest. Using an assay to determine the differences in the N-terminal ATP-binding site between GRP94 and HSP90, it was eventually revealed that GRP94 could be selectively targeted (131). The GRP94-selective inhibitor, BnIm, was developed by replacing the cis-amide with a bioisosteric imidazole ring that mimicked the amide heteroatoms. No degradation of HSP90α/HSP90β client proteins was observed in BnIm-treated cancer cells, indicating its selectivity towards GRP94 vs. the cytosolic isoforms. Additionally, no cytotoxic effects were observed (132). In addition, researchers established the structure-activity relationships of the BnIm scaffold, and further enhancements were made by replacing the imidazole ring with other heterocycles and by modifying the benzyl appendage. This second generation of compounds demonstrated a two-fold higher affinity than BnIm and a 32-fold higher selectivity for GRP94 than HSP90α (133). PU-WS13 is a purine derivative that disrupts the cell surface oncoprotein HER2. This compound substantially decreases HER2 levels in overexpressing HER2 breast cancer cells (134). Compound 54 has a phenyl ring at the meta-position, an isopropyl appendage at the fourth carbon of the benzene ring, and a cyclohexanol with an amine linker at the ortho-position of the benzamide scaffold, exhibiting >1,000-fold affinity to GRP94 than to HSP90α (135).

Pan-inhibitors of HSP90 have demonstrated negative efficacy with regard to its TRAP1 inhibitory effect due to its poor mitochondrial permeability, where TRAP1 is located. Shepherdin is the first peptidomimetic with the ability to permeate into the mitochondria and target TRAP1 (136). There is a highly positively charged moiety at the N-terminus of shepherdin which can interrupt mitochondrial integrity, making it swell and subsequently release cytochrome c (137). For mitochondrial penetration, investigators used the mitochondrial permeating TPP moiety to replace the corresponding ammonium group on PU-H71 to develop the novel new compound SMTIN-P01. SMTIN-P01 induced membrane depolarization and cytochrome c release, resulting in cytotoxicity to cancer cells (138). DN401 is the most selective TRAP1 inhibitor developed to date. Its selectivity for TRAP1 is >9-fold compared with the other isoforms. It was developed by modifying the pyridine ring to a pyrazolopyrimidine scaffold with a pyridinyl appendage (139).

To date, although a few HSP90 inhibitors have been produced that demonstrate selective affinity to HSP90 and disrupt its biological activities, no HSP90 inhibitor has been applied in clinical cancer therapies, which indicates that the full prospects of HSP90 inhibitors have not been administered as a monotherapy in cancer treatment. The inhibitors that have entered clinical trials have had to be postponed or terminated due to their moderate effects. More importantly, resistance to HSP90 inhibitors has been demonstrated as the major reason for their limited effects as a monotherapy (140). HSF1 is a crucial factor for the underlying resistance to NTD inhibitors. It forms homotrimers after binding to the inhibited HSP90 and induces the pro-survival heat shock response (102). Additionally, the overexpression of the multidrug resistance efflux pump P-glycoprotein 1 overcomes the anticancer effects of benzoquinone-based HSP90 inhibitors (141). The overexpression of UDP glucuronosyltransferase 1A results in resistance to resorcinol-based HSP90 inhibitors (142). Based on these findings, combination therapy strategies have been investigated.

A previous study revealed that CRC cells treated with the resorcinol-containing inhibitor, AUY-922, exhibited a higher sensitivity to 5-FU-based chemotherapy both in vitro and in vivo, which supports the combined use of AUY-922 with 5-FU as a feasible therapeutic strategy (141). A phase I clinical study testing the efficacy of the combination therapy AUY-922 and capecitabine demonstrated that in 19 patients with advanced CRC, 63% of these patients demonstrated a partial response or stable disease when treated with the combination therapy (143). The administration of AUY922 in combination with doxorubicin resulted in increased levels of caspase-3 expression, a biomarker of mitochondrial apoptosis, as well as decreased levels of VEGF mRNA, an effect that was not observed in monotherapy treatments (144,145). The combination of irinotecan and 17-AAG was also assessed in a phase I study in patients with solid tumors. Of the 27 patients, a decrease in tumor volume was observed in 6 patients, 5 of 10 patients with p53-mutant tumor had stable disease and 2 of 6 patients with p53-wild-type tumor presented with stable disease (146). Additionally, the multifunctional nanoceria platform loaded with both doxorubicin and STA-9090 as a combination therapy was previously reported in NSCLC. The results reported >80% of NSCLC cell death levels within 48 h in vitro. STA-9090 synergizes with and enhances the therapeutic efficacy of doxorubicin, minimizing the potential cardiotoxicity of doxorubicin via reactive oxygen species (ROS) production. Furthermore, apoptosis, necrosis and migration assays supported the negative effects of STA-9090 on the proliferation and migration of cancer cells (147).

CH5164840 combined with the EGFR inhibitor, erlotinib, has demonstrated improved antitumor effects in EGFR-overexpressing xenograft models. Additionally, ERK signaling was suppressed by the combined application of erlotinib and CH5164840 in vivo (148). 17-AAG has been demonstrated as a positive factor for prognosis in breast cancer cells treated with the VEGF inhibitor, bevacizumab, in a preclinical study (149). The combination of AUY922 and erlotinib was tested in phase I trials that consisted of 18 patients with EGFR-mutant lung cancer. Partial responses were observed; however, the toxic effects were a major limiting factor (150). A combination of STA-9090 and Ziv-Aflibercept, an antiangiogenic agent, was evaluated in patients with advanced adenocarcinoma (three colon adenocarcinomas, one small bowel adenocarcinoma, and one rectal adenocarcinoma). Of 5 patients, 3 achieved stable disease, although dose-related toxicity was observed (151). 17-DMAG in combination with the anti-EGFR agent lapatinib overcame the acquired lapatinib resistance in an ER-positive HER2-overexpressing breast cancer cell line. Suppression of cell proliferation and HSP90 expression was observed in monotherapy and combination therapy, with the latter demonstrating a comparably increased synergistic effect (152). 17-AAG in combination with trastuzumab in HER2-positive breast cancer did not demonstrate suitable antitumor efficacy, whereas the combination therapy demonstrated antitumor efficacy in ALK-mutated lung cancers (50,153). 3-methyladenine (3-MA) is an autophagy inhibitor that selectively inhibits the P13K signaling pathway. The effect of combining 17-AAG with 3-MA evaluated in preclinical trials revealed that the combination therapy resulted in a notable increase in apoptosis and a lower level of autophagy vs. monotherapy (154). The HSP90 molecular chaperone mechanism regulates the Raf kinase signaling pathway; thus, the inhibition of HSP90 affects the Raf kinase signaling pathway. Based on this mechanism, a combination of 17-AAG and Raf kinase inhibitor sorafenib was evaluated in a phase I trial. Antitumor efficacy was observed in 9 out of 12 renal cancer patients and 4 out of 6 melanoma patients (155). A phase Ib trial of TAS-116 combined with nivolumab investigated the tumor response and corresponding adverse response at the same dose in CRC and other solid tumors patients. Positive tumor responses were observed and the optimum concentration for safety profiles and antitumor activity of TAS-116 was 160 mg (156). FW-04-806 is reported to promote the antitumor efficacy of lapatinib in inhibiting cell proliferation and inducing cell apoptosis, and in particular, in reducing HER3 levels which were increased by lapatinib to inhibit HER2. The results highlighted the potential of this combination therapy for HER2-positive breast cancer (99). Bortezomib, a proteasome inhibitor, has demonstrated improved anticancer effects when combined with IPI-504, KW-2478 and PU-H71. The HSP90 inhibitors can overcome intrinsic and acquired resistance to the proteasome inhibitor in mantle cell lymphoma, multiple myeloma and Ewing sarcoma. In addition, the synergistic effect of HSP90 inhibitors and bortezomib is likely to reduce the toxic effects of HSP90 inhibitors, due to a reduction in the required doses (85,95,157,158).

In melanoma cells, HSP90 inhibitors have been demonstrated to be suitable candidates for increasing the sensitivity of tumor cells to T-cells out of a list of 850 bioactive drugs. In addition, the inhibition of HSP90 enhances the antitumor effects of anti-CTLA4 and anti-PD-1 therapy in vivo (159). Furthermore, several HSP90 clients such as mutated EGFR, rearranged ALK, HIF-1α and JAK2 have been revealed to play essential roles in regulating immune checkpoint blockade by promoting PD1 and PD-L1 expression (160). These results suggested that HSP90 inhibitors can be a complementary strategy to immune checkpoint blockade for cancer therapy. In a preclinical study, combining STA-9090 and the anti-PD-L1 antibody STI-A1015 demonstrated better efficacy in colon carcinoma and melanoma in vivo than monotherapies (160). The combination of 17-DMAG and agonists of EphA2 has also been found to improve the recruitment of therapeutic T-cells by reconditioning the tumor microenvironment, leading to an increase in antigen presentation and tumor cell recognition (161).

In addition to the three classifications of combination therapy discussed above, efforts have been made in de novo therapeutic strategies. Radiotherapy is one of the most frequently used cancer treatments. HSP90 client proteins, such as BRCA1, BRCA2, CHK1, DNA-PKcs, ATM, FANCA and the MRE11/RAD50/NBN complex are involved in DNA damage response pathways and have become a major cause of resistance to radiotherapy. Based on the preclinical study that demonstrated the favorable effect of AT13387 in combination with radiotherapy in vitro (162), an in vivo study was performed on mice and positive results were obtained: AT13387 increased sensitivity to radiotherapy, enhanced apoptosis, attenuation of migratory capacity, and a reduced DNA damage response were observed (163). A functional antioxidant nanomedicine composed of nanoceria encapsulated with a two-drug cocktail of lactonic sophorolipids, a constituent of natural sophorolipid known to inhibit histone deacetylase activity, and the HSP90 inhibitor, STA9090, were evaluated in NSCLC. The combination resulted in a marked reduction of cell viability and suppression of cell migration; nanoceria without any encapsulated drugs did not display any additional toxic burden (164). A novel multifunctional nano-platform for targeted delivery of heat, ROS, and 17AAG/17DMAG simultaneously was proposed for prostate cancer treatment (165). The common adverse effects of geldanamycin derivatives are hepatoxicity, renal failure, and gastrointestinal toxicities. Moreover, poor water solubility is a major limiting factor for its clinical use. In that study, nano-platforms were formulated to allow targeted delivery of HSP90 inhibitors, thus improving therapeutic efficacy whilst minimizing their off-site toxic effects (165).

An abundance of HSP90 inhibitors have been discovered over the past decades, with certain inhibitors demonstrating excellent antitumor efficacy both in vitro and in vivo and entered clinical trials. However, limited clinical effects and insurmountable toxicity have forced these trials to be postponed or terminated. Phage display technology is the most commonly used and robust in vitro method to select specific peptides or antibodies against almost any antigen (166). Phage technology screens out the peptides required from a complex mixture pool of billions of displayed peptides on phages in a combinatorial library via the high affinity of peptides to phages with a specific target (167). Peptides that are applied as a targeting tool in cancer treatment may demonstrate advantages in high affinity, favorable absorbability, endogenous degradability and ease of synthesis, with fewer adverse reactions, improved safety profiles, and ease of modifications with a variety of linker chemistries (168,169). Peptide scFv47 was screened from commercially available Tomlinson I and J phage display libraries and was characterized as an HSP90α-selective binder. Experiments in vitro revealed the ability of scFv47 to bind specifically to HSP90α and inhibit ATPase in human breast cancer cells (170). In a recent study, a potentially druggable peptide with strongly selective binding to the N-terminal of HSP90 was screened using a T7-phage display system from an undisclosed-cryptand. This peptide was demonstrated to exhibit strong antibody-like affinity (KD, 62 nM) to the N-terminal of HSP90 driven by enthalpy and demonstrated HSP90-inhibitory biological activity by binding to the ATPase site in the NTD. Notably, it is the first reported strong NTD-specific homing peptide against HSP90 screened using a T7-phage display system from the library of an undisclosed-cryptand36 with two lariat arms (171). It is hypothesized that the HSP90-homing peptide obtained for target recognition is not the final achievement, these homing peptides may form the basis of novel antibody-based HSP90 targeted strategies for anticancer treatment.