The search for therapeutic targets in lung cancer: Preclinical and human studies of carcinoembryonic antigen-related cell adhesion molecule 5 expression and its associated molecular landscape

OBJECTIVES

CEACAM5 is a cell-surface glycoprotein expressed on epithelial cells of some solid tumors. Tusamitamab ravtansine (SAR408701), a humanized antibody-drug conjugate targeting CEACAM5, is in clinical development for nonsquamous non-small cell lung cancer (NSQ-NSCLC) with CEACAM5 high expression (HE), defined as membranous CEACAM5 immunohistochemistry staining at ≥ 2+ intensity in ≥ 50% of tumor cells.

MATERIALS AND METHODS

We investigated correlations between CEACAM5 expression by immunohistochemistry, CEACAM5 protein expression by ELISA, and CEACAM5 RNA expression by RNA-seq in NSQ-NSCLC patient-derived xenograft (PDX) models, and tumor responses to tusamitamab ravtansine in these models. We assessed prevalence of CEACAM5 HE, clinicopathologic characteristics and molecular markers in patients with NSQ-NSCLC in clinical cohorts.

RESULTS

In a lung PDX set of 10 NSQ-NSCLC specimens, correlations between CEACAM5 by IHC, ELISA and RNA-seq ranged from 0.72 to 0.88. In a larger lung PDX set, higher H-scores were present in NSQ- (n = 93) vs SQ-NSCLC (n = 128) models, and in 12 of these NSQ-NSCLC models, more tumor responses to tusamitamab ravtansine occurred in CEACAM5 HE (5/8; 62.5%) versus moderate or negative expression (1/4; 25%), including 3 with KRAS mutations among the 6 responders. In clinical NSQ-NSCLC samples, CEACAM5 HE prevalence was (52/214; 24.3%) in primary tumors and (6/17; 35.3%) in metastases. In NSQ-NSCLC primary tumors, CEACAM5 HE prevalence was significantly higher in KRAS-altered versus wild-type (35.0% vs 19.5%; P = 0.028) and in programmed cell death ligand 1 (PD-L1) negative (tumor cells 0%)/low (1-49%) versus high (≥50%) (33.3%, 26.1%, 5.0%; P = 0.031), but not significantly different in EGFR-mutated versus wild-type (20.0% vs 25.7%, P = 0.626).

CONCLUSIONS

In NSQ-NSCLC tumors, CEACAM5 HE prevalence was 24.3% overall and was higher with KRAS altered and with PD-L1 negative/low tumors but similar regardless of EGFR mutation status. These findings support targeting CEACAM5 and the clinical development of tusamitamab ravtansine for patients with NSQ-NSCLC with CEACAM5 HE.

Isatuximab plus atezolizumab in patients with advanced solid tumors: results from a phase I/II, open-label, multicenter study

BACKGROUND

The anti-CD38 antibody isatuximab is approved for the treatment of relapsed/refractory multiple myeloma, but there are no data on its efficacy in solid tumors. This phase I/II study (NCT03637764) assessed the safety and activity of isatuximab plus atezolizumab (Isa + Atezo), an anti-programmed death-ligand 1 (PD-L1) antibody, in patients with immunotherapy-naive solid tumors: epithelial ovarian cancer (EOC), glioblastoma (GBM), hepatocellular carcinoma (HCC), and squamous cell carcinoma of the head and neck (SCCHN).

PATIENTS AND METHODS

Phase I assessed safety, tolerability, pharmacokinetics, pharmacodynamics, and the recommended phase II dose (RP2D) of isatuximab 10 mg/kg intravenously (i.v.) every week for 3 weeks followed by once every 3 weeks + atezolizumab 1200 mg i.v. every 3 weeks. Phase II used a Simon's two-stage design to assess the overall response rate or progression-free survival rate at 6 months (GBM cohort). Interim analysis was carried out at 6 months following first dose of the last enrolled patient in each cohort. Pharmacodynamic biomarkers were tested for CD38, PD-L1, tumor-infiltrating immune cells, and FOXP3+ regulatory T cells (Tregs) in the tumor microenvironment (TME).

RESULTS

Overall, 107 patients were treated (EOC, n = 18; GBM, n = 33; HCC, n = 27; SCCHN, n = 29). In phase I, Isa + Atezo showed an acceptable safety profile, no dose-limiting toxicities were observed, and RP2D was confirmed. Most patients experienced ≥1 treatment-emergent adverse event (TEAE), with ≤48.5% being grade ≥3. The most frequent TEAE was infusion reactions. The study did not continue to stage 2 based on prespecified targets. Tumor-infiltrating CD38+ immune cells were reduced and almost cleared after treatment. Isa + Atezo did not significantly modulate Tregs or PD-L1 expression in the TME.

CONCLUSIONS

Isa + Atezo had acceptable safety and tolerability. Clinical pharmacodynamic evaluation revealed efficient target engagement of isatuximab via treatment-mediated reduction of CD38+ immune cells in the TME. Based on clinical data, CD38 inhibition does not improve responsiveness to PD-L1 blockade in these patients.

Abstract CT517: Amcenestrant, an oral selective estrogen receptor (ER) degrader (SERD), in ER+/HER2- advanced breast cancer (aBC): combined biomarker analyses from a Phase 1/2 study in postmenopausal women and a Phase 1 study in postmenopausal Japanese women

Background: Amcenestrant is an optimized oral SERD that has shown safety and clinical benefit in patients (pts) with wild-type (WT) and mutated ESR1 aBC. Here we provide a pooled analysis of biomarkers from two studies in heavily-pretreated, postmenopausal women with ER+/HER2- aBC.

Methods: Pooled analysis included study data from AMEERA-1 (NCT03284957), in which pts received single-agent amcenestrant in dose escalation (20-600 mg QD or 300 mg BID) and dose expansion (400 mg QD), and from AMEERA-2 (NCT03816839), in which Japanese pts received single-agent amcenestrant at 400 mg QD or 300 mg BID. Mutational profiling of plasma circulating cell-free DNA was conducted by droplet digital polymerase chain reaction (ddPCR) for 12 pathogenic ESR1 mutations at baseline (BL) and on treatment (OT; Cycle 2, Day 28), and by next-generation sequencing (NGS; 77-gene panel) at BL and end of treatment (EOT). Clinical benefit (CB; complete response + partial response [PR] + stable disease ≥ 24 weeks) was assessed in pooled pts who received amcenestrant≥ 150 mg QD or BID. In tumor tissue, ER, progesterone receptor (PgR), Ki67 and Bcl-2 expression levels over time were assessed by immunohistochemistry; functional ER pathway activity was assessed by gene set variation analysis with RNA-seq data (ER activity score).

Results: Among pts with data for ESR1 mutation status at BL (N = 80; ddPCR), 41 pts were WT and 39 pts had at least one ESR1 mutation, with D538G and Y537S the most prevalent. CB was observed in pts with both WT (13 pts) and mutated ESR1 (12 pts) at BL. The most frequent pathogenic mutations detected in pts (N = 79; NGS) at BL were ESR1 (49%), followed by PI3KCA (46%)and TP53 (33%). In response-evaluable pts with ESR1 mutational profiling at BL and OT (N = 40; ddPCR), 17 of 19 pts with ESR1 mutation at BL showed a decrease in ESR1 mutation allele frequency, independently of clinical benefit. Eight pts showed an increase (7/25 no CB and 1/15 CB). Among 17 pts with biopsies, pts with ER or PgR -/low (H-score: 0-3) at BL did not derive benefit from treatment. In the remaining patients with ER high (H-score ≥ 140), no difference between pts with and without CB was observed in ER, Bcl-2 or ER activity score, while pts with CB had a tendency for lower Ki67. ER and PgR decreased upon treatment independently of CB, while ER activity score and Ki67 mostly decreased in pts with CB and mostly increased in pts without CB.

Conclusions: In postmenopausal women with ER+/HER2- aBC treated with single-agent amcenestrant, low Ki67 at BL and the pharmacodynamic decrease of Ki67 and ER activation score by amcenestrant trended toward an association with CB, whereas increase in ESR1 mutation allele frequency on treatment trended toward an association with lack of CB. Clinical benefit was observed in both pts with WT and mutated ESR1 at BL.

Citation Format: Hiroji Iwata, Aditya Bardia, Simon Lord, Hannah M. Linden, Mario Campone, Kenji Tamura, Kan Yonemori, Toru Mukohara, Cécile Combeau, Nils Ternes, Dimitri Carene, Jeff Ming, Joon Sang Lee, Marina Celanovic, Anne-Laure Bauchet, Monsif Bouaboula, Tomoyuki Tanaka, Yumiko Kawabata, Sarat Chandarlapaty. Amcenestrant, an oral selective estrogen receptor (ER) degrader (SERD), in ER+/HER2- advanced breast cancer (aBC): combined biomarker analyses from a Phase 1/2 study in postmenopausal women and a Phase 1 study in postmenopausal Japanese women [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT517.

Abstract CT154: Isatuximab (Isa) plus atezolizumab (Atezo) in patients (pts) with advanced malignancies: Results from a Phase 1/2 open-label multicenter study

Background: The CD38 monoclonal antibody Isa is approved for the treatment of multiple myeloma, but there are no data on its efficacy in solid tumors. A mechanism of immune resistance to anti-PD-1/PD-L1 therapy caused by CD38 expression in tumor cells has been described in the preclinical setting, providing a rationale for testing a combination of Isa+Atezo (anti-PD-L1 antibody) in the clinic to prevent therapy resistance and further enhance antitumor efficacy. The present study assessed safety and efficacy of Isa+Atezo in pts with epithelial ovarian cancer (EOC; n = 18), glioblastoma multiforme (GBM; n = 33), hepatocellular carcinoma (HCC; n = 27), or squamous cell carcinoma of the head and neck (SCCHN; n = 29), not amenable to local therapy with curative intent. Methods: Phase 1 (NCT03637764) assessed safety and confirmed the recommended Phase 2 dose (RP2D) of Isa according to a 21-day dose-limiting toxicity (DLT) observation period. Pts received Isa 10 mg/kg intravenously (IV) every week for 3 weeks followed by once every 3 weeks + Atezo 1200 mg IV every 3 weeks. Phase 2 used a Simon's 2-stage design to assess the overall response rate (ORR; EOC, HCC, SCCHN cohorts) or progression-free survival at 6 months (PFS-6; GBM cohort). Interim analysis was performed at 6 months following first treatment of the last enrolled pt in each cohort. Pharmacodynamics (PD), including analysis of CD38, PD-L1 and tumor-infiltrating lymphocytes in the tumor microenvironment (TME), were also evaluated. Results: In Phase 1, Isa+Atezo had acceptable safety and tolerability, with no new safety signals, in pts with EOC, GBM, HCC, or SCCHN. No DLTs were observed and RP2D were confirmed. Overall, 99% of pts experienced TEAEs, with >40% being Grade ≥3. The most frequently reported TEAE of any grade for all cohorts was infusion-related reactions (EOC, 83.3%; GBM, 54.5%; HCC, 63.0%; SCCHN, 31.0%). In the GBM Phase 2 cohort, the PFS-6 was 3.1%; no patients responded in Stage 1. In the EOC, HCC, and SCCHN Phase 2 cohorts, ORR was 5.6%, 7.4%, and 13.8%, respectively; therefore, the study did not continue to Stage 2. Tumor-infiltrating CD38+ immune cells were reduced by ~20% after treatment. Isa+Atezo did not lead to significant modulation of Tregs or PD-L1 in the TME. Conclusions: Isa+Atezo had acceptable safety and tolerability. Clinical PD evaluation suggested the efficient target engagement effect of Isa by demonstrating treatment-mediated reduction of CD38+ immune cells in the TME. However, significant reduction of CD38+ Tregs and restoration of T-cell proliferation was not observed. Based on clinical data, CD38 inhibition does not seem to influence response to anti-PD-L1 agents in these pts. Although no new safety signals were observed, efficacy did not fulfil criteria to expand enrollment despite the evidence of target engagement of Isa.

Citation Format: Maria Vieito Villar, Matteo Simonelli, Ferry A.L.M. Eskens, Marta Gil-Martin, Chia-Jui Yen, Radka Obermannova, Yee Chao, Vittorina Zagonel, Bohuslav Melichar, Victor Moreno, Ming-Lung Yu, Alberto Bongiovanni, Emiliano Calvo, Sylvie Rottey, Jean-Pascal Machiels, Antonio González-Martín, Luis Paz-Ares, Chih-Long Chang, Warren Mason, Chia-Chi Lin, David Reardon, Elena Garralda, Armando Santoro, Robin Meng, Giovanni Abbadessa, Fatima Menas, Helen Lee, Qianying Liu, Cécile Combeau, Nils Ternes, Christophe Massard. Isatuximab (Isa) plus atezolizumab (Atezo) in patients (pts) with advanced malignancies: Results from a Phase 1/2 open-label multicenter study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT154.

Abstract B145: PK/PD evaluation of an anti-CEACAM5 antibody drug conjugate, in a colon patient-derived-xenografted mice model

CEACAM5 is an oncofetal protein that is expressed in a number of tumors of epithelial origin such as colorectal cancer (CRC), lung and gastric cancer. Differential expression of CEACAM5 protein in tumor tissues compared to normal tissues make this target attractive for targeting with an antibody-drug conjugate (ADC).

The ADC that is characterized in this study contains a specific anti-CEACAM5 antibody and the maytansinoid derivative DM4, a potent antimitotic agent that inhibits microtubule assembly. DM4 is covalently bound to the humanized IgG1 (hIgG) antibody through a N-succinimidyl-4-(2-pyridyldithio) butyrate (SPDB) linker that is stable in plasma and cleavable inside cells. After binding and internalization into CEACAM5-positive cancer cells, this ADC is degraded, releasing cytotoxic DM4 metabolites.

In the present study the Pharmacokinetic (PK), Pharmacodynamic (PD) and efficacy relationships of this ADC were investigated against subcutaneous colon Patient-Derived-Xenograft (PDX) in severe combined immunodeficient (SCID) mice. The modulation of several biomarkers after a single administration of this compound at 3 dose levels were quantified in the tumor by immunoassay (phosphorylated Histone H3 (pHH3), a marker of cells arrested in mitosis and free human CEACAM5 expression). The spatial localization in the tumor of the biomarkers (pHH3, free human CEACAM5, DM4 and hIgG) was analyzed by immunohistochemistry (IHC). A population pharmacokinetic analysis of the ADC was performed first, followed by population PK/PD analysis for pHH3 induction and tumor growth inhibition (TGI).

This study has shown both by immunoassay and IHC that a single administration of this anti-CEACAM5 ADC induced in the xenografted tumor a dose dependent increase of pHH3 expression compared to control, with a maximal induction observed at 24-48 hours. The levels of DM4 and hIgG detected by IHC in the tumor were dose dependent and progressively decreased with time. DM4 and hIgG part of the ADC were essentially immunodetected at the membrane level of tumor cells and sub-localization was observed in vesicular intracellular compartments at later time point which confirms ADC tumor cell penetration. The PK/PD relationship analysis indicated that compound concentration producing 50% of the maximum pHH3 stimulation was 2.3 μg/mL and the threshold concentration for tumor eradication was 8 μg/mL in this colon PDX model. Both immunoassay and IHC assessment confirmed that single administration of the ADC had no impact on CEACAM5 expression at the tumor cell membrane.

Overall these data have confirmed mechanism of action of this anti-CEACAM5 ADC and its attractiveness as a potent compound to induce dose dependent CEACAM5 expressing tumor cell death. This compound is now being explored in a phase 1 clinical trial in cancer patients.

Citation Format: Christophe Henry, Céline Nicolazzi, Céline Amara, Anne-Marie Lefebvre, Christelle Larois, Florence Attenot, Claire Brillac, Marion Bouillon-Pichault, Nathalie Fagniez, Stéphanie Decary, Cécile Combeau, Pierre-François Berne, Souâd Naimi, Marielle Chiron, Véronique Blanc. PK/PD evaluation of an anti-CEACAM5 antibody drug conjugate, in a colon patient-derived-xenografted mice model. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B145.

Abstract 1688: A novel anti-CEACAM5 maytansinoid-antibody-drug conjugate for the treatment of colorectal, lung and gastric tumors

Carcinoembryonic antigen cell adhesion molecule 5, CEACAM5 (CEA, CD66e) is a well known tumor marker, in particular in colorectal carcinomas, where circulating CEA is used to monitor response to chemotherapy. This GPI anchored glycoprotein belongs to the CEA-related cell adhesion molecule (CEACAM) family and shares domains identity to other members, like CEACAM6. CEACAM5 is expressed in non-human primate and also shares identity to different members, making it difficult to find an antibody both selective to human CEACAM5 and cross-reacting solely with monkey CEACAM5.

CEACAM5 has been described in the literature as a poorly internalizing surface protein, but interestingly, antibodies with different uptake capacities have been found.

CEACAM5 is highly expressed at the surface of tumor cells in several epithelial tumors, including CRC, lung and gastric tumors and displays a limited expression in normal tissue where it is found solely at the luminal surface of columnar absorptive cells. This prompted us to develop an anti-CEACAM5 antibody-drug conjugate (ADC) for the treatment of CEACAM5-positive tumors.

We generated multiple anti-CEACAM5 antibodies by immunization of Balb/c mice with recombinant human and monkey CEACAM5 extracellular domain and CEACAM5-positive tumor cells. We selected a highly specific CEACAM5 antibody that cross-reacts with monkey. The Alexa488-labeled anti CEACAM5 antibody internalizes in tumor cells and is processed in lysosomes leading to free Alexa488 molecules. Although the internalization rate was shown to be moderate, the high number of CEACAM5 at the surface of tumor cells allowed to produce a high number of free Alexa488, suggesting that the antibody could be suited for conjugation to a cytotoxic molecule.

Indeed, conjugation of the antibody to the cytotoxic maytansinoid, DM4, with the cleavable SPDB linker generated SAR408701, which kills different CEACAM5-positive tumor cells at sub-nM concentration. SAR408701 in vivo efficacy was evaluated in CRC, lung and gastric patient-derived xenografts (PDXs), following a single injection of ADC at low doses (2.5-5 mg/kg). The conjugate was able to elicit strong and specific antitumor efficacy in a number of PDX models representative of the CEACAM5-positive patient population. Repeating the administrations resulted in most cases in a more pronounced antitumor efficacy even at doses that were otherwise marginally active as single dose.

SAR408701 was well tolerated in cynomologus monkey and displayed similar toxicity profile as other SPDB-DM4 ADCs. Based on preclinical efficacy data and the absence of target mediated toxicity in monkey, SAR408701 is expected to have anticancer activity with a favorable therapeutic index, warranting its evaluation in patients with CEACAM5-positive tumors.

Citation Format: Stéphanie Decary, Pierre-François Berne, Céline Nicolazzi, Anne-Marie Lefebvre, Tarik Dabdoubi, Beatrice Cameron, Catherine Devaud, Catherine Prades, Hervé Bouchard, Alhassan Cassé, Christophe Henry, Céline Amara, Paul Ferrari, Laetitia Maçon, Eric Lacoste, Cécile Combeau, Eric Beys, Souad Naimi, Francis Blanche, Carlos Garcia-Echeverria, Jean-François Mayaux, Véronique Blanc. A novel anti-CEACAM5 maytansinoid-antibody-drug conjugate for the treatment of colorectal, lung and gastric tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1688. doi:10.1158/1538-7445.AM2015-1688

SAR156497, an exquisitely selective inhibitor of aurora kinases

The Aurora family of serine/threonine kinases is essential for mitosis. Their crucial role in cell cycle regulation and aberrant expression in a broad range of malignancies have been demonstrated and have prompted intensive search for small molecule Aurora inhibitors. Indeed, over 10 of them have reached the clinic as potential anticancer therapies. We report herein the discovery and optimization of a novel series of tricyclic molecules that has led to SAR156497, an exquisitely selective Aurora A, B, and C inhibitor with in vitro and in vivo efficacy. We also provide insights into its mode of binding to its target proteins, which could explain its selectivity.

Tricyclic series of heat shock protein 90 (Hsp90) inhibitors part I: discovery of tricyclic imidazo[4,5-c]pyridines as potent inhibitors of the Hsp90 molecular chaperone

A novel class of heat shock protein 90 (Hsp90) inhibitors was developed after a low throughput screen (LTS) of a focused library containing approximately 21K compounds selected by virtual screening. The initial [1-{3-H-imidazo[4-5-c]pyridin-2-yl}-3,4-dihydro-2H-pyrido[2,1-a]isoindole-6-one] (1) compound showed moderate activity (IC(50) = 7.6 μM on Hsp82, the yeast homologue of Hsp90). A high-resolution X-ray structure shows that compound 1 binds into an "induced" hydrophobic pocket, 10-15 Å away from the ATP/resorcinol binding site. Iterative cycles of structure-based drug design (SBDD) and chemical synthesis led to the design and preparation of analogues with improved affinity. These optimized molecules make productive interactions within the ATP binding site as reported by other Hsp90 inhibitors. This resulted in compound 8, which is a highly potent inhibitor in biochemical and cellular assays (K(d) = 0.35 nM on Hsp90; IC(50) = 30 nM on SKBr3 mammary carcinoma cells) and in an in vivo leukemia model.

RPR112378 and RPR115781: two representatives of a new family of microtubule assembly inhibitors

A screening program aimed at the discovery of new antimicrotubule agents yielded RPR112378 and RPR115781, two natural compounds extracted from the Indian plant Ottelia alismoides. We report their isolation, structural determination, and mechanisms of action. RPR112378 is an efficient inhibitor of tubulin polymerization (IC(50) = 1.2 microM) and is able to disassemble preformed microtubules. Regarding tubulin activity, RPR115781 is 5-fold less active than RPR112378. Tubulin-RPR112378 complexes, when isolated by gel filtration, were able to block further tubulin addition to growing microtubules, a mechanism that accounts for the substoichiometric effect of the drug. RPR112378 was found to prevent colchicine binding but not vinblastine binding to tubulin. Although colchicine binding is known to induce an increase of tubulin GTPase activity, no such increase was observed with RPR112378. We show that RPR112378 is a highly cytotoxic compound and that RPR115781 is 10, 000-fold less active as an inhibitor of KB cell growth. Part of the cytotoxicity of RPR112378 is probably caused by a reaction of addition with sulfhydryl groups, an observation that has not been made with RPR115781. In conclusion, these molecules represent a new class of inhibitors of microtubule assembly with potential therapeutic value.

Gamma-tubulin during the differentiation of spermatozoa in various mammals and man

The distribution of gamma-tubulin as a marker of microtubule organizing centres (MTOC) was studied during spermiogenesis in rodents and in rabbit, monkey and man. A polyclonal antibody directed against human gamma-tubulin was used both for indirect immunofluorescence (IIF) and post-embedding immunogold procedures. In all species, gamma-tubulin was detected in the proximal and distal centrioles of round spermatids. In elongating spermatids, gamma-tubulin was predominantly found in the pericentriolar material (PCM) of both centrioles and particularly around the adjunct of the proximal centriole. At this level, some labelling was also associated with manchette microtubules, but other parts of the manchette and the nuclear ring were never labelled. We propose a role for distal centriole gamma-tubulin in axoneme nucleation and centriolar adjunct gamma-tubulin in manchette nucleation. The disappearance of gamma-tubulin in mature spermatozoa indicates that sperm aster nucleation should be dependent on oocyte gamma-tubulin. Remnants of gamma-tubulin in some human spermatozoa suggest that paternal gamma-tubulin also could contribute to sperm aster formation.

[3H](azidophenyl)ureido taxoid photolabels peptide amino acids 281-304 of alpha-tubulin

The taxoid binding site on porcine brain tubulin was covalently labeled, in the presence or absence of Taxotere, with the photoaffinity reagent [3H]-p-(azidophenyl)ureido taxoid derivative [3H]TaxAPU [Combeau, C., Commercon, A., Mioskowski, C., Rousseau, B., Aubert, F., & Goeldner, M. (1994) Biochemistry 33, 6676-6683]. After disulfide reduction and carboxymethylation, the alkylated tubulin samples were treated with trypsin and the mixtures of peptides were first fractionated by gel filtration over Sephadex G50. Anion exchange chromatography of the radioactive areas showed, for one area, three major radioactive signals which were further analyzed by reversed phase C18 HPLC, leading to well-resolved radioactive peaks. Microsequencing of these different peaks gave a complete sequence of a tryptic fragment on alpha-tubulin (alpha-281-304) and two partial peptide sequences of a tryptic fragment on beta-tubulin (beta-217-229) in addition to sequences of mixture of peptides. The radioactive signals were lost while concentrating the samples for microsequencing, preventing the identification of the modified amino acids. These results identify the first peptide on alpha-tubulin which binds to the taxoids and confirm the involvement of both alpha- and beta-tubulin in the taxoid binding site.

Preclinical evaluation of docetaxel (Taxotere)

Progress in cancer chemotherapy has been made owing to the discovery and development of drugs that have new structures, new mechanisms of action, and high levels of experimental antitumor activity. Docetaxel (Taxotere; Rhône-Poulenc Rorer, Antony, France) is prepared by semisynthesis from 10-deacetyl baccatin III, an inactive taxoid precursor extracted from the needles of the European yew Taxus baccata. Docetaxel has been found to promote tubulin assembly in microtubules and to inhibit their depolymerization. As predicted by its unique biochemical mechanism of action, docetaxel acts as a mitotic spindle poison and induces a mitotic block in proliferating cells. In vitro, the docetaxel concentrations required to reduce murine and human cell survival by 50% range from 4 to 35 ng/mL, and the cytotoxic effects are greater on proliferating cells than on nonproliferating cells. Docetaxel also is cytotoxic at clinically relevant concentrations against fresh human tumor biopsy specimens (breast, lung, ovarian, colorectal cancer, melanoma) in a soft agar cloning system. Docetaxel has significant in vivo antitumor activity in the different models generally used for the preclinical evaluation of drugs. Eleven of 12 murine transplantable tumors in syngeneic mice have been found to be sensitive to intravenous docetaxel with complete regressions of advanced-stage tumors. Activity also has been observed with human tumor xenografts in nude mice at an advanced stage. In combination studies, synergism has been observed in vivo with 5-fluorouracil, cyclophosphamide, etoposide, vinorelbine, and methotrexate. Preclinical toxicity in mice and dogs has been evaluated by using one and five daily intravenous doses, respectively. The dog was found to be the more sensitive species. The dose-limiting toxicities are hematologic and gastrointestinal in both species. Neurotoxicity also has been observed at high dosages in mice.

[Taxoids: structural and experimental properties]

Paclitaxel (Taxol) and docetaxel (Taxotere) are the first representatives of a new class of antitumor compounds. These two taxoids are clinically active against breast, ovarian and lung cancers. Taxoids are highly complex diterpenoids form natural origin. Preclinical and clinical developments have been made possible after a long and sustained chemical effort: paclitaxel is extracted from the barks of the Pacific yew tree Taxus brevifolia whereas docetaxel is prepared by hemisynthesis starting from 10-deacetyl-baccatin III, an inactive precursor found in the needles of the European yew tree Taxus baccata. These two drugs are active in various in vitro and in vivo preclinical models (cell lines, cloning of human tumor stem cells, murine grafted tumors, human xenografts). Taxoids constitute a new class of antimitotic agents different from vinca-alkaloids: on the one hand, paclitaxel and docetaxel can be considered as inhibitors of the reaction of depolymerization of microtubules into tubulin; on the other hand, vinca-alkaloids inhibit reaction of polymerization of tubulin into microtubules. An active program of medicine chemistry is done in various pharmaceutical and academic Institutions with two objectives: knowledge of structure-activity relationships and selection of new candidates for clinical trials.

Synthesis and structure-activity relationships of new antitumor taxoids. Effects of cyclohexyl substitution at the C-3' and/or C-2 of taxotere (docetaxel)

Synthesis and cytotoxicity of the new analogs (11-13) of docetaxel possessing cyclohexyl groups instead of phenyl groups at the C-3' and/or C-2 benzoate positions are described. The C-2 cyclohexanecarboxylate analog of paclitaxel (15) is also synthesized for comparison. The potency of these new taxoids were examined for their inhibitory activity for microtubule disassembly and also for their cytotoxicity against murine P388 leukemia cell line as well as doxorubicin-resistant P388 leukemia cell line (P388/Dox). It is found that 3'-dephenyl-3'-cyclohexyldocetaxel (11) (0.72T) and 2-(hexahydro)docetaxel (12) (0.85T) possess strong inhibitory activity for microtubule disassembly equivalent to docetaxel (0.7T), which is more potent than paclitaxel (1.0T). The results clearly indicate that phenyl or an aromatic group at C-3' or C-2 is not a requisite for strong binding to the microtubules. This finding has opened an avenue for development of new nonaromatic analogs of docetaxel and paclitaxel. 3'-Dephenyl-3'-cyclohexyl-2-(hexahydro)docetaxel (13) (2T) turns out to be a substantially weaker inhibitor. The cytotoxicities of 11-13 against P388 are, however, in the same range that is 8-12 times weaker than docetaxel and 4-6 times weaker than paclitaxel, i.e., 13 shows equivalent cytotoxicity to that of 11 or 12 in spite of much lower microtubule disassembly inhibitory activity. The cytotoxicities of these new taxoids against the P388/Dox cell line are only 2-2.5 times lower than that of docetaxel. The potency of 2-(hexahydro)paclitaxel (15) for these assays is much lower than the docetaxel counterpart 12. The significant loss of activity in vivo against B16 melanoma is observed for 11-13, i.e., 11 is only marginal (T/C = 38% at 20 mg/kg/day), and 12 and 13 are inactive (T/C = 76% and 79%, respectively). This could be ascribed to faster metabolism, faster excretion or other bioavailability problems.

Predominant labeling of beta- over alpha-tubulin from porcine brain by a photoactivatable taxoid derivative

An [(azidophenyl)ureido]taxoid (TaxAPU) was synthesized in a radiolabeled form by coupling an aminotaxoid to tritiated N-methyl-N-(chloroformyl)-p-azidoaniline. TaxAPU was used to photolabel polymerized porcine brain tubulin. This newly synthesized probe possesses taxoid properties as demonstrated by its effect, in the absence of light, on the kinetics of tubulin assembly and microtubule disassembly and on the critical concentration of tubulin. TaxAPU apparently competes with Taxol for the same binding site with an equilibrium dissociation constant of 6 microM. The photoactivation of 266 nm of the radiolabeled probe in the presence of microtubules led to a covalent incorporation of radioactivity. Analysis of the radiolabeled polypeptides by electrophoresis under denaturing conditions revealed a specific incorporation of tritium in both the alpha-and beta-subunits of tubulin. A dependence on probe concentration was observed for the irreversible radioactivity incorporated into both subunits and maintained essentially a ratio of 2.5:1 between beta/alpha. Therefore, TaxAPU constitutes a true photoaffinity probe for the taxoid binding site on microtubules. Our results complement those reported by Rao et al. (1992) of photo-cross-linking experiments with unmodified Taxol.

Actin polymerization: regulation by divalent metal ion and nucleotide binding, ATP hydrolysis and binding of myosin

Actin filaments are major dynamic components of the cytoskeleton of eukaryotic cells. Assembly of filaments from monomeric actin occurs with expenditure of energy, the tightly bound ATP being irreversibly hydrolyzed during polymerization. This dissipation of energy perturbs the laws of reversible helical polymerization defined by Oosawa and Asakura (1975), and affects the dynamics of actin filaments. We have shown that ATP hydrolysis destabilizes actin-actin interactions in the filament. The destabilization is linked to the liberation of Pi that follows cleavage of gamma-phosphate. Pi release therefore plays the role of a conformational switch. Because ATP hydrolysis is uncoupled from polymerization, the nucleotide content of the filaments changes during the polymerization process, and filaments grow with a stabilizing "cap" of terminal ADP-Pi subunits. The fact that the dynamic properties of F-actin are affected by ATP hydrolysis results in a non-linear dependence of the rate of filament elongation on monomer concentration. Possible modes of regulation of filament assembly may be anticipated from the basic properties of actin. We have shown that the tightly bound divalent metal ion (Ca2+ or Mg2+) interacts with the beta- and gamma-phosphates of ATP bound to actin, and that the Me-ATP bidentate chelate is bound to G-actin in the A configuration. The nature of the bound metal ion affects the conformation of actin and the rate of ATP hydrolysis. In motile living cells, a large pool of actin is maintained unpolymerized by interaction with G-actin binding proteins such as thymosin beta 4 and its variants or profilin. Part of this pool is released to increase the F-actin pool upon cell stimulation. The role of G-actin polymerizing proteins may be crucial in defining the patterns of filament assembly in these situations. The myosin head (myosin subfragment-1) may be considered as a model actin polymerizing protein, may be the closest model to the short tailed myosin I family. The mechanism of assembly of decorated filaments from G-actin and myosin subfragment-1 has therefore been examined.

Interaction between G-actin and myosin subfragment-1 probed by covalent cross-linking

The topography of rapid equilibrium complexes formed between G-actin and myosin subfragment-1, which are the first kinetic intermediates in the polymerization process into F-acto-S1 filaments, has been probed by chemical cross-linking. In the absence of ATP, cross-linking of G-actin-S1 complexes by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) yielded a major 165-170-kDa and a fainter 200-205-kDa doublet polypeptide. The actin:S1 molar ratio was 1 in the EDC-cross-linked complexes, using either double labeling techniques or the method combining EDC + N-hydroxysuccinimide. Chemical cleavages of the covalently cross-linked complexes by formic acid and N-hydroxylamine (Sutoh, K. (1983) Biochemistry 22, 1579-1585) showed that in the main cross-linked 165-kDa polypeptide, the 1-12 acidic N-terminal region of actin was covalently linked to the lysine-rich region connecting the central 50-kDa domain to the C-terminal 20-kDa domain of S1, as in F-acto-S1 complexes. G-actin, but not F-actin, was covalently cross-linked to S1 by N,N'-paraphenylenedimaleimide (p-PDM). A major 195-kDa and a minor 165-kDa polypeptide were obtained, could be separated from actin and S1 by DEAE-cellulose chromatography, and did not exhibit actin-activated Mg-ATPase activity. Both EDC-cross-linked and p-PDM-cross-linked complexes between G-actin and S1 could be incorporated into F-acto-S1 decorated filaments. The C-terminal cysteine 374 of actin is involved in the p-PDM cross-linked 195-kDa complex. Accordingly, a covalent photocross-linked 200-kDa conjugate was formed between S1 heavy chain and benzophenone-G-actin, obtained by covalent modification of Cys374 by benzophenonemaleimide (Tao, T., Lamkin, M., and Scheiner, C. J. (1985) Arch. Biochem. Biophys. 240, 627-634). These results demonstrate that (i) G-actin-S1 and F-actin-S1 complexes display a large similarity in the EDC-cross-linked electrostatic close contacts and (ii) a change in the environment of Cys374 is linked to the polymerization into F-actin-S1 decorated filaments.

Covalent modification of G-actin by pyridoxal 5'-phosphate: polymerization properties and interaction with DNase I and myosin subfragment 1

Pyridoxal 5'-phosphate (PLP), a lysine-specific reagent, has been used to modify G-actin. At pH 7.5, PLP reacted with 1.7-2 lysines on G-actin. Limited proteolytic digestion experiments indicated that, in agreement with previous works, essentially lysine-61 was modified in a 1:1 fashion by PLP, other lysines being much less reactive. A PLP-derivatized affinity label of ATP binding sites, AMPPLP, reacted with two additional lysines that do not appear to be located in the ATP site on G-actin. PLP-G-actin did not polymerize spontaneously up to 30 microM; however, it retained other essential native properties of G-actin. PLP-actin bound to the barbed ends of actin filaments with an equilibrium dissociation constant of 4 microM and prevented dilution-induced depolymerization like a capping protein. PLP-actin copolymerized with unmodified actin. The stability of F-actin copolymers decreased with the fraction of PLP-actin incorporated, consistent with a model within which the actin-PLP-actin interactions in the copolymer are 50-fold weaker, and PLP-actin-PLP-actin interactions are 200-fold weaker than regular actin-actin interactions. PLP-actin bound DNase I with an equilibrium association constant of 2 nM-1, i.e., 10-fold lower than that of unmodified actin. PLP modification did not affect the binding of G-actin to myosin subfragment 1. However, polymerization of PLP-actin by myosin subfragment 1 was not observed in low ionic strength buffers, whereas PLP-F-actin-S1 filaments, in which the stoichiometry PLP-actin:S1 is 1:1, were formed with an apparent critical concentration of 4.5 microM in the presence of 0.1 M KCl.

Myosin subfragment-1 interacts with two G-actin molecules in the absence of ATP

The interaction between G-actin and myosin subfragment-1 (S1) has been monitored by pyrenyl-actin fluorescence and light scattering. In low ionic strength buffer and in the absence of ATP the polymerization of G-actin induced by myosin subfragment-1 is preceded by the formation of binary GS and ternary G2S complexes in which S1 interacts tightly in rapid equilibrium (K greater than 10(7) M-1) with one and two G-actin molecules, respectively. Pyrenyl fluorescence of G-actin is enhanced 4-fold in GS and 3-fold in G2S. At concentrations of G-actin and S1 in the micromolar range and above, G2S is the predominant species at G-actin/S1 ratios equal to or greater than 1. The isomer of myosin subfragment-1 carrying the A1 light chain, S1(A1), forms a tighter ternary complex than the isomer S1(A2). Actin-bound ATP is not hydrolyzed upon formation of GS and G2S. In the presence of one molar equivalent or more of myosin subfragment-1/mol of G-actin, in low ionic strength buffer containing no nucleotides, G-actin polymerizes faster in the presence of S1(A1) than in the presence of S1(A2). The interaction of S1 with G-actin is inhibited by the binding of ATP or ADP to S1, ATP having a higher affinity for S1 than ADP. The possible structural similarity of the G2S complex to the F-acto-S1 complex in the rigor state and the potential significance of a ternary (actin)2-myosin interaction for actomyosin-based motility are discussed.

Characterization of the aluminum and beryllium fluoride species bound to F-actin and microtubules at the site of the gamma-phosphate of the nucleotide

Aluminum fluoride and beryllium fluoride complexes have previously been shown to bind tightly to F-ADP-actin and GDP-microtubules in competition with Pi and to mimic the XDP-Pi transient state of the polymerization. The structure of the bound complexes is investigated here in further detail. Using a fluoride ion-specific electrode, the number of fluoride atoms per aluminum or beryllium atom in the bound complex could be determined. The results indicate that AIF-4 and either BeF2(OH)-.H2O or BeF3-.H2O are the tightly bound species in both F-actin and microtubules. The dependences of the binding on pF and pH are consistent with this conclusion. The possible geometries of aluminum and beryllium fluorides in the gamma-phosphate subsite of the nucleotide are discussed in correlation with the catalytic mechanism of nucleotide hydrolysis.

Probing the mechanism of ATP hydrolysis on F-actin using vanadate and the structural analogs of phosphate BeF-3 and A1F-4

The binding of phosphate analogs to F-ADP-actin filaments and their effect on the dynamics of the polymer have been investigated. Orthovanadate binds to F-actin with the same affinity as phosphate and, at low saturation levels, stabilizes the filament structure in a Pi-like fashion; at higher concentration, it promotes destabilization of the filament. BeF-3 and A1F-4 bind to F-ADP-actin in competition with Pi, with a stoichiometry of 1 mol/mol of F-actin subunit and an affinity 3 orders of magnitude higher than Pi (KD = 2 and 25 microM for BeF-3 and A1F-4, respectively). BeF-3 and A1F-4 mimic Pi in stabilizing F-actin and slow down the rate of actin dissociation from filaments 150-fold. Only 1% of F-ADP-BeF3 subunits provide extensive stabilization of the filament. A quantitative analysis of the stabilization by BeF-3 is proposed. While Pi appears in rapid equilibrium with F-ADP-actin, BeF-3 binds to and dissociates from F-ADP-actin at very slow rates (k+ = 4 M-1 S-1; k = 8.10(-6) S-1). In addition, although functionally similar to the reconstituted F-ADP-Pi species, F-ADP-BeF3 has a different conformation as indicated by the 17% quenching of pyrenyl fluorescence linked to BeF-3 binding. We suggest that BeF-3 may be a good analog of the transition state F-ADP-P* and that Pi release following cleavage of ATP on F-actin might be rate-limited by the isomerization of F-ADP-P*.