Mathematical Modeling of In Vivo Alpha Particle Generators and Chelator Stability.

Background: Targeted α particle therapy using long-lived in vivo α particle generators is cytotoxic to target tissues. However, the redistribution of released radioactive daughters through the circulation should be considered. A mathematical model was developed to describe the physicochemical kinetics of 212Pb-labeled pharmaceuticals and its radioactive daughters. Materials and Methods: A bolus of 212Pb-labeled pharmaceuticals injected in a developed compartmental model was simulated. The contributions of chelated and free radionuclides to the total released energy were investigated for different dissociation fractions of 212Bi for different chelators, for example, 36% for DOTA. The compartmental model was applied to describe a 212Bi retention study and to assess the stability of the 212Bi-1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane (212Bi-DOTAM) complex after ß- decay of 212Pb. Results: The simulation of the injection showed that α emissions contribute 75% to the total released energy, mostly from 212Po (72%). The simulation of the 212Bi retention study showed that (16 ± 5)% of 212Bi atoms dissociate from the 212Bi-DOTAM complexes. The fractions of energies released by free radionuclides were 21% and 38% for DOTAM and DOTA chelators, respectively. Conclusion: The developed α particle generator model allows for simulating the radioactive kinetics of labeled and unlabeled pharmaceuticals being released from the chelating system due to a preceding disintegration.

Bone Marrow Relative Biological Effectiveness for a 212Pb-labeled Anti-HER2/neu Antibody.

PURPOSE: We have determined the in vivo relative biological effectiveness (RBE) of an alpha-particle-emitting radiopharmaceutical therapeutic agent (212Pb-labeled anti-HER2/neu antibody) for the bone marrow, a potentially dose-limiting normal tissue. METHODS AND MATERIALS: The RBE was measured in mice using femur marrow cellularity as the biological endpoint. External beam radiation therapy (EBRT), delivered by a small-animal radiation research platform was used as the reference radiation. Alpha-particle emissions were delivered by 212Bi after the decay of its parent nuclide 212Pb, which was conjugated onto an anti-HER2/neu antibody. The alpha-particle absorbed dose to the marrow after an intravenous administration (tail vein) of 122.1 to 921.3 kBq 212Pb-TCMC-7.16.4 was calculated. The mice were sacrificed at 0 to 7 days after treatment and the radioactivity from the femur bone marrow was measured. Changes in marrow cellularity were assessed by histopathology. RESULTS: The dose response for EBRT and 212Pb-anti-HER2/neu antibody were linear-quadratic and linear, respectively. On transforming the EBRT dose-response relationship into a linear relationship using the equivalent dose in 2-Gy fractions of external beam radiation formalism, we obtained an RBE (denoted RBE2) of 6.4, which is independent of cellularity and absorbed dose. CONCLUSIONS: Because hematologic toxicity is dose limiting in almost all antibody-based RPT, in vivo measurements of RBE are important in helping identify an initial administered activity in phase 1 escalation trials. Applying the RBE2 and assuming typical antibody clearance kinetics (biological half-life of 48 hours), using a modified blood-based dosimetry method, an average administered activity of approximately 185.5 MBq (5.0 mCi) per patient could be administered before hematologic toxicity is anticipated.

212Pb-conjugated anti-rat HER2/neu antibody against a neu-N derived murine mammary carcinoma cell line: cell kill and RBE in vitro.

PURPOSE: In the current work, the RBE of a 212Pb-conjugated anti-HER2/neu antibody construct has been evaluated, in vitro, by colony formation assay. The RBE was estimated by comparing two absorbed dose-survival curves: the first obtained from the conjugated 212Pb experiments (test radiation), the second obtained by parallel experiments of single bolus irradiation of external beam (reference radiation). MATERIALS AND METHODS: Mammary carcinoma NT2.5 cells were treated with (0-3.70) kBq/ml of radiolabeled antibody. Nonspecific binding was assessed with addition of excess amount of unlabeled antibody. The colony formation curves were converted from activity concentration to cell nucleus absorbed dose by simulating the decay and transport of all daughter and secondary particles of 212Pb, using the Monte Carlo code GEANT 4. RESULTS: The radiolabeled antibody yielded an RBE of 8.3 at 37% survival and a survival independent RBE (i.e. RBE2) of 9.9. Unbound/untargeted 212Pb-labeled antibody, as obtained in blocking experiments yielded minimal alpha-particle radiation to cells. Conclusions: These results further highlight the importance of specific targeting toward achieving tumor cell kill and low toxicity to normal tissue.

Targeted α-Emitter Therapy with 212Pb-DOTAMTATE for the Treatment of Metastatic SSTR-Expressing Neuroendocrine Tumors: First-in-Humans Dose-Escalation Clinical Trial.

Peptide receptor radiotherapy with somatostatin analogs has been successfully used for years as a treatment for somatostatin-overexpressing tumors. Treatment of neuroendocrine tumors (NETs) with the ß-particle emitter 177Lu-DOTATATE is currently considered the standard of care for subjects with gastroenteropancreatic NETs. Despite the success of 177Lu-DOTATATE, there remains significant room for improvement in terms of both safety and efficacy. Targeted α-emitter therapy with isotopes such as 212Pb has the potential to improve both. Here, we present the preliminary results of the phase 1 first-in-humans dose-escalation trial evaluating 212Pb-DOTAMTATE (a bifunctional metal chelator [DOTAM] and the SSTR-targeting peptide [TATE]) in patients with somatostatin receptor-positive NETs. Methods: Twenty subjects with histologically confirmed NETs, prior positive somatostatin analog scans, and no prior history of 177Lu/90Y/111In peptide receptor radiotherapy, with different primary sites of the disease, were enrolled. Treatment began with single ascending doses of 212Pb-DOTAMTATE, with subsequent cohorts receiving an incremental 30% dose increase, which was continued until a tumor response or a dose-limiting toxicity was observed. This was followed by a multiple ascending dose regimen. The recommended phase 2 dose regimen consisted of 4 cycles of 2.50 MBq/kg (67.6 µCi/kg) of 212Pb-DOTAMTATE administered at 8-wk intervals, intravenously. Results: Ten subjects received the highest dose, 2.50 MBq/kg/cycle (67.6 µCi/kg/cycle). Treatment was well tolerated, with the most common treatment-emergent adverse events being nausea, fatigue, and alopecia. No serious treatment-emergent adverse events were related to the study drug, and no subjects required treatment delay or a dose reduction. An objective radiologic response of 80% was observed for the first 10 subjects treated at the recommended phase 2 dose. Conclusion: Targeted α-therapy with 212Pb-DOTAMTATE has been shown to be well tolerated. Preliminary efficacy results are highly promising. If these results are confirmed in a larger, multicenter clinical trial, 212Pb-DOTAMTATE would provide a substantial benefit over currently Food and Drug Administration-approved therapies for patients with metastatic or inoperable SSTR-expressing NETs regardless of the grade and location of the primary tumor.

Preclinical study of 212Pb alpha-radioimmunotherapy targeting CD20 in non-Hodgkin lymphoma.

BACKGROUND: Despite therapeutic advances, Non-Hodgkin lymphoma (NHL) relapses can occur. The development of radioimmunotherapy (RIT) with α-emitters is an attractive alternative. In this study, we investigated the potential of α-RIT in conjunction with 212Pb-rituximab for the treatment of NHL. METHODS: EL4-hCD20-Luc cells (mouse lymphoma cell line) were used for in vitro and in vivo studies. Biodistribution and efficacy studies were performed on C57BL/6 mice injected intravenously with 25 × 103 cells. RESULTS: 212Pb-rituximab (0.925-7.4 kBq/mL) inhibit proliferation of EL4-hCD20-Luc cells in vitro. Biodistribution of 203/212Pb-rituximab in mice showed a significant tumour uptake and suggested that the liver, spleen, and kidneys were the organs at risk. For efficacy studies, mice were treated at either 11 days (early stage) or 20-30 days after injection of tumour cells (late stage). Treatment with 277.5 kBq 212Pb-rituximab significantly prolonged survival. Even at an advanced tumour stage, significant tumour regression occurred, with an increase in the median survival time to 28 days, compared with 9 days in the controls. CONCLUSIONS: These results show the efficacy of 212Pb-rituximab in a murine syngeneic lymphoma model, in terms of significant tumour regression and increased survival, thereby highlighting the potency of α-RIT for the treatment of NHL.

Targeted alpha therapy for chronic lymphocytic leukaemia and non-Hodgkin's lymphoma with the anti-CD37 radioimmunoconjugate 212Pb-NNV003.

Relapse of chronic lymphocytic leukaemia and non-Hodgkin's lymphoma after standard of care treatment is common and new therapies are needed. The targeted alpha therapy with 212Pb-NNV003 presented in this study combines cytotoxic α-particles from 212Pb, with the anti-CD37 antibody NNV003, targeting B-cell malignancies. The goal of this study was to explore 212Pb-NNV003 for treatment of CD37 positive chronic lymphocytic leukaemia and non-Hodgkin's lymphoma in preclinical mouse models.An anti-proliferative effect of 212Pb-NNV003 was observed in both chronic lymphocytic leukaemia (MEC-2) and Burkitt's lymphoma (Daudi) cells in vitro. In biodistribution experiments, accumulation of 212Pb-NNV003 was 23%ID/g and 16%ID/g in Daudi and MEC-2 tumours 24 h post injection. In two intravenous animal models 90% of the mice treated with a single injection of 212Pb-NNV003 were alive 28 weeks post cell injection. Median survival times of control groups were 5-9 weeks. There was no significant difference between different specific activities of 212Pb-NNV003 with regards to therapeutic effect or toxicity. For therapeutically effective activities, a transient haematological toxicity was observed. This study shows that 212Pb-NNV003 is effective and safe in preclinical models of CD37 positive chronic lymphocytic leukaemia and non-Hodgkin's lymphoma, warranting future clinical testing.

Drugs That Modify Cholesterol Metabolism Alter the p38/JNK-Mediated Targeted and Nontargeted Response to Alpha and Auger Radioimmunotherapy.

PURPOSE: For the development of new anticancer therapeutic radiopharmaceuticals, including alpha particle emitters, it is important to determine the contribution of targeted effects in irradiated cells, and also of nontargeted effects in nonirradiated neighboring cells, because they may affect the therapeutic efficacy and contribute to side effects. EXPERIMENTAL DESIGN: Here, we investigated the contribution of nontargeted cytotoxic and genotoxic effects in vitro and in vivo (in xenografted mice) during alpha (212Pb/212Bi, 213Bi) and Auger (125I) radioimmunotherapy (RIT). RESULTS: Between 67% and 94% (alpha RIT) and 8% and 15% (Auger RIT) of cancer cells were killed by targeted effects, whereas 7% to 36% (alpha RIT) and 27% to 29% (Auger RIT) of cells were killed by nontargeted effects. We then demonstrated that the nontargeted cell response to alpha and Auger RIT was partly driven by lipid raft-mediated activation of p38 kinase and JNK. Reactive oxygen species also played a significant role in these nontargeted effects, as demonstrated by NF-κB activation and the inhibitory effects of antioxidant enzymes and radical scavengers. Compared with RIT alone, the use of RIT with ASMase inhibitor (imipramine) or with a lipid raft disruptor (e.g., methyl-beta-cyclodextrin or filipin) led to an increase in clonogenic cell survival in vitro and to larger tumors and less tissue DNA damage in vivo. These results were supported by an inhibitory effect of pravastatin on Auger RIT. CONCLUSIONS: Cell membrane-mediated nontargeted effects play a significant role during Auger and alpha RIT, and drugs that modulate cholesterol level, such as statins, could interfere with RIT efficacy.

Preclinical Investigation of 212Pb-DOTAMTATE for Peptide Receptor Radionuclide Therapy in a Neuroendocrine Tumor Model.

Somatostatin analogues have been examined as a treatment for somatostatin receptor overexpressing tumors for years; specifically, octreotate (TATE) and octreotide (TOC). Several versions of these analogues coupled to beta or gamma nuclides are currently used as imaging agents, as treatments with peptide receptor radionuclide therapy (PRRT) for patients with neuroendocrine tumors or are being explored in preclinical and clinical settings. Our study describes the use of 212Pb-DOTAMTATE, the octreotate analogue, in combination with 212Pb, the parent of an alpha emitter. Preclinical studies demonstrated tumor targeting of 212Pb-DOTAMTATE of >20% ID/g up to 24 hours post drug injection. The addition of kidney protection agents, including l-lysine and l-arginine decreases drug accumulation in the kidneys and the addition of ascorbic acid to the chelation mixture reduces oxidation of the drug product. 212Pb-DOTAMTATE displays a favorable toxicity profile with single-dose injections of 20 µCi showing 100% survival and with nontoxic cumulative doses up to 45 µCi, when fractionated into three smaller doses of 15 µCi. In an initial efficacy study, a single 10 µCi injection of 212Pb-DOTAMTATE extended the mean survival 2.4-fold. Efficacy was enhanced by giving three treatment cycles of 212Pb-DOTAMTATE and reducing the time between injections to two weeks. Efficacy was optimized further by the addition of a chemo-sensitizing agent, 5-fluorouracil, given in combination with three cycles of 10 µCi 212Pb-DOTAMTATE. These conditions led to 79% of the animals being tumor free at the end of the 31-week study suggesting that 212Pb-DOTAMTATE alone or in combination with a chemotherapeutic may have positive clinical implications.

Safety and Outcome Measures of First-in-Human Intraperitoneal α Radioimmunotherapy With 212Pb-TCMC-Trastuzumab.

PURPOSE: One-year monitoring of patients receiving intraperitoneal (IP) Pb-TCMC-trastuzumab to provide long-term safety and outcome data. A secondary objective was to study 7 tumor markers for correlation with outcome. METHODS: Eighteen patients with relapsed intra-abdominal human epidermal growth factor receptor-2 expressing peritoneal metastases were treated with a single IP infusion of Pb-TCMC-trastuzumab, delivered <4 h after 4 mg/kg IV trastuzumab. Seven tumor markers were studied for correlation with outcome. RESULTS: Six dose levels (7.4, 9.6, 12.6, 16.3, 21.1, 27.4 MBq/m) were well tolerated with early possibly agent-related adverse events being mild, transient, and not dose dependent. These included asymptomatic, abnormal laboratory values. No late renal, liver, cardiac, or other toxicity was noted up to 1 year. There were no clinical signs or symptoms of an immune response to Pb-TCMC-trastuzumab, and assays to detect an immune response to this conjugate were negative for all tested. Tumor marker studies in ovarian cancer patients showed a trend of decreasing Cancer antigen 72-4 (CA 72-4) aka tumor-associated glycoprotein 72 (TAG-72) and tumor growth with increasing administered radioactivity. Other tumor markers, including carbohydrate antigen (CA125), human epididymis protein 4 (HE-4), serum amyloid A (SAA), mesothelin, interleukin-6 (IL-6), and carcinoembryonic antigen (CEA) did not correlate with imaging outcome. CONCLUSIONS: IP Pb-TCMC-trastuzumab up to 27 MBq/m seems safe for patients with peritoneal carcinomatosis who have failed standard therapies. Serum TAG-72 levels better correlated to imaging changes in ovarian cancer patients than the more common tumor marker, CA125.

Toxicological Studies of 212Pb Intravenously or Intraperitoneally Injected into Mice for a Phase 1 Trial.

Faced with the novelty of a 212Pb-labeled monoclonal antibody (mAb) for clinical translation, concerns were expressed by the Food and Drug Administration (FDA) regarding 212Pb prematurely released from the mAb-chelate conjugate. The objective of this study was to simulate the worst case scenario of such a failure. Groups of Balb/c mice (n = 9-20) were administered 212Pb by intraperitoneal (0.0925-1.85 MBq) or intravenous (0.0925-1.11 MBq) injection and then euthanized at 7 or 90 days to assess acute or chronic effects. Weights were recorded prior to injection of the 212Pb and at the end of the observation periods. Blood samples were collected for clinical chemistry and blood cell analysis. Thirty tissues were harvested and formalin fixed for histopathological examination. Treatment related effects of the 212Pb were observed in the bone marrow, spleen, kidneys and the liver. Histological alterations in these organs were considered mild to moderate, indicating low grade toxicity, and not considered severe enough to affect function. This data was presented to the FDA and determined to be acceptable. The clinical trial with 212Pb-TCMC-trastuzumab was approved in January 2011 and the trial opened at the University of Alabama at Birmingham (UAB) in July.

Dose escalation and dosimetry of first-in-human α radioimmunotherapy with 212Pb-TCMC-trastuzumab.

UNLABELLED: Our purpose was to study the safety, distribution, pharmacokinetics, immunogenicity, and tumor response of intraperitoneal (212)Pb-TCMC-trastuzumab (TCMC is S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane) in patients with human epidermal growth factor receptor type 2 (HER-2)-expressing malignancy. METHODS: In a standard 3 + 3 phase 1 design for dose escalation, (212)Pb-TCMC-trastuzumab was delivered intraperitoneally less than 4 h after administration of trastuzumab (4 mg/kg intravenously) to patients with peritoneal carcinomatosis who had failed standard therapies. RESULTS: Five dosage levels (7.4, 9.6, 12.6, 16.3, and 21.1 MBq/m(2)) showed minimal toxicity at more than 1 y for the first group and more than 4 mo for others. The lack of substantial toxicity was consistent with the dosimetry assessments (mean equivalent dose to marrow, 0.18 mSv/MBq). Radiation dosimetry assessment was performed using pharmacokinetics data obtained in the initial cohort (n = 3). Limited redistribution of radioactivity out of the peritoneal cavity to circulating blood, which cleared via urinary excretion, and no specific uptake in major organs were observed in 24 h. Maximum serum concentration of the radiolabeled antibody was 22.9% at 24 h (decay-corrected to injection time) and 500 Bq/mL (decay-corrected to collection time). Non-decay-corrected cumulative urinary excretion was 6% or less in 24 h (2.3 half-lives). Dose rate measurements performed at 1 m from the patient registered less than 5µSv/h (using portable detectors) in the latest cohort, significantly less than what is normally observed using nuclear medicine imaging agents. Antidrug antibody assays performed on serum from the first 4 cohorts were all negative. CONCLUSION: Five dose levels of intraperitoneal (212)Pb-TCMC-trastuzumab treatment of patients with peritoneal carcinomatosis showed little agent-related toxicity, consistent with the dosimetry calculations.

Pharmacokinetics and imaging of 212Pb-TCMC-trastuzumab after intraperitoneal administration in ovarian cancer patients.

PURPOSE: Study distribution, pharmacokinetics, and safety of intraperitoneal (IP) 212Pb-TCMC-trastuzumab in patients with HER-2-expressing malignancy. EXPERIMENTAL DESIGN: IP 212Pb-TCMC-trastuzumab was delivered, after 4 mg/kg intravenous (IV) trastuzumab, to 3 patients with HER-2-expressing cancer who had failed standard therapies. Patients were monitored for toxicity and pharmacokinetics/dosimetry parameters. RESULTS: Imaging studies after 0.2 mCi/m2 (7.4 MBq/m2) show little redistribution out of the peritoneal cavity and no significant uptake in major organs. Peak blood level of the radiolabeled antibody, determined by decay corrected counts, was <23% injected dose at 63 hours; maximum blood radioactivity concentration was 6.3nCi/mL at 18 hours. Cumulative urinary excretion was ≤6% in 2.3 half-lives. The maximum external exposure rate immediately post-infusion at skin contact over the abdomen averaged 7.67 mR/h and dropped to 0.67 mR/h by 24 hours. The exposure rates at the other positions monitored (axilla, chest, and femur) decreased as a function of distance from the abdomen. The data points correlate closely with 212Pb physical decay (T1/2=10.6 hours). Follow-up >6 months showed no evidence of agent-related toxicity. CONCLUSIONS: Pharmacokinetics and imaging after 0.2 mCi/m2 IP 212Pb-TCMC-trastuzumab in patients with HER-2-expressing malignancy showed minimal distribution outside the peritoneal cavity, ≤6% urinary excretion, and good tolerance.

Comparison between internalizing anti-HER2 mAbs and non-internalizing anti-CEA mAbs in alpha-radioimmunotherapy of small volume peritoneal carcinomatosis using 212Pb.

BACKGROUND AND PURPOSE: We assessed the contribution of antibody internalization in the efficacy and toxicity of intraperitoneal α-radioimmunotherapy (RIT) of small volume carcinomatosis using (212)Pb-labeled monoclonal antibodies (mAbs) that target HER2 (internalizing) or CEA (non-internalizing) receptors. MATERIALS AND METHODS: Athymic nude mice bearing 2-3 mm intraperitoneal tumor xenografts were intraperitoneally injected with similar activities (370, 740 and 1480 kBq; 37 MBq/mg) of (212)Pb-labeled 35A7 (anti-CEA), trastuzumab (anti-HER2) or PX (non-specific) mAbs, or with equivalent amounts of unlabeled mAbs, or with NaCl. Tumor volume was monitored by bioluminescence and survival was reported. Hematologic toxicity and body weight were assessed. Biodistribution of (212)Pb-labeled mAbs and absorbed dose-effect relationships using MIRD formalism were established. RESULTS: Transient hematological toxicity, as revealed by white blood cells and platelets numbering, was reported in mice treated with the highest activities of (212)Pb-labeled mAbs. The median survival (MS) was significantly higher in mice injected with 1.48 MBq of (212)Pb-35A7 (non-internalizing mAbs) (MS = 94 days) than in animals treated with the same activity of (212)Pb-PX mAbs or with NaCl (MS = 18 days). MS was even not reached after 130 days when follow-up was discontinued in mice treated with 1.48 MBq of (212)Pb-trastuzumab. The later efficacy was unexpected since final absorbed dose resulting from injection of 1.48 MBq, was higher for (212)Pb-35A7 (35.5 Gy) than for (212)Pb-trastuzumab (27.6 Gy). These results also highlight the lack of absorbed dose-effect relationship when mean absorbed dose was calculated using MIRD formalism and the requirement to perform small-scale dosimetry. CONCLUSIONS: These data indicate that it might be an advantage of using internalizing anti-HER2 compared with non-internalizing anti-CEA (212)Pb-labeled mAbs in the therapy of small volume xenograft tumors. They support clinical investigations of (212)Pb-mAbs RIT as an adjuvant treatment after cytoreductive surgery in patients with peritoneal carcinomatosis.

Significant systemic therapeutic effects of high-LET immunoradiation by 212Pb-trastuzumab against prostatic tumors of androgen-independent human prostate cancer in mice.

The purpose of this study was to determine therapeutic effects and systemic toxicity of 212Pb-trastuzumab in an orthotopic model of human prostate cancer cells in nude mice. TCMC-Trastuzumab was radiolabeled with 212Pb. The 212Pb-trastuzumab generated from the procedure was intact and had high binding affinity with a dissociation constant (of 3.9±0.99 nM. PC-3MM2 cells, which expressed a lower level of HER2 both in culture and in tumors, were used in therapy studies. A single intravenous injection of 212Pb-trastuzumab reduced tumor growth by 60-80%, reduced aortic lymph node metastasis, and prolonged the survival of tumor-bearing mice. Treatment with 212Pb-trastuzumab did not cause significant changes in body weight, serum glutamic pyruvic transaminase (SGPT), blood urea nitrogen (BUN), hematological profiles, and histological morphology of several major organs of tumor-bearing mice. These findings suggest that a systemic delivery of 212Pb-trastuzumab could be an effective modality for management of advanced human prostate cancer.

Bacterial origin recognition complexes direct assembly of higher-order DnaA oligomeric structures.

Eukaryotic initiator proteins form origin recognition complexes (ORCs) that bind to replication origins during most of the cell cycle and direct assembly of prereplication complexes (pre-RCs) before the onset of S phase. In the eubacterium Escherichia coli, there is a temporally similar nucleoprotein complex comprising the initiator protein DnaA bound to three high-affinity recognition sites in the unique origin of replication, oriC. At the time of initiation, this high-affinity DnaA-oriC complex (the bacterial ORC) accumulates additional DnaA that interacts with lower-affinity sites in oriC, forming a pre-RC. In this paper, we investigate the functional role of the bacterial ORC and examine whether it mediates low-affinity DnaA-oriC interactions during pre-RC assembly. We report that E. coli ORC is essential for DnaA occupation of low-affinity sites. The assistance given by ORC is directed primarily to proximal weak sites and requires oligomerization-proficient DnaA. We propose that in bacteria, DnaA oligomers of limited length and stability emerge from single high-affinity sites and extend toward weak sites to facilitate their loading as a key stage of prokaryotic pre-RC assembly.

Mutational analysis reveals Escherichia coli oriC interacts with both DnaA-ATP and DnaA-ADP during pre-RC assembly.

Prior to initiating DNA synthesis, Escherichia coli oriC switches from ORC, comprising initiator DnaA bound at three high-affinity sites, to pre-RC, when additional DnaA molecules interact with low-affinity sites. Two types of low-affinity sites exist: R boxes that bind DnaA-ATP and DnaA-ADP with equal affinity, and I-sites with a three- to fourfold preference for DnaA-ATP. To assess the regulatory role of weak DnaA interactions during pre-RC assembly in vivo, we compared the behaviour of plasmid-borne wild-type oriC with mutants having an increased or decreased number of DnaA-ATP discriminatory I-sites. Increasing the number of discriminatory sites by replacing R5M with I2 inactivated extrachromosomal oriC function. Mutants with no discriminatory sites perturbed host growth and rapidly replaced wild-type chromosomal oriC, but normal function returned if one I-site was restored at either the I2, I3 or R5M position. These observations are consistent with assembly of E. coli pre-RC in vivo from mixtures of DnaA-ATP and DnaA-ADP, with I-site interactions coupling pre-RC assembly to DnaA-ATP levels.

SeqA blocking of DnaA-oriC interactions ensures staged assembly of the E. coli pre-RC.

DnaA occupies only the three highest-affinity binding sites in E. coli oriC throughout most of the cell cycle. Immediately prior to initiation of chromosome replication, DnaA interacts with additional recognition sites, resulting in localized DNA-strand separation. These two DnaA-oriC complexes formed during the cell cycle are functionally and temporally analogous to yeast ORC and pre-RC. After initiation, SeqA binds to hemimethylated oriC, sequestering oriC while levels of active DnaA are reduced, preventing reinitiation. In this paper, we investigate how resetting of oriC to the ORC-like complex is coordinated with SeqA-mediated sequestration. We report that oriC resets to ORC during sequestration. This was possible because SeqA blocked DnaA binding to hemimethylated oriC only at low-affinity recognition sites associated with GATC but did not interfere with occupation of higher-affinity sites. Thus, during the sequestration period, SeqA repressed pre-RC assembly while ensuring resetting of E. coli ORC.