Evaluation of farnesyl:protein transferase and geranylgeranyl:protein transferase inhibitor combinations in preclinical models.

Farnesyl:protein transferase (FPTase) inhibitors (FTIs) were originally developed as potential anticancer agents targeting the ras oncogene and are currently in clinical trials. Whereas FTIs inhibit the farnesylation of Ha-Ras, they do not completely inhibit the prenylation of Ki-Ras, the allele most frequently mutated in human cancers. Whereas farnesylation of Ki-Ras is blocked by FTIs, Ki-Ras remains prenylated in FTI-treated cells because of its modification by the related prenyltransferase, geranylgeranyl:protein transferase type I (GGPTase-I). Hence, cells transformed with Ki-ras tend to be more resistant to FTIs than Ha-ras-transformed cells. To determine whether Ki-ras-transformed cells can be targeted by combining an FTI with a GGPTase-I inhibitor (GGTI), we evaluated potent, selective FTIs, GGTIs, and dual prenylation inhibitors (DPIs) that have both FTI and GGTI activity. We find that in human PSN-1 pancreatic tumor cells, which harbor oncogenic Ki-ras, and in other tumor lines having either wild-type or oncogenic Ki-ras, treatment with an FTI/GGTI combination or with a DPI blocks Ki-Ras prenylation and induces markedly higher levels of apoptosis relative to FTI or GGTI alone. We demonstrate that these compounds can inhibit their enzyme targets in mice by monitoring pancreatic and tumor tissues from treated animals for inhibition of prenylation of Ki-Ras, HDJ2, a substrate specific for FPTase, and Rap1A, a substrate specific for GGPTase-I. Continuous infusion (72 h) of varying doses of GGTI in conjunction with a high, fixed dose of FTI causes a dose-dependent inhibition of Ki-Ras prenylation. However, a 72-h infusion of a GGTI, at a dose sufficient to inhibit Ki-Ras prenylation in the presence of an FTI, causes death within 2 weeks of the infusion when administered either as monotherapy or in combination with an FTI. DPIs are also lethal after a 72-h infusion at doses that inhibit Ki-Ras prenylation. Because 24 h infusion of a high dose of DPI is tolerated and inhibits Ki-Ras prenylation, we compared the antitumor efficacy from a 24-h FTI infusion to that of a DPI in a nude mouse/PSN-1 tumor cell xenograft model and in Ki-ras transgenic mice with mammary tumors. The FTI and DPI were dosed at a level that provided comparable inhibition of FPTase. The FTI and the DPI displayed comparable efficacy, causing a decrease in growth rate of the PSN-1 xenograft tumors and tumor regression in the transgenic model, but neither treatment regimen induced a statistically significant increase in tumor cell apoptosis. Although FTI/GGTI combinations elicit a greater apoptotic response than either agent alone in vitro, the toxicity associated with GGTI treatment in vivo limits the duration of treatment and, thus, may limit the therapeutic benefit that might be gained by inhibiting oncogenic Ki-Ras through dual prenyltransferase inhibitor therapy.

Design and biological activity of (S)-4-(5-([1-(3-chlorobenzyl)-2-oxopyrrolidin-3-ylamino]methyl)imidazol-1-ylmethyl)benzonitrile, a 3-aminopyrrolidinone farnesyltransferase inhibitor with excellent cell potency.

The synthesis, structure-activity relationships, and biological properties of a novel series of imidazole-containing inhibitors of farnesyltransferase are described. Starting from a 3-aminopyrrolidinone core, a systematic series of modifications provided 5h, a non-thiol, non-peptide farnesyltransferase inhibitor with excellent bioavailability in dogs. Compound 5h was found to have an unusually favorable ratio of cell potency to intrinsic potency, compared with other known FTIs. It exhibited excellent potency against a range of tumor cell lines in vitro and showed full efficacy in the K-rasB transgenic mouse model.

Anions modulate the potency of geranylgeranyl-protein transferase I inhibitors.

We have identified and characterized potent and specific inhibitors of geranylgeranyl-protein transferase type I (GGPTase I), as well as dual inhibitors of GGPTase I and farnesyl-protein transferase. Many of these inhibitors require the presence of phosphate anions for maximum activity against GGPTase I in vitro. Inhibitors with a strong anion dependence were competitive with geranylgeranyl pyrophosphate (GGPP), rather than with the peptide substrate, which had served as the original template for inhibitor design. One of the most effective anions was ATP, which at low millimolar concentrations increased the potency of GGPTase I inhibitors up to several hundred-fold. In the case of clinical candidate l-778,123, this increase in potency was shown to result from two major interactions: competitive binding of inhibitor and GGPP, and competitive binding of ATP and GGPP. At 5 mm, ATP caused an increase in the apparent K(d) for the GGPP-GGPTase I interaction from 20 pm to 4 nm, resulting in correspondingly tighter inhibitor binding. A subset of very potent GGPP-competitive inhibitors displayed slow tight binding to GGPTase I with apparent on and off rates on the order of 10(6) m(-)1 s(-)1 and 10(-)3 s(-)1, respectively. Slow binding and the anion requirement suggest that these inhibitors may act as transition state analogs. After accounting for anion requirement, slow binding, and mechanism of competition, the structure-activity relationship determined in vitro correlated well with the inhibition of processing of GGPTase I substrate Rap1a in vivo.

Therapeutic intervention and signaling.

Significant advances in our understanding of intracellular signal transduction pathways have emerged within the past several years. It is now apparent that, under certain circumstances, particular isoforms of Ras can be prenylated by geranylgeranyl protein transferase as well as farnesyl protein transferase. New pathways controlling growth factor-dependent inhibition of apoptosis involving phosphoinositide 3'-hydroxykinase and the protein kinase Akt have also been clarified.

Cloning and mutagenesis of the p110 alpha subunit of human phosphoinositide 3'-hydroxykinase.

Activation of phosphoinositide 3'-hydroxykinase (P13K) is required for mitogenic signal transduction by several growth factors and oncogenes. P13K is a heterodimer consisting of a p85 regulatory subunit and a p110 catalytic subunit. In the current study, we report the cloning and characterization of the p110 alpha catalytic subunit of human P13K. This clone is highly homologous (> 99% amino acid identity) to bovine brain p110 alpha, but contains 10 amino acid differences from the human p110 alpha sequence previously reported. Comparison of this sequence with known Ser/Thr kinases and p110 homologs highlighted several conserved residues within the putative kinase domain. Mutational analysis of these residues (Asp915, (Asp933 + Phe934)) yielded P13K mutants with virtually complete loss of phosphoinositide phosphorylating activity. Expression of the wild-type p110 alpha protein in CHO cells is sufficient to activate the serum response element derived from the promoter of c-fos, an immediate early gene product. In contrast, the catalytically impaired p110 alpha mutants as well as the p85 alpha subunit of P13K were inactive in the fos assay. These studies suggest that the mitogenic signal transduction pathway mediated by P13K is dependent upon the enzymatic activity of the p110 alpha subunit of P13K.

Oncogene products as therapeutic targets for cancer.

Enormous progress has been made in the last several years in delineating signal transduction pathways associated with cell proliferation and apoptosis. The components of these pathways, which include both oncogenes and tumor suppressors, may provide viable targets for therapeutic intervention for the treatment of cancer and other diseases. This review highlights some of these recent biologic and pharmacologic advances, focusing on the ras pathway and on p53-dependent apoptosis.

Interfacial catalysis by phosphoinositide 3'-hydroxykinase.

Phosphorylation of phosphoinositides by phosphoinositide 3'-hydroxykinase (PI3K) occurs at a lipid/water interface. We have determined that highly purified recombinant human P13K binds tightly to vesicle interfaces composed primarily of phosphatidylinositol (PI) or 1,2-dimyristoyl-sn-glycero-3-phosphomethanol (DMPM). The rate of desorption of PI3K from the vesicle interface is slow and does not significantly affect the observed product formation kinetics. Observations which demonstrate that PI3K is tightly bound to the vesicle lipid/water interface include the following: (1) product formation plateaus rapidly, even in the presence of active enzyme and excess substrate; (2) total product formation is proportional to the amount of PI3K; (3) initial product formation rates are unaffected by bulk lipid concentration but are dependent on the interfacial substrate concentration; and (4) PI3K partitions with lipid vesicles in sedimentation gradients. This enzymatic profile has been referred to as catalysis in the "scooting" mode (Berg et al., 1991). A kinetic analysis of PI3K catalysis in the scooting mode is presented. The interfacial Km,app for PI was determined to be approximately 6.0 mol % in PI/DMPM vesicles. The ratio of specificity constants (kcat/Km) for PI, phosphatidylinositol 4-monophosphate (PIP), and phosphatidylinositol 4,5-diphosphate (PIP2) utilization was determined to be near unity. These results provide a rigorous enzymological framework for the kinetic analysis of PI3K inhibitors.

An SH3 domain is required for the mitogenic activity of microinjected phospholipase C-gamma 1.

Phospholipase activity is elevated in dividing cells. In response to growth factor stimulation, phospholipase C-gamma (PLC-gamma) binds to activated tyrosine kinase receptors via SH2 binding domains, resulting in phosphorylation of PLC-gamma and activation of its enzyme activity. These observations suggest that PLC-gamma participates in the signal transduction pathway employed by growth factors to promote mitogenesis. Consistent with this hypothesis, microinjection of purified bovine PLC-gamma into quiescent fibroblasts has been previously reported to initiate a mitogenic response [Smith et al. (1989) Proc. Natl. Acad. Sci. 86, 3659]. We have reproduced this result using recombinant rat PLC-gamma protein. Surprisingly, however, a catalytically inactive mutant of PLC-gamma, H335Q, also elicited a full mitogenic response. The capacity to induce mitogenesis by microinjection of PLC-gamma was mapped to the 'Z' domain of the protein, which contains PLC-gamma's SH2 and SH3 motifs. Inactivation of the phosphorylated tyrosine binding properties of both SH2 domains had no effect on the mitogenic activity of the Z-domain peptide. However, deletion of the SH3 domain resulted in a complete loss of activity. These results suggest that PLC-gamma's mitogenic properties do not require the enzyme's phospholipase activity, but are instead mediated by a novel pathway for mitogenic stimulation which is dependent upon an intact SH3 domain.

Phase I clinical study of the recombinant oncotoxin TP40 in superficial bladder cancer.

Transforming growth factor alpha-Pseudomonas exotoxin-40 (TP40) is a hybrid fusion protein that selectively binds to cancer cells that express the epidermal growth factor receptor. TP40 is then internalized and kills these cells by virtue of its Pseudomonas exotoxin-derived domains. We studied the safety and short-term antitumor activity of intravesical TP40 in 43 patients with refractory superficial bladder cancer. These patients had resected Ta/T1 disease (n = 19), visible Ta or T1 lesions (n = 11), or carcinoma in situ (n = 13). Patients were treated with increasing dose levels of TP40 at 0.15, 0.3, 0.6, 1.2, 2.4, 4.8, or 9.6 mg/week for 6 weeks and evaluated by comparing pretreatment and posttreatment cystoscopic examinations, cytology, and histopathology. All TP40 doses were well tolerated. No evidence of antitumor activity was seen in any of the patients with Ta or T1 lesions. However, 8 of 9 patients with evaluable carcinoma in situ were judged by histopathology of multiple biopsy specimens to exhibit clinical improvement following TP40 therapy. In most of these responsive patients, cystoscopic examination supported the histopathological findings, although cytology of urine and bladder washings persistently demonstrated malignant cells. Therefore, TP40 appears to be a well-tolerated biological agent that may prove to have utility in treating carcinoma in situ of the bladder.

Identification of a novel retinoblastoma gene product binding site on human papillomavirus type 16 E7 protein.

Transformation of mammalian cells by human papillomavirus type 16 appears to require binding of the viral E7 protein to the cellular retinoblastoma growth suppressor gene product (pRB). Binding of E7 protein to pRB inhibits several of pRB's biochemical properties, including association with the transcription factor E2F. Fragments of E7 protein derived from its conserved region 2 (CR2) domain bind to pRB and are sufficient to inhibit binding of full-length E7 protein to pRB. However, these CR2 fragments exhibit reduced affinity for pRB compared to the full-length protein and do not inhibit formation of the pRB-E2F complex. These observations suggest the existence of additional contact sites between the E7 protein and pRB. In the current study we have identified a region of E7, distinct from the CR2 domain, which is sufficient to bind pRB. This new pRB binding motif encompasses the zinc-binding conserved region 3 (CR3) domain of E7. Studies with a series of pRB deletion mutants suggest that pRB residues between amino acids 803 and 841 are necessary for binding to the E7 CR3 domain. An E7 CR3 peptide inhibits binding of E2F to pRB, indicating that E2F and E7(31-98) bind to pRB at the same or overlapping sites. These results are consistent with a model in which optimal binding of E7 to pRB requires at least two distinct contact sites: the previously identified high affinity interaction between the E7 CR2 domain and the pRB "pocket" region, and a second interaction between the E7 CR3 domain and the COOH-terminal region of pRB. The latter interaction is sufficient for E7's inhibition of E2F binding to pRB.

Cloning and characterization of E2F-2, a novel protein with the biochemical properties of transcription factor E2F.

E2F is a mammalian transcription factor that appears to play an important role in cell cycle regulation. While at least two proteins (E2F-1 and DP-1) with E2F-like activity have been cloned, studies from several laboratories suggest that additional homologs may exist. A novel protein with E2F-like properties, designated E2F-2, was cloned by screening a HeLa cDNA library with a DNA probe derived from the DNA binding domain of E2F-1 (K. Helin, J. A. Lees, M. Vidal, N. Dyson, E. Harlow, and A. Fattaey, Cell 70:337-350, 1992). E2F-2 exhibits overall 46% amino acid identity to E2F-1. Both the sequence and the function of the DNA and retinoblastoma gene product binding domains of E2F-1 are conserved in E2F-2. The DNA binding activity of E2F-2 is dramatically enhanced by complementation with particular sodium dodecyl sulfate-polyacrylamide gel electrophoresis-purified components of HeLa cell E2F, and anti-E2F-2 antibodies cross-react with components of purified HeLa cell E2F. These observations are consistent with a model in which E2F binds DNA as a heterodimer of two distinct proteins, and E2F-2 is functionally and immunologically related to one of these proteins.

Transcription factor E2F binds DNA as a heterodimer.

E2F is a mammalian transcription factor that appears to play an important role in cell cycle control. DNA affinity column-purified E2F from HeLa cells reproducibly exhibits multiple protein bands when analyzed by SDS/PAGE. After electrophoretic purification, electroelution, and refolding of the individual protein components, the E2F DNA binding activity of the individual proteins was poor. However, upon mixing the individual components together, a dramatic (100- to 1000-fold) increase in specific DNA binding activity was observed. The five protein bands isolated can be separated into two groups based on apparent molecular mass. Optimal reconstitution of activity requires one of the two proteins found in the group of larger molecular mass (approximately 60 kDa) and one of the three proteins in the smaller-sized group (approximately 50 kDa). The reconstituted heterodimer is identical to authentic affinity-purified E2F by three criteria: DNA-binding specificity, DNA pattern, and binding to the retinoblastoma gene product. A recently cloned protein with E2F-like activity, RBP3/E2F-1, is related to the protein components of the group of larger molecular mass, as determined by Western blot analysis and reconstitution experiments. These data suggest that E2F, like many other transcription factors, binds DNA as an oligomeric complex composed of at least two distinct proteins.

Protein domains governing interactions between E2F, the retinoblastoma gene product, and human papillomavirus type 16 E7 protein.

Human papillomaviruses (HPVs) are the etiological agents for genital warts and contribute to the development of cervical cancer in humans. The HPV E7 gene product is expressed in these diseases, and the E7 genes from HPV types 16 and 18 contribute to transformation in mammalian cells. Mutation and deletion analysis of this gene suggests that the transforming activity of the protein product resides in the same domain as that which is directly involved in complex formation with the retinoblastoma gene product (pRB). This domain is one of two conserved regions (designated CRI and CRII) shared by E7 and other viral oncoproteins which bind pRB, including adenovirus E1A protein. Binding of HPV type 16 E7 protein to pRB has previously been shown to affect pRB's ability to bind DNA and to form complexes with other cellular proteins. In the current study, we map the functional interaction between E7 protein and pRB by monitoring the association between a 60-kDa version of the pRB, pRB60, and the cellular transcription factor E2F. We observe that CRII of E7 (amino acids 20 to 29), which completely blocks binding of full-length E7 protein, is necessary but not sufficient to inhibit E2F/pRB60 complex formation. While CRI of E1A (amino acids 37 to 55) appears to be sufficient to compete with E2F for binding to pRB60, the equivalent region of E7 is neither necessary nor sufficient. Only E7 fragments that contained both CRII and at least a portion of the zinc-binding domain (amino acids 60 to 98) inhibited E2F/pRB60 complex formation. These results suggest that pRB60 associates with E7 and E2F through overlapping but distinct domains.

Retinoblastoma protein binding properties are dependent on 4 cysteine residues in the protein binding pocket.

The retinoblastoma gene product (pRB) participates in regulating mammalian cell replication. The mechanism responsible for pRB's growth regulatory activity is uncertain. However, pRB is known to bind viral transforming proteins including the papilloma virus E7 protein, cellular proteins, and DNA. pRB contains a critical domain termed the "binding pocket" which is required for binding activities. This binding pocket contains 8 cysteine residues. A naturally occurring mutation affecting one of these cysteines is known to eliminate pRB's protein and DNA binding activities. To investigate the cysteine residues in pRB's binding pocket, each residue was mutated to alanine, phenylalanine, or serine. These mutant genes were used to prepare pRBs harboring specific amino acid substitutions. Individual mutations at positions 407, 553, 666, and 706 depressed pRB binding to E7 protein, DNA, and a conformation-specific anti-pRB antibody, XZ133. Combinations of these inhibitory mutations exhibited additive inhibitory effects on pRB's binding properties. Mutations at positions 438, 489, 590, 712, and 853 did not affect pRB binding to E7 protein, DNA, or the XZ133 antibody. Combination of these five neutral mutations yielded a pRB species with full E7 protein, DNA, and XZ133 binding activities. These studies indicate that the cysteine residues at positions 407, 553, 666, and 706 contribute to the E7 protein and DNA binding properties of pRB and appear to do so by maintaining pRB's normal conformation.

Human papillomavirus type 16 E7 protein inhibits DNA binding by the retinoblastoma gene product.

The human papillomavirus E7 gene can transform murine fibroblasts and cooperate with other viral oncogenes in transforming primary cell cultures. One biochemical property associated with the E7 protein is binding to the retinoblastoma tumor suppressor gene product (pRB). Biochemical properties associated with pRB include binding to viral transforming proteins (E1A, large T, and E7), binding to cellular proteins (E2F and Myc), and binding to DNA. The mechanism by which E7 stimulates cell growth is uncertain. However, E7 binding to pRB inhibits binding of cellular proteins to pRB and appears to block the growth-suppressive activity of pRB. We have found that E7 also inhibits binding of pRB to DNA. A 60-kDa version of pRB (pRB60) produced in reticulocyte translation reactions or in bacteria bound quantitatively to DNA-cellulose. Recombinant E7 protein used at a 1:1 or 10:1 molar ratio with pRB60 blocked 50 or greater than 95% of pRB60 DNA-binding activity, respectively. A mutant E7 protein (E7-Ala-24) with reduced pRB60-binding activity exhibited a parallel reduction in its blocking of pRB60 binding to DNA. An E7(20-29) peptide that blocks binding of E7 protein to pRB60 restored the DNA-binding activity of pRB60 in the presence of E7. Peptide E7(2-32) did not block pRB60 binding to DNA, while peptide E7(20-57) and an E7 fragment containing residues 1 to 60 partially blocked DNA binding. E7 species containing residues 3 to 75 were fully effective at blocking pRB60 binding to DNA. These studies indicate that E7 protein specifically blocks pRB60 binding to DNA and suggest that the E7 region responsible for this property lies between residues 32 and 75. The functional significance of these observations is unclear. However, we have found that a point mutation in pRB60 that impairs DNA-binding activity also blocks the ability of pRB60 to inhibit cell growth. This correlation suggests that the DNA-binding activity of retinoblastoma proteins contributes to their biological properties.

Purification and characterization of a functionally homogeneous 60-kDa species of the retinoblastoma gene product.

The retinoblastoma susceptibility gene (RB) encodes a 928-amino acid protein (pRB) that is hypothesized to function in a pathway that restricts cell proliferation. The immortalizing proteins from three distinct DNA tumor viruses (SV40 large T antigen, adenovirus E1a, and human papilloma virus Type 16 E7) have been shown to interact with RB protein through two noncontiguous regions comprised of amino acids 393-572 (domain A) and 646-772 (domain B). We constructed a truncated form of RB (RB p60) that retains these two domains but eliminates the N-terminal 386 amino acids of RB. RB p60 was expressed in Escherichia coli in inclusion bodies. After solubilization, it was refolded in the presence of magnesium chloride, and the active protein was isolated with an E7 peptide affinity column. The protein that elutes from this column is functionally homogenous in its ability to bind immobilized E7 protein. Thermal denaturation studies provide additional evidence for the conformational homogeneity of the isolated protein. This purification scheme allows the isolation of significant amounts of RB p60 protein that is suitable for structural and functional studies.

Characterization of functional HPV-16 E7 protein produced in Escherichia coli.

Human papillomaviruses (HPVs) are the etiologic agents responsible for genital warts and are contributing factors in the pathogenesis of human cervical cancer. The HPV E7 gene is transcriptionally active in these diseases and has been shown to transform mammalian cells in vitro. We have expressed and purified the HPV-16 E7 gene product in Escherichia coli. The isolated E7 protein contains zinc in a 1:1 molar ratio. X-ray absorption fine structure studies demonstrated that the zinc is coordinated by 4 sulfur ligands. We sequentially derivatized the E7 cysteines to differentiate between solvent-exposed, metal-bound, and disulfide-associated cysteines. Our results demonstrate that Cys24 and Cys68 are accessible to solvent, while cysteines in the two conserved Cys-X-X-Cys motifs are likely involved in binding zinc. We observed no evidence for the existence of disulfide bonds in recombinant E7 protein under the conditions tested.

Activity of a recombinant transforming growth factor-alpha-Pseudomonas exotoxin hybrid protein against primary human tumor colony-forming units.

Transforming growth factor-alpha-Pseudomonas exotoxin-40 (TP40) is a recombinant fusion protein. TP40 consists of the entire human transforming growth factor-alpha (TGF alpha) protein fused to a 40,000 Da. segment of the Pseudomonas exotoxin A protein. TP40 is a bifunctional molecule that possesses the epidermal growth factor (EGF) receptor binding properties of TGF alpha and the cell killing properties of Pseudomonas exotoxin A. These properties make TP40 a selective cytotoxic agent that kills EGF receptor bearing cells. TP40 has been shown to effectively kill human tumor cell lines that possess EGF receptors in vitro and in nude mice. In the present study, TP40 was tested against tumors taken directly from patients and grown in a soft agar human tumor cloning system. A total of 107 patients' tumors (taken from patients with tumors refractory to chemotherapy) were tested with a continuous exposure to 0.5-50 nM concentrations of the agent. TP40 exhibited a clear dose response effect against a wide variety of human solid tumor colony-forming units with greater than or equal to 84% of evaluable tumors responding at a drug concentration greater than or equal to 24 nM. When used as a continuous exposure, concentrations of TP40 as low as 5 nM demonstrated substantial in vitro activity. This activity included cytotoxicity against breast, colorectal, endometrial, head and neck, non small-cell lung, gastric, sarcoma, and pancreatic cancer tumor colony-forming units. Additional in vivo testing of this compound is warranted.

Specific N-methylations of HPV-16 E7 peptides alter binding to the retinoblastoma suppressor protein.

Complex formation between the human papilloma virus type 16 E7 protein (HPV-16 E7) and the retinoblastoma growth suppressor protein (RB) is believed to contribute to the process of cellular transformation that leads to cervical carcinoma. Genetic analysis of the HPV-16 E7 protein has shown that the segment of E7 homologous to the conserved region 2 of adenovirus 5 E1A protein is involved in both RB binding and E7-mediated cell transformation. We have previously shown that a peptide colinear with HPV-16 E7 residues 21-29 was able to block immobilized species of E7 from binding to RB protein. The current study reports the effects of different chemical modifications of this peptide. One type of modification, methylation of the alpha-amino nitrogens contributed by Leu22, Tyr25, and Leu28, resulted in a 45-fold increase in E7/RB binding antagonist activity. This increased antagonist activity is sequence-specific since methylation of the amino groups contributed by Tyr23, Cys24, or Glu26 resulted in a profound loss of binding antagonist activity. Using a newly developed binding assay we determined that the apparent dissociation constant for recombinant HPV-16 E7 protein binding to recombinant human RB protein is 1.3 nM. The peptide Ac[N-MeLeu22,N-Me-Tyr25,N-MeLeu28]-(21-29)-E7 amide was determined to be a competitive inhibitor of HPV-16 E7 binding to RB with a Ki value of 32 nM.