Characterization of Binding Properties of ICAM-1 Peptides to LFA-1: Inhibitors of T-cell Adhesion

Intercellular adhesion molecule-1 suppresses TMZ chemosensitivity in acquired TMZ-resistant gliomas by increasing assembly of ABCB1 on the membrane

AIMS

Despite aggressive treatment, the recurrence of glioma is an inevitable occurrence, leading to unsatisfactory clinical outcomes. A plausible explanation for this phenomenon is the phenotypic alterations that glioma cells undergo aggressive therapies, such as TMZ-therapy. However, the underlying mechanisms behind these changes are not well understood.

METHODS

The TMZ chemotherapy resistance model was employed to assess the expression of intercellular adhesion molecule-1 (ICAM1) in both in vitro and in vivo settings. The potential role of ICAM1 in regulating TMZ chemotherapy resistance was investigated through knockout and overexpression techniques. Furthermore, the mechanism underlying ICAM1-mediated TMZ chemotherapy resistance was examined using diverse molecular biological methods, and the lipid raft protein was subsequently isolated to investigate the cellular subcomponents where ICAM1 operates.

RESULTS

Acquired TMZ resistant (TMZ-R) glioma models heightened production of intercellular adhesion molecule-1 (ICAM1) in TMZ-R glioma cells. Additionally, we observed a significant suppression of TMZ-R glioma proliferation upon inhibition of ICAM1, which was attributed to the enhanced intracellular accumulation of TMZ. Our findings provide evidence supporting the role of ICAM1, a proinflammatory marker, in promoting the expression of ABCB1 on the cell membrane of TMZ-resistant cells. We have elucidated the mechanistic pathway by which ICAM1 modulates phosphorylated moesin, leading to an increase in ABCB1 expression on the membrane. Furthermore, our research has revealed that the regulation of moesin by ICAM1 was instrumental in facilitating the assembly of ABCB1 exclusively on the lipid raft of the membrane.

CONCLUSIONS

Our findings suggest that ICAM1 is an important mediator in TMZ-resistant gliomas and targeting ICAM1 may provide a new strategy for enhancing the efficacy of TMZ therapy against glioma.

Fuzapladib in a randomized controlled multicenter masked study in dogs with presumptive acute onset pancreatitis

BACKGROUND

Currently, no specific treatment is available for acute onset pancreatitis (AP), and management relies on symptomatic and supportive standard of care (SOC). Fuzapladib is a novel leukocyte function-associated antigen type-1 (LFA-1) activation inhibitor, blocking activation and subsequent adhesion and migration of neutrophils, potentially decreasing the risk of pancreatitis progression and systemic inflammation.

OBJECTIVE

Evaluate the safety and clinical response of dogs with AP after 3 days of administration of fuzapladib.

ANIMALS

Sixty-one client-owned dogs with presumptive AP.

METHODS

Randomized, masked, and placebo controlled multicenter study. Sixty-one dogs with AP were included for safety assessment, whereas 35 evaluable cases (fuzapladib, n = 16; placebo, n = 19) were included for clinical evaluation. Clinical improvement was assessed based on the change in the modified clinical activity index (MCAI) score on Day 3 compared to Day 0. Secondary variables included canine acute pancreatitis clinical severity index (CAPCSI) scores and serum concentrations of canine pancreatic lipase immunoreactivity, cytokines, and C-reactive protein.

RESULTS

Fuzapladib was well tolerated by all treated dogs. Mean change in MCAI scores was significantly higher in the fuzapladib-treated (-7.75) than the placebo group (-5.68; P = .02, 95% confidence interval [CI] for the difference, -4.33, -0.35), suggesting clinical improvement in fuzapladib-treated dogs. No significant difference was found in any of the secondary variables between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of fuzapladib to dogs was safe, and a favorable response was detected in 2 clinical activity scores. Effects of fuzapladib on survival and duration of hospitalization were not studied.

Immune Modulation by Antigenic Peptides and Antigenic Peptide Conjugates for Treatment of Multiple Sclerosis

The immune system defends our body by fighting infection from pathogens utilizing both the innate and adaptive immune responses. The innate immune response is generated rapidly as the first line of defense. It is followed by the adaptive immune response that selectively targets infected cells. The adaptive immune response is generated more slowly, but selectively, by targeting a wide range of foreign particles (i.e., viruses or bacteria) or molecules that enter the body, known as antigens. Autoimmune diseases are the results of immune system glitches, where the body's adaptive system recognizes self-antigens as foreign. Thus, the host immune system attacks the self-tissues or organs with a high level of inflammation and causes debilitation in patients. Many current treatments for autoimmune diseases (i.e., multiple sclerosis (MS), rheumatoid arthritis (RA)) have been effective but lead to adverse side effects due to general immune system suppression, which makes patients vulnerable to opportunistic infections. To counter these negative effects, many different avenues of antigen specific treatments are being developed to selectively target the autoreactive immune cells for a specific self-antigen or set of self-antigens while not compromising the general immune system. These approaches include soluble antigenic peptides, bifunctional peptide inhibitors (BPI) including IDAC and Fc-BPI, polymer conjugates, and peptide-drug conjugates. Here, various antigen-specific methods of potential treatments, their efficacy, and limitations will be discussed along with the potential mechanisms of action.

Improving Brain Delivery of Biomolecules via BBB Modulation in Mouse and Rat: Detection using MRI, NIRF, and Mass Spectrometry

There is an urgent need to develop new and alternative methods to deliver functional biomolecules to the brain for diagnosis and treatment of brain diseases. The goal of this study was to evaluate the activity of blood-brain barrier (BBB) modulators (i.e., HAV and ADT peptides) to deliver functional biomolecules (i.e., galbumin, IRdye800cw-cLABL, and cIBR7) to the brains of mice and rats. HAV6, cHAVc3, and ADTC5 peptides but not HAV4 peptide significantly enhanced the brain delivery of 65 kDa galbumin compared to control in Balb/c mice as quantified by magnetic resonance imaging (MRI). Ten-minute pretreatment with ADTC5 peptide still significantly increased brain delivery of galbumin; however, no enhancement was observed after 10-min pretreatment with HAV6. There was no enhancement of galbumin deposition following 40-min pretreatment with ADTC5 or HAV6, suggesting a short duration of the BBB opening for large molecules. ADTC5 peptide also improved the brain delivery of IRdye800cw-cLABL peptide about 3.5-fold compared to control in Balb/c mice as detected by near infrared fluorescence (NIRF). The BBB modulator activity of ADTC5 to deliver cIBR7 peptide was also evaluated in vivo using Sprague-Dawley rats. The amount of cIBR7 in the brain was detected by LC-MS/MS. ADTC5 peptide enhanced the delivery of cIBR7 peptide into rat brain about 4-fold compared to control and the intact cIBR7 can be efficiently extracted and detected in rat brain. In conclusion, HAV and ADT peptides enhance the brain delivery of functional peptides (e.g., cLABL and cIBR7) and protein (e.g., 65 kDa galbumin) in two animal models, and the duration of the BBB opening for a large molecule (e.g., galbumin) was short.

Conjugates of Cell Adhesion Peptides for Therapeutics and Diagnostics Against Cancer and Autoimmune Diseases

Overexpressed cell-surface receptors are hallmarks of many disease states and are often used as markers for targeting diseased cells over healthy counterparts. Cell adhesion peptides, which are often derived from interacting regions of these receptor-ligand proteins, mimic surfaces of intact proteins and, thus, have been studied as targeting agents for various payloads to certain cell targets for cancers and autoimmune diseases. Because many cytotoxic agents in the free form are often harmful to healthy cells, the use of cell adhesion peptides in targeting their delivery to diseased cells has been studied to potentially reduce required effective doses and associated harmful side-effects. In this review, multiple cell adhesion peptides from extracellular matrix and ICAM proteins were used to selectively direct drug payloads, signal-inhibitor peptides, and diagnostic molecules, to diseased cells over normal counterparts. RGD constructs have been used to improve the selectivity and efficacy of diagnostic and drug-peptide conjugates against cancer cells. From this precedent, novel conjugates of antigenic and cell adhesion peptides, called Bifunctional Peptide Inhibitors (BPIs), have been designed to selectively regulate immune cells and suppress harmful inflammatory responses in autoimmune diseases. Similar peptide conjugations with imaging agents have delivered promising diagnostic methods in animal models of rheumatoid arthritis. BPIs have also been shown to generate immune tolerance and suppress autoimmune diseases in animal models of type-1 diabetes, rheumatoid arthritis, and multiple sclerosis. Collectively, these studies show the potential of cell adhesion peptides in improving the delivery of drugs and diagnostic agents to diseased cells in clinical settings.

Specific targeting of the IL-23 receptor, using a novel small peptide noncompetitive antagonist, decreases the inflammatory response

IL-23 is part of the IL-12 family of cytokines and is composed of the p19 subunit specific to IL-23 and the p40 subunit shared with IL-12. IL-23 specifically contributes to the inflammatory process of multiple chronic inflammatory autoimmune disorders, including psoriasis, multiple sclerosis, inflammatory bowel disease, and rheumatoid arthritis. So far, one antibody targeting the shared p40 subunit of IL-12 and IL-23, Ustekinumab, is approved clinically to treat psoriasis. However, there are no treatments inhibiting specifically the IL-23 proinflammatory response. We have developed small IL-23R-specific antagonists by designing all D-peptides arising from flexible regions of IL-23R. Of these peptides, we selected 2305 (teeeqqly), since in addition to its soluble properties, it inhibited IL-23-induced STAT3 phosphorylation in spleen cells. Peptide 2305 specifically binds to IL-23R/IL-12Rβ1-expressing HEK-293 cells and not to cells devoid of the receptor. Peptide 2305 showed functional selectivity by modulating IL-23-induced gene expression in IL-23R/IL-12Rβ1-expressing cells and in Jurkat cells; 2305 does not inhibit IL-12-induced cytokine expression in IL-12Rβ-IL-12Rβ2-HEK-293 cells. Finally, compared with anti-p40 treatment, 2305 effectively and selectively inhibits IL-23-induced inflammation in three in vivo mouse models: IL-23-induced ear inflammation, anti-CD40-induced systemic inflammatory response, and collagen-induced arthritis. We, hereby, describe the discovery and characterization of a potent IL-23R small-peptide modulator, 2305 (teeeqqly), that is effective in vivo. 2305 may be more convenient, less cumbersome, less costly, and most importantly, more specific than current biologics for the treatment of inflammatory conditions, and conceivably complement the actual therapies for these chronic and debilitating inflammatory diseases.

Controlling immune response and demyelination using highly potent bifunctional peptide inhibitors in the suppression of experimental autoimmune encephalomyelitis

In this study, we investigated the efficacy of new bifunctional peptide inhibitors (BPIs) in suppressing experimental autoimmune encephalomyelitis (EAE) in an animal model. BPI [e.g. proteolipid protein-cyclo(1,8)-CPRGGSVC-NH2 (PLP-cIBR)] is a conjugate between the PLP139-151 peptide derived from proteolipid protein (PLP) and the cIBR7 peptide derived from domain-1 (D1) of intercellular adhesion molecule-1 (ICAM-1). PLP-cIBR is designed to bind to major histocompatibility complex (MHC)-II and leucocyte function-associated antigen-1 (LFA-1) simultaneously to inhibit the formation of the immunological synapse and alter the differentiation and activation of a subpopulation of T cells, thus inducing immunotolerance. The results show that PLP-cIBR is highly potent in ameliorating EAE, even at low concentrations and less frequent injections. Mice treated with PLP-cIBR had a higher secretion of cytokines related to regulatory and/or suppressor cells compared to phosphate-buffered saline (PBS)-treated mice. In contrast, T helper type 1 (Th1) cytokines were higher in mice treated with PBS compared to PLP-cIBR, suggesting that it suppressed Th1 proliferation. Also, we observed significantly less demyelination in PLP-cIBR-treated mice compared to the control, further indicating that PLP-cIBR promoted protection against demyelination.

Methotrexate (MTX)-cIBR conjugate for targeting MTX to leukocytes: conjugate stability and in vivo efficacy in suppressing rheumatoid arthritis

Methotrexate (MTX) has been used to treat rheumatoid arthritis at low doses and leukemia at high doses; however, this drug can produce severe side effects. Our hypothesis is that MTX side effects can be attenuated by directing the drug to the target cells (i.e., leukocytes) using (cyclo(1,12)PenPRGGSVLVTGC) peptide (cIBR). To test this hypothesis, MTX was conjugated to the N-terminus of cIBR peptide to give MTX-cIBR conjugate. MTX-cIBR (5.0 mg/kg) suppressed joint arthritis in adjuvant arthritis rats and prevented periarticular inflammation and bone resorption of the limb joints. In vitro, the toxicity of MTX-cIBR peptide against Molt-3 T cells was inhibited by anti-lymphocyte function-associated antigen-1 (LFA-1) antibody and cIBR peptide in a concentration-dependent manner, suggesting that the uptake of MTX-cIBR was partially mediated by LFA-1. Chemical stability studies indicated that MTX-cIBR was most stable at pH 6.0. The MTX portion of MTX-cIBR was unstable under acidic conditions, whereas the cIBR portion was unstable under basic conditions. In biological media, MTX-cIBR had short half lives in rat plasma (44 min) and homogenized rat heart tissue (38 min). This low plasma stability may contribute to the low in vivo efficacy of MTX-cIBR; therefore, there is a need to design a more stable conjugate to improve the in vivo efficacy.

The latest developments in synthetic peptides with immunoregulatory activities

In the past few years, many researches have provided us with much data demonstrating the abilities of synthetic peptides to impact immune response in vitro and in vivo. These peptides were designed according to the structure of some important protein molecules which play a key role in immune response, so they act with specific targets. The class I and II MHC-derived peptides inhibit the TCR recognition of antigen peptide-MHC complex. Rationally designed CD80 and CD154-binding peptides block the interaction between cell surface costimulatory molecules on antigen-presenting cells (APCs) and T cells. Some peptides were designed to inhibit the activities of cell signal proteins, including JNK, NF-κB and NFAT. Some peptide antagonists competitively bind to important cytokines and inhibit their activities, such as TNF-α, TGF-β and IL-1β inhibitory peptides. Adhesion molecule ICAM-1 derived peptides block the T cell adhesion and activation. These immunoregulatory peptides showed therapeutic effect in several animal models, including collagen-induced arthritis (CIA), autoimmune cystitis model, murine skin transplant model and cardiac allograft model. These results give us important implications for the development of a novel therapy for immune mediated diseases.

cIBR effectively targets nanoparticles to LFA-1 on acute lymphoblastic T cells

Leukocyte function associated antigen-1 (LFA-1) is a primary cell adhesion molecule of leukocytes required for mediating cellular transmigration into sites of inflammation via the vascular endothelium. A cyclic peptide, cIBR, possesses high affinity for LFA-1, and conjugation to the surface of poly(DL-lactic-co-glycolic acid) nanoparticles can specifically target and deliver the encapsulated agents to T cells expressing LFA-1. The kinetics of targeted nanoparticle uptake by acute lymphoblastic leukemia T cells was investigated by flow cytometry and microscopy and compared to untargeted nanoparticles. The specificity of targeted nanoparticles binding to the LFA-1 integrin was demonstrated by competitive inhibition using free cIBR peptide or using the I domain of LFA-1 to inhibit the binding of targeted nanoparticles. The uptake of targeted nanoparticles was concentration and energy dependent. The cIBR-conjugated nanoparticles did not appear to localize with lysosomes whereas untargeted nanoparticles were detected in lysosomes in 6 h and steadily accumulated in lysosomes for 24 h. Finally, T-cell adhesion to epithelial cells was inhibited by cIBR nanoparticles. Thus, nanoparticles displaying the cIBR ligand may offer a useful targeted drug delivery system as an alternative treatment of inflammatory diseases involving transmigration of leukocytes.

Effect of modification of the physicochemical properties of ICAM-1-derived peptides on internalization and intracellular distribution in the human leukemic cell line HL-60

The objective of this work is to test the hypothesis that increasing the hydrophilicity of DOX-peptide conjugates may modify their entry mechanisms into HL-60 cells from passive diffusion to receptor-mediated uptake. To test this hypothesis, the entry mechanisms and the intracellular disposition of DOX-cIBR7, DOX-PEGcIBR7, FITC-cIBR, and FITC-cIBR7 were evaluated in HL-60 cells. To increase the hydrophilicity of the peptide, the cIBR peptide (cyclo(1,12)Pen-PRGGSVLVTGC) was modified to cIBR7 peptide (cyclo(1,8)CPRGGSVC) by removing the hydrophobic residues at the C-terminus. DOX-cIBR7 conjugate, which has higher hydrophilicity than DOX-cIBR, was synthesized. Second, a hydrophilic linker (11-amino-3,6,9-trioxaundecanate linker) was incorporated between DOX and cIBR7 to generate DOX-PEGcIBR7 with higher hydrophilicity than DOX-cIBR7. As controls, FITC-cIBR and FITC-cIBR7 were used to check for any endocytic uptake process of the peptide. As previously found with DOX-cIBR, DOX-cIBR7, and DOX-PEGcIBR7, conjugates enter the cells via passive diffusion and not via the energy-dependent endocytic process. This result suggests that an increase in hydrophilicity does not influence the entry mechanism of the DOX-peptide conjugates. In contrast to the DOX-cIBR7 conjugate, the FITC-cIBR7 conjugate showed energy-dependent cellular entry into the cells and followed an endocytic pathway similar to that for dextran. Finally, the entry of DOX-cIBR7 and DOX-PEGcIBR into the cell cytosol was shown to be due to the properties of DOX and not to those of the peptide.

Solution structure of a novel T-cell adhesion inhibitor derived from the fragment of ICAM-1 receptor: cyclo(1,8)-Cys-Pro-Arg-Gly-Gly-Ser-Val-Cys

This study is aimed at elucidating the structure of a novel T-cell adhesion inhibitor, cyclo(1,8)-CPRGGSVC using one- and two-dimensional (2D) (1)H NMR and molecular dynamics (MD) simulation. The peptide is derived from the sequence of its parent peptide cIBR (cyclo(1,12)-PenPRGGSVLVTGC), which is a fragment of intercellular adhesion molecule-1 (ICAM-1). Our previous results show that the cyclo(1,8)-CPRGGSVC peptide binds to the LFA-1 I-domain and inhibits heterotypic T-cell adhesion, presumably by blocking the LFA-1/ICAM-1 interactions. The structure of the peptide was determined using NMR and MD simulation in aqueous solution. Our results indicate that the peptide adopts type-I beta-turn conformation at the Pro2-Arg3-Gly4-Gly5 (PRGG) sequence. The beta-turn structure at the PRGG motif is well conserved in cIBR peptide and ICAM-1 receptor, which suggests the importance of the PRGG motif for the biological activity of cyclo(1,8)-CPRGGSVC peptide. Meanwhile, the Gly5-Ser6-Val7-Cys8-Cys1 (GSVCC) sequence forms a "turn-like" random coil structure that does not belong to any structured motif. Therefore, cyclo(1,8)-CPRGGSVC peptide has only one structured region at the PRGG sequence, which may play an important role in the binding of the peptide to the LFA-1 I-domain. The conserved beta-turn conformation of the PRGG motif in ICAM-1, cIBR, and cyclo(1,8)-CPRGGSVC peptides can potentially be used to design peptidomimetics. (c) 2009 Wiley Periodicals, Inc. Biopolymers 91: 633-641, 2009.This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com.

Immunological and inflammatory functions of the interleukin-1 family

More than any other cytokine family, the interleukin (IL)-1 family is closely linked to the innate immune response. This linkage became evident upon the discovery that the cytoplasmic domain of the IL-1 receptor type I is highly homologous to the cytoplasmic domains of all Toll-like receptors (TLRs). Thus, fundamental inflammatory responses such as the induction of cyclooxygenase type 2, increased expression of adhesion molecules, or synthesis of nitric oxide are indistinguishable responses of both IL-1 and TLR ligands. Both families nonspecifically affect antigen recognition and lymphocyte function. IL-1beta is the most studied member of the IL-1 family because of its role in mediating autoinflammatory diseases. Although the TLR and IL-1 families evolved to assist in host defense against infection, unlike the TLR family, the IL-1 family also includes members that suppress inflammation, both specifically within the IL-1 family but also nonspecifically for TLR ligands and the innate immune response.

Development of a novel noncompetitive antagonist of IL-1 receptor

IL-1 is a major proinflammatory cytokine which interacts with the IL-1 receptor I (IL-1RI) complex, composed of IL-1RI and IL-1R accessory protein subunits. Currently available strategies to counter pathological IL-1 signaling rely on a recombinant IL-1 receptor antagonist, which directly competes with IL-1 for its binding site. Presently, there are no small antagonists of the IL-1RI complex. Given this void, we derived 15 peptides from loops of IL-1R accessory protein, which are putative interactive sites with the IL-1RI subunit. In this study, we substantiate the merits of one of these peptides, rytvela (we termed "101.10"), as an inhibitor of IL-1R and describe its properties consistent with those of an allosteric negative modulator. 101.10 (IC(50) approximately 1 nM) blocked human thymocyte proliferation in vitro, and demonstrated robust in vivo effects in models of hyperthermia and inflammatory bowel disease as well as topically in contact dermatitis, superior to corticosteroids and IL-1ra; 101.10 did not bind to IL-1RI deficient cells and was ineffective in vivo in IL-1RI knockout mice. Importantly, characterization of 101.10, revealed noncompetitive antagonist actions and functional selectivity by blocking certain IL-1R pathways while not affecting others. Findings describe the discovery of a potent and specific small (peptide) antagonist of IL-1RI, with properties in line with an allosteric negative modulator.

Structural modifications of ICAM-1 cyclic peptides to improve the activity to inhibit heterotypic adhesion of T cells

Lymphocyte function-associated antigen-1 (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) interaction plays an important role in the formation of the immunological synapse between T cells and antigen-presenting cells. Blocking of LFA-1/ICAM-1 interactions has been shown to suppress the progression of autoimmune diseases. cIBR peptide (cyclo(1,12)PenPRGGSVLVTGC) inhibits ICAM-1/LFA-1 interaction by binding to the I-domain of LFA-1. To increase the bioactivity of cIBR peptide, we systemically modified the structure of the peptide by (i) replacing the Pen residue at the N-terminus with Cys, (ii) cyclization using amide bond formation between Lys-Glu side chains, and (iii) reducing the peptide size by eliminating the C-terminal residue. We found that the activity of cIBR peptide was not affected by replacing Phe with Cys. Peptide cyclization by forming the Lys-Glu amide bond also increased the activity of cIBR peptide, presumably due to the resistance of the amide bond to the reducing nature of glutathione in plasma. We also found that a reduced derivative of cIBR with eight residues (cyclo(1,8)CPRGGSVC) has a bioactivity similar to that of the larger cIBR peptides. Our findings suggest that, by systemically modifying the structure of cIBR peptide, the biological activity of these derivatives can be optimized for future use to inhibit T-cell adhesion in in vivo models of autoimmune diseases.

ICAM-1 peptide inhibitors of T-cell adhesion bind to the allosteric site of LFA-1. An NMR characterization

We have used nuclear magnetic resonance to characterize the binding site of two intercellular adhesion molecule-1 derived cyclic peptides, cIBC and cIBR, to the I-domain of leukocyte function-associated antigen-1. These peptides inhibit the leukocyte function-associated antigen-1/intercellular adhesion molecule-1 interaction known to play a key role in autoimmune diseases and cancer metastasis. Perturbation of the chemical shifts and intensities of the nuclear magnetic resonance signals corresponding to a number of residues of the I-domain of leukocyte function-associated antigen-1 show that both peptides bind to the I-domain allosteric site, the binding site of I-domain allosteric inhibitors such as lovastatin, and therefore the peptides probably also act as allosteric inhibitors of leukocyte function-associated antigen-1. Molecular models of the interaction of these two cyclic peptides with leukocyte function-associated antigen-1 I-domain show that the binding mode of the three molecules are analogous: the hydrophobic residues of the peptides remain buried and occupy the same positions as the apolar groups of lovastatin, while the peptides regions containing the most polar residues are flexible and primarily exposed to the solvent. These results suggest an allosteric mechanism for the inhibitory effect on T-cell adhesion displayed by both peptides, which exhibit potential as therapeutic agents.

Mechanism of Internalization of an ICAM-1-Derived Peptide by Human Leukemic Cell Line HL-60: Influence of Physicochemical Properties on Targeted Drug Delivery

Peptide-mediated targeted delivery offers an attractive strategy for selective delivery of cytotoxic drugs to cancer cells. In this work, we have investigated the mechanism of internalization of cIBR peptide [cyclo(1,12)PenPRGGSVLVTGC] that is conjugated with fluorescein isothiocyanate (FITC) and doxorubicin (DOX) to give FITC-cIBR and DOX-cIBR conjugates, respectively. Internalization mechanisms of FITC-cIBR and DOX-cIBR were studied in LFA-1-expressing cells (HL-60) and LFA-1-deficient cells (HUVEC) under the following conditions: (a) at two different temperatures (4 and 37 degrees C), (b) in the presence of ATP-depleting agents (sodium azide and 2-deoxy- d-glucose), and (c) in the presence of a microtubule-disrupting agent (nocodazole). At 37 degrees C, FITC-cIBR was internalized by HL-60 cells and located in the endosomes; however, it was not internalized by LFA-1-deficient HUVEC. Incubation of FITC-cIBR at 4 degrees C or in the presence of nocodazole inhibited its endocytosis in HL-60 cells. The ATP inhibitors inhibited the internalization of FITC-cIBR but maintained its binding to cell surface receptors. In contrast, DOX-cIBR was diffusely distributed in the cytoplasm of LFA-1-expressing HL-60 cells following incubation at 37 degrees C. No inhibitory processes could block the entry or change the distribution pattern of DOX-cIBR into HL-60 cells, suggesting that DOX-cIBR uptake was not mediated by receptors such as LFA-1. DOX-cIBR was still found inside HUVEC, but with a distribution pattern somewhat different from that in HL-60 cells. The major entry mechanism of DOX-cIBR could be via passive diffusion because DOX-cIBR has an octanol/water distribution coefficient (Log D) of 1.15. Thus, DOX-cIBR is more lipophilic than FITC-cIBR with a Log D of 0.57. Therefore, the change in the hydrophobicity of the conjugate may alter the mechanism of entry of DOX-cIBR compared to that of FITC-cIBR. This study suggests that alteration of the physicochemical properties of drug-peptide conjugates can change the mode of uptake from receptor-mediated uptake to passive diffusion.