Are Broadly Neutralizing Antibodies against HIV-1 Unusual? (47.32)

To date, AIDS vaccines cannot induce broadly(b)-neutralizing (Nt) antibodies (Abs) against HIV-1. It has been suggested that HIV’s Nt epitopes mimic “self” epitopes, and that bNt Abs are rare because they develop from autoreactive B cell clones that are normally deleted or made anergic by self antigens (Agns); such Abs are thought to have long CDR-H3 loops (H3s). Thus, self-tolerance must be broken to accumulate self-reactive naïve B cells whose Abs recognize Nt, HIV-1 epitopes. It has been reported that most of the bNt monoclonal (M)Abs have very long H3s, and that most react with self molecules (e.g., cardiolipin, CL). Alternatively, bNt Abs may arise out of “normal” Ab responses, with HIV Agns selecting bNt Abs with long H3s, and/or long H3s may develop via processes driven by persistent Agn.

We have begun to test these models by several approaches. Using autoAgn ELISAs and microarrays, bNt MAbs were shown not to be unusually self-reactive, compared to genuine autoreactive MAbs or autoimmune sera; also, bNt sera did not react significantly with CL. Finally, sequence analysis of ~700 human MAbs indicated that the bNt MAbs are no different than non-Nt MAbs against HIV-1 or other chronic viruses, or autoimmune anti-protein MAbs; but they are very different from autoimmune MAbs against non-protein autoAgns. Taken together, our results support the conclusion that HIV-bNt Abs are not unusual in their reactivity or structures.

Difference of NPY and its receptor gene expressions between obesity and obesity-resistant rats in response to high-fat diet

AIM

To investigate the effect of high-fat diet on expressions of hypothalamic neuropeptide Y (NPY) and Y1, Y2, Y5 receptors and the mechanism relative to the susceptibility of obesity.

METHODS

Thirty-six female SD rats were randomly divided into high-fat diet group (HF, n=27) and chow food group (CF, n=9), and given either HF or CF diet for 13 weeks. Then the HF diet group was subdivided into dietary induced obesi-ty (DIO) and dietary induced obesity resistant (DIO-R) rat according to the final body weight. Body weight, caloric intake, energy efficiency, visceral fat pads were measured and compared. The levels of plasma and hypothalamic NPY were determined by Radioimmunoassay. Real-time Polymerase Chain Reaction (PCR) was used to measure the gene expression of NPY and its receptors in hypothalamus.

RESULTS

Body weight, caloric intake, energy efficiency and visceral fat pads in DIO rats were higher significantly than those in CF or DIO-R rats (p<0.01). No difference in plasma NPY level was detected among the three groups (p>0.05); the hypothalamic NPY level was significantly higher in DIO group than in CF or DIO-R group (p<0.01). Gene expression levels of NPY and Y1, Y2, Y5 receptors were higher significantly in DIO group than in CF or DIO-R group (p<0.01), while no significant difference was found between DIO-R and CF rats (p>0.05), except that Y2 receptor was lower in DIO-R rats (p<0.01).

CONCLUSIONS

SD rat fed with a HF diet showed different susceptibility to obesity, and up-regulated hypothalamic NPY and Y1, Y2, Y5 receptor gene expressions were closely associated with being predisposed to obesity and overeating of DIO rats.

Left ventricular hypertrophy in patients with renal artery stenosis

Left ventricular hypertrophy (LVH) has been shown to be 3 times more prevalent in patients with renal artery stenosis (RAS) compared to essential hypertension, but factors that predict LVH in this population are not known. We identified 66 patients with unilateral renal artery stenosis and an interpretable electrocardiogram (ECG). LVH by either Cornell voltage-duration product or Sokolow-Lyon voltage criteria was present in 18 of the 66 patients (27%). The mean intra-aortic blood pressure was 100 +/- 14 mm Hg in patients with LVH, and 104 +/- 23 mm Hg in those without LVH (P = 0.37). The average stenosis by quantitative computerized angiography was 68 +/- 17% in patients with LVH, and 64 +/- 13% in those without LVH (P = 0.34). The mean translesional pressure gradient was 11 +/- 15 mm Hg in patients with LVH, and 13 +/- 20 mm Hg in those without LVH (P = 0.60). Using linear regression models, there was no correlation between intra-aortic blood pressure, percentage of stenosis, or translesional pressure gradient and either Cornell voltage-duration product or Sokolow-Lyon voltage criteria. In summary, LVH using ECG criteria was present in 27% of patients with unilateral RAS but was not associated with blood pressure at the time of the procedure or severity of renal artery stenosis.

Further evidence that the chromogranin B gene confers predisposition to schizophrenia: a family-based association study in Chinese

The Chromogranin B (CHGB) gene has been proposed as a candidate gene for predisposition to schizophrenia due to its location on the genome, the evidence of genetic studies, and its functional role in schizophrenia. To investigate its association with schizophrenia using case-control analysis, we genotyped eight single nucleotide polymorphisms (SNPs) and performed transmission disequilibrium tests (TDT) using 192 Han Chinese trios. The G allele of IVS4 + 808A > G showed a trend of over-transmission from heterozygous parents to affected offspring (P = 0.06), although no significant over-transmission was found for individual markers. Furthermore, a significant transmission was observed for the common haplotype G-G-A-G-C (P = 0.0018). Overall, our results suggest that at least one locus in or close to the CHGB gene confers risk of the disorder and strengthen the evidence that CHGB is a promising susceptibility gene for schizophrenia in Chinese population.

Anticancer Activity of 5-Benzylidene-2-Phenylimino-1, 3-Thiazolidin-4-one (BPT) Analogs

We recently identified two compounds of 5-benzylidene-2-phenylimino-1,3-thiazolidin-4-one (BPT) analog, 5-(4-methylbenzylidene)-2-phenylamino-1,3-thiazolidin-4-one (MMPT) and 5-(2,4-dihydroxybenzylidene)-2-phenylimino-1,3-thiazolidin-4-one (DBPT), that can effectively induce apoptosis in cancer cells but not in normal cells, independently of P-glycoprotein status. To further investigate the antitumor activity of BPT analogs, we obtained 18 commercially available analogs of BPT and synthesized 7 analogs in our lab, and analyzed their antitumor activity in various cancer cells, including paclitaxel- and vinorelbine-sensitive and -resistant human lung cancer cells. Two of the compounds were more potent than MMPT or DBPT in induction of apoptosis in certain cancer cell lines and remained tumor selective. Seven compounds did not induce any cytotoxic effects in any of the cell lines tested at the highest concentration tested (31 microM). The other compounds induced cytotoxic effects in some cancer cells but not in others or were less potent than MMPT and DBPT. Cell uptake studies showed that analogs that effectively induced cell killing in paclitaxel- and vinorelbine-resistant cells could be taken up easily by those cells despite their high levels of P-glycoprotein expression. These data further demonstrate that thiazolidinone analogs are not P-glycoprotein substrates and could be useful for treatment of P-glycoprotein overexpressing refractory cancers.

Coronary artery fistulae

Coronary artery fistulae are abnormal communications between a coronary artery and a cardiac chamber or major vessel (vena cava, pulmonary veins, pulmonary artery). They are usually diagnosed by coronary arteriography. Clinical presentations are variable depending on the type of fistula, shunt volume, site of the shunt, and presence of other cardiac conditions. In this article, we review the literature regarding etiology, incidence, clinical manifestation, image studies, and management.

Admixture and population stratification in African Caribbean populations

Throughout biomedical research, there is growing interest in the use of ancestry informative markers (AIMs) to deconstruct racial categories into useful variables. Studies on recently admixed populations have shown significant population substructure due to differences in individual ancestry; however, few studies have examined Caribbean populations. Here we used a panel of 28 AIMs to examine the genetic ancestry of 298 individuals of African descent from the Caribbean islands of Jamaica, St. Thomas and Barbados. Differences in global admixture were observed, with Barbados having the highest level of West African ancestry (89.6%+/- 2.0) and the lowest levels of European (10.2%+/- 2.2) and Native American ancestry (0.2%+/- 2.0), while Jamaica possessed the highest levels of European (12.4%+/- 3.5) and Native American ancestry (3.2%+/- 3.1). St. Thomas, USVI had ancestry levels quite similar to African Americans in continental U.S. (86.8%+/- 2.2 West African, 10.6%+/- 2.3 European, and 2.6%+/- 2.1 Native American). Significant substructure was observed in the islands of Jamaica and St. Thomas but not Barbados (K=1), indicating that differences in population substructure exist across these three Caribbean islands. These differences likely stem from diverse colonial and historical experiences, and subsequent evolutionary processes. Most importantly, these differences may have significant ramifications for case-control studies of complex disease in Caribbean populations.

Experimental study on in vitro tumor cell killing by focused bi-frequency ultrasound activated hematoporphyrin derivatives

OBJECTIVE

As ultrasound has become an important therapeutic technique on tumor, in order to find the cell-killing effect of focused bi-frequency ultrasound activated hematoporphyrin derivatives, experimental study was processed in vitro.

METHODS

Tumor cells K562 and SW-480 were killed by focused bi-frequency ultrasound activated hematoporphyrin (Hp) derivatives under optimized ultrasonic parameters and the cell-killing effect was tested by MTT (Thiazolyl blue: 3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide) technique.

RESULTS

The lower frequency (0.6 MHz) was better than the higher (1.1 MHz) in activating hematoporphyrin derivatives; the bi-frequency was better than mono-frequency, and the cell-killing efficiency was 2-3 times of sum of the two kinds of ultrasounds which constitute bi-frequency. Incubation for the 16 h was better than 4 h after the cells exposed to the ultrasound.

CONCLUSION

The method of killing tumor cells by focused bi-frequency ultrasound activated hematoporphyrin derivatives is presented and evaluated in this paper.

Long-Term Results of Surgical Treatment of Tetralogy of Fallot in Adults

BACKGROUND

The long-term results of patients with tetralogy of Fallot surgically treated in adulthood were evaluated to define the real benefit of surgical correction.

METHODS

Between August 1990 and February 2004, 57 patients older than 18 years of age with tetralogy of Fallot received total correction. Forty-two patients (73.7 %) required transannular patch.

RESULTS

Hospital and late mortality were 7.0 % (n = 4) and 5.7 % (n = 3), respectively. One patient was reoperated on to close residual ventricular septal defect. Four patients were lost. Of the remaining 49 patients, the mean follow-up was 65 +/- 38 months (range 11 - 173 months). Actuarial survival was 97.4 +/- 2.5 %, 91 +/- 7 % and 72.8 +/- 17.1 % at 5, 10 and 14 years, respectively. At the latest follow-up, 35 (76.1 %) of the surviving patients presently have NYHA functional class I ( P < 0.01).

CONCLUSION

The overall survival of surgically treated adult patients with tetralogy of Fallot is acceptable. The greatest benefit of total correction at this age is the functional improvement.

Polymorphisms within the CTLA4 gene are associated with infant atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is one of the most common childhood disorders. It can have a significant impact on the physical and psychological well-being of affected individuals. Although environmental triggers are important, AD also has a strong genetic component. Identifying genes associated with AD may help to understand better the basis of this disorder and its relationship with other allergic disorders such as asthma.

OBJECTIVES

Polymorphisms in the gene encoding the inhibitory CTLA4 receptor, an important regulator of T cells, are associated with asthma as well as autoimmune disorders. We have now tested whether polymorphisms in the CTLA4 gene are also associated with early childhood AD.

METHODS

A family-based cohort of 112 children and their parents was recruited from Western Sydney, Australia. All children were seen by a paediatric dermatologist and presented with AD within the first 3 years of life. Using the transmission disequilibrium test, individual and haplotypic associations with the +49 and CT60 polymorphisms in exon 1 and the 3' nontranslated DNA of the CTLA4 gene were tested.

RESULTS

Single tests of association revealed significant association of the +49(A) [P = 0.037, odds ratio (OR) 1.59, 95% confidence interval (CI) 1-2.55] and borderline significance of the CT60(A) alleles (P = 0.055, OR 1.51, 95% CI 1-2.38). Significant association of the +49(A)/CT60(A) haplotype was detected (P = 0.002, OR 1.78, 95% CI 1.2-2.65).

CONCLUSIONS

Polymorphisms within the gene encoding CTLA4 were associated with early onset infant AD. This is in agreement with findings from asthmatic cohorts, suggesting that the +49(A)/CT60(A) haplotype is a genetic risk factor common to asthma and AD.

Rebound intracranial hypertension after treatment of spontaneous intracranial hypotension

Spontaneous intracranial hypotension (SIH) is characterized by orthostatic headache and the cause is usually cerebrospinal fluid leaks in spine level. Most patients with SIH have a benign course. Epidural blood patch (EBP) is the treatment of choice when initial conservative managements are ineffective. We reported a patient with SIH diagnosed by using magnetic resonance imaging and radionuclide cisternography. Acute rebound intracranial hypertension developed after EBP and was successfully treated with intravenous osmotic agent.

Bony metastatic disease responses to sorafenib (BAY 43–9006) independent of PSA in patients with metastatic androgen independent prostate cancer

4506

Background: Sorafenib is a novel bis-aryl urea, multi-kinase inhibitor, approved for the treatment of advanced renal cell carcinoma. It inhibits b- and c-Raf kinase, PDGFR, c-kit, VEGFR, Flt-3 and p38 and demonstrates anti-proliferative and anti-angiogenic activity. Studies have shown a role for anti-angiogenic therapy for androgen-independent prostate cancer (AIPC). Evidence suggests that the Ras-Raf-MAPK-ERK signaling pathway is dysregulated in AIPC and might be targeted by sorafenib. Methods: 22 patients (pts) with progressive metastatic AIPC enrolled in an open-label, single arm phase II study. The primary objective was to determine if sorafenib is associated with a 50% 4 month probability of progression free survival as determined by clinical, radiographic, and PSA criteria. Sorafenib was given continuously at a dose of 400 mg orally twice daily in 28-day cycles. Clinical assessment and PSA measurement occurred every cycle with radiographic measurements every 2 cycles. Results: Baseline patient characteristics included a median (range) age of 64 (51–78), Gleason 8 (6–9), PSA 226.4 ug/l (2–1905), Alk Phos 108 u/l (48–259), Hb 12.7 g/dl (10.2–15.1). 60% of patients had received one prior chemotherapy regimen. Of the 19 pts with progressive disease, 10 progressed only by PSA criteria in the absence of evidence of clinical and radiographic progression. Two pts were found to have dramatic disappearance of bone metastatic lesions as demonstrated by bone scan, even though they met PSA progression criteria at the time the scans were obtained. Toxicities likely related to treatment include: one grade 3 hypertension and hand-foot syndrome; grade 1/2 toxicities: fatigue, anorexia, hypertension, skin rash, nausea, and diarrhea. Conclusion: Sorafenib in AIPC is relatively well tolerated with 2 patients demonstrating evidence of improved bony metastatic lesions. Interpretation of this study is complicated by discordant radiographic and PSA responses. PSA may not be an adequate biomarker for monitoring sorafenib activity. Further study of sorafenib in metastatic AIPC using clinical and radiographic endpoints is warranted.

No significant financial relationships to disclose.

Routine interval computed tomography in detecting new soft tissue disease in patients with androgen-independent prostate cancer (AIPC) and only bone metastasis

4621

Background: Bone is the most common metastatic site in AIPC and the only site of metastasis for many patients. However, some pts with only bone metastasis will eventually develop soft tissue disease. Thus many clinical trials include CT monitoring to exclude new soft tissue disease as the only manifestation of disease progression. The true incidence of the asymptomatic development of soft tissue metastases without PSA or bone scan progression is unknown. Here we have identified pts with bone metastasis only at the time of protocol enrollment. We analyzed their disease progression pattern to determine the utility of routine interval CT scans. Methods: We reviewed 100 pts with metastatic AIPC from three phase II clinical trials (docetaxel/thalidomide, docetaxel/vaccine, and ketoconazole/alendronate) between 1995 and 2004. Pts were assessed every 4 weeks with PSA measurement. CT and bone scans were performed 2 months after start of the trial and then at least every 3 months. Median follow-up time was 8 months (1–44 months). The pts with bone metastasis only were examined for the occurrence of soft tissue diseases by subsequent CT scans in association with bone scan and PSA. Results: Of 100 pts with metastatic AIPC, 33% had soft tissue disease alone or soft tissue and bone disease. 67% had no soft tissue involvement identified by CT but had bone metastasis revealed by bone scan. Of the pts with only bone metastasis, 3% (2 pts) developed new soft tissue disease. One pt developed new right illiac fossa lymphadenopathy after 8 months, and another a peri-rectal mass after 12 months. The one with new lymphadenopathy also had multiple new bone lesions identified by bone scan and PSA progression. The one with a peri-rectal mass had PSA progression and a palpable abnormality. Conclusions: The incidence of asymptomatic new soft tissue disease as the only manifestation of disease progression appears to be quite low in pts with only bone metastasis. Scheduled interval examination with CT to exclude new soft tissue disease does not appear to be warranted. CT scans should probably only be done at the time of disease progression shown by bone scan/PSA or when there are clinical signs or symptoms of new soft tissue disease.

No significant financial relationships to disclose.

Predictive biomarker discovery and validation for the targeted chemotherapeutic ixabepilone

3011

Background: Ixabepilone is a microtubule stabilizing agent with significant therapeutic value in breast cancer (BC) patients. To identify predictive biomarkers capable of identifying patients likely to receive optimal benefit from ixabepilone treatment, preclinical and clinical studies were carried out. Several biomarkers discovered using preclinical models were validated in a neoadjuvant BC clinical study ( CA163080 ) and one, estrogen receptor 1 (ER), was shown to double the pathological complete response (pCR) rate in patients treated with ixabepilone. To identify candidate sets of biomarkers that could further increase the pCR rate we have performed post-hoc analyses of the preclinical and clinical data. Methods: Eighteen BC cell lines were classified as sensitive or resistant (S/R) based on the IC50 values for ixabepilone treatment. Gene expression profiling of the BC cell lines was conducted and genes correlated with the S/R classification were identified using a k-Nearest Neighbors algorithm. Patients in clinical study CA163080 underwent a pretreatment core needle biopsy from which RNA was isolated and gene expression profiles generated (data available on 134 patients). Analyses using the preclinical and clinical markers were conducted using various statistical tools. Results: Several markers used in combination with ER were found to be capable of tripling the pCR to ixabepilone in CA163080. In addition to ER other predictive markers were identified that were as predictive as ER, including several genes whose expression is anti-correlated with ER and are part of the ER pathway. Finally, various sub-group analyses were performed and manifested the importance of clinical sample variation that needs to be considered for the analysis. Conclusions: Several single biomarkers identified from preclinical studies were validated in the clinical study CA163080 , demonstrating the utility of this approach. Such markers can be used in combination to better identify patients likely to respond to ixabepilone in future clinical trials. Furthermore, molecular response markers that can be tied to the mechanism of drug resistance can be used for further developing chemotherapy in drug development.

[Table: see text]

Synthesis and characterization of aspartic acid complexes of antimony and bismuth triiodide

New bioinorganic complexes of the aspartic acid with the antimony or bismuth triiodide were synthesized by a direct solid-solid reaction at room temperature. The formula of the complex is MI(3)[OOCCH(2)CH(NH(2))CO](2.5) x 2.5H(2)O (M = Sb, Bi). The complex may be a dimer with bridge structure. The crystal structure of the complexes belongs to a triclinic system. The lattice parameters are a = 0.9883 nm, b = 1.4284 nm, c = 2.0114 nm, alpha = 94.46 degrees , beta = 99.76 degrees and gamma = 100.1 degrees for the complex of antimony and a = 0.9756 nm, b = 1.4560 nm, c = 1.9875 nm, alpha = 94.18 degrees , beta = 97.25 degrees and gamma = 101.16 degrees for the complex of bismuth. The infrared spectra and thermal analyses can demonstrate the complex formation between the aspartic acid and the antimony or bismuth ion.

Transcriptional up-regulation of histamine receptor-1 in epithelial, mucus and inflammatory cells in perennial allergic rhinitis

BACKGROUND

Histamine receptors play an important role in the pathogenesis of nasal allergy. Activation of histamine receptor 1 (H1R) and 2 (H2R) can cause allergic symptoms which can be blocked effectively by antihistamines. H1R and H2R transcript levels have been found to be up-regulated in perennial - but not in seasonal - allergic rhinitis (AR). The present study aimed to explore H1R and H2R expression in complex tissues of the nasal mucosa of perennial allergic rhinitis (PAR).

METHODS

Ten patients with PAR and 13 non-AR subjects were recruited for the study by medical history, physical examination and laboratory screening tests. In this study, we have analysed single cells dissected from the nasal mucosa biopsies by laser-assisted microdissection. H1R mRNA expression was analysed in different cell types such as epithelial, endothelial, mucus and inflammatory cells isolated from the nasal mucosa of PAR in comparison with non-AR subjects.

RESULTS

H1R mRNA gene expression level was significantly increased in the nasal mucosa of PAR in comparison with non-AR (P<0.0001). H1R mRNA was significantly elevated in epithelial (P<0.001) and mucus cells (P<0.05) of PAR in comparison with non-AR whereas H1R gene expression levels in endothelial cells between both groups were not changed (P=0.23). Interestingly, inflammatory cells in the nasal mucosa of PAR patients were also strongly expressed H1R mRNA (P<0.001).

CONCLUSION

The present study indicates that PAR alters the expression of H1R mRNA in epithelial, mucus and inflammatory cells of the nasal mucosa and but not in endothelial cells. Therefore, epithelial, mucus and inflammatory cells may play an important role in histamine-mediated allergic airway inflammation in PAR.

Breast cancer resistance protein-mediated topotecan resistance in ovarian cancer cells

Overexpression of breast cancer resistance protein (BCRP) and mitoxantrone (MX) resistance protein can confer resistance to a variety of cytostatic drugs, such as MX, topotecan (TPT), doxorubicin, and daunorubicin. This study investigates the role of BCRP in resistance of ovarian cancer to TPT treatment. We have developed TPT-resistant human ovarian cancer cell line. Intracellular concentration of fluorescent dye rhodamine 123 (Rh123) was measured by flow cytometry. The expression of several membrane transporter proteins including P-glycoprotein (P-gp), multidrug resistance protein 1 (MRP1), and BCRP were determined by reverse transcription-polymerase chain reaction and Western blotting. The Rh123 concentration in parental cells was approximately three times of those in TPT-resistant cells. In contrast to undetectable level of P-gp messenger RNA (mRNA) and minimal level of MRP1 expression in TPT-resistant cells, overexpression of both the BCRP mRNA and the protein was detected in these cells. Introduction of antisense-phosphorothioate oligonucleotide derived from BCRP mRNA into TPT-resistant cells resulted in a significant increase in the concentration of intracellular Rh123. These results suggested a novel mechanism in which a reduced intracellular drug concentration may be mediated by BCRP gene products in human ovarian cancer cells.

Inhibitors of histone deacetylases promote hematopoietic stem cell self-renewal

BACKGROUND

Histone deacetylases (HDAC) are associated with a variety of transcriptional repressors that control cellular differentiation and proliferation. HDAC inhibitors such as trichostatin A, trapoxin and chlamydocin could be useful tools to modulate these cellular processes. We investigated their effect on the self-renewal of hematopoietic stem cells (HSC) during ex vivo culture.

METHODS

Purified murine HSC with the phenotype c-Kit+,Thy-1.1(lo), Lin(-/lo), Sca-1+ were cultured for 4 days with IL-3, IL-6 and c-Kit ligand without or with HDAC inhibitors, after which their degree of phenotypic differentiation in culture was assessed by flow cytometric analysis. To explore whether HDAC inhibitors could have a beneficial role in human HSC transplantation, mobilized peripheral blood CD34+ cells were cultured with thrombopoietin mimetic peptide, flt3 ligand, and c-Kit ligand, without or with various HDAC inhibitors. The fluorescent dye, carboxyfluorescein-diacetate succinimidylester (CFSE), was used to track division of cell subsets, and engrafting ability was evaluated in a non-obese diabetic (NOD) -SCID xenotransplantation model.

RESULTS

Murine HSC cultured with HDAC inhibitors maintained a more primitive phenotype than control cultures. The number of human HSC expressing Thy-1 increased up to seven-fold during a 5-day culture with HDAC inhibitors compared with control cultures. Chlamydocin was the most effective of the HDAC inhibitors tested at promoting Thy-1 expression on human cells. CFSE tracking showed that the increase in Thy-1+ cells resulted from cell division. In a NOD-SCID repopulation assay, cells exposed to chlamydocin for 24 h displayed an average four-fold higher engrafting ability over control cells.

DISCUSSION

Our studies suggest that HDAC inhibitors can induce ex vivo expansion of human HSC, and may improve engraftment in hematopoietic transplant patients when cell dose is limiting.