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.

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*.