Chemotherapy in a pregnant rat model. 2.5-fluorouracil: nonlinear kinetics and placental transfer

Unraveling the Mechanisms of Clinical Drugs-Induced Neural Tube Defects Based on Network Pharmacology and Molecular Docking Analysis

Chemotherapeutic agents such as methotrexate (MTX), raltitrexed (RTX), 5-fluorouracil (5-FU), hydroxyurea (HU), and retinoic acid (RA), and valproic acid (VPA), an antiepileptic drug, all can cause malformations in the developing central nervous system (CNS), such as neural tube defects (NTDs). However, the common pathogenic mechanisms remain unclear. This study aimed to explore the mechanisms of NTDs caused by MTX, RTX, 5-FU, HU, RA, and VPA (MRFHRV), based on network pharmacology and molecular biology experiments. The MRFHRV targets were integrated with disease targets, to find the potential molecules related to MRFHRV-induced NTDs. Protein-protein interaction analysis and molecular docking were performed to analyze these common targets. Utilizing the kyoto encyclopedia of genes and genomes (KEGG) signaling pathways, we analyzed and searched the possible causative pathogenic mechanisms by crucial targets and the signaling pathway. Results showed that MRFHRV induced NTDs through several key targets (including TP53, MAPK1, HSP90AA1, ESR1, GRB2, HDAC1, EGFR, PIK3CA, RXRA, and FYN) and multiple signaling pathways such as PI3K/Akt pathway, suggesting that abnormal proliferation and differentiation could be critical pathogenic contributors in NTDs induced by MRFHRV. These results were further validated by CCK8 assay in mouse embryonic stem cells and GFAP staining in embryonic brain tissue. This study indicated that chemotherapeutic and antiepileptic agents induced NTDs might through predicted targets TP53, MAPK1, GRB2, HDAC1, EGFR, PIK3CA, RXRA, and FYN and multiple signaling pathways. More caution was required for the clinical administration for women with childbearing potential and pregnant.

Cancer during Pregnancy: A Review of Preclinical and Clinical Transplacental Transfer of Anticancer Agents

The occurrence of cancer during pregnancy is observed in 1 in 1000 pregnancies and is expected to increase given the trend of delaying childbearing. While breast cancer is the most common, the incidence of other cancers, such as cervical, ovarian, and lung cancers as well as hemopathies and melanomas, is also increasing. Thus, cancer occurrence in pregnant women raises questions of management during pregnancy and, especially, assessment of the treatment benefit-risk ratio to ensure optimal management for the mother while ensuring the safety of the fetus. Chemotherapy remains a cornerstone of cancer management. If the use of anticancer agents appears possible during pregnancy, while avoiding the first trimester, the extent of placental transfer of different anticancer agents varies considerably thereafter. Furthermore, the significant physiological pharmacokinetic variations observed in pregnant women may have an impact on the placental transfer of anticancer agents. Given the complexity of predicting placental transfer of anticancer agents, preclinical studies are therefore mandatory. The aim of this review was to provide updated data on in vivo and ex vivo transplacental transfer of anticancer agents used in the management of the most common pregnancy-associated cancers to better manage these highly complex cases.

Gastric cancer during pregnancy: A report on 13 cases and review of the literature with focus on chemotherapy during pregnancy

Introduction

Gastric cancer during pregnancy is extremely rare and data on optimal treatment and possible chemotherapeutic regimens are scarce. The aim of this study is to describe the obstetric and maternal outcome of women with gastric cancer during pregnancy and review the literature on antenatal chemotherapy for gastric cancer.

Material and methods

Treatment and outcome of patients registered in the International Network on Cancer, Infertility and Pregnancy database with gastric cancer diagnosed during pregnancy were analyzed.

Results

In total, 13 women with gastric cancer during pregnancy were registered between 2002 and 2018. Median gestational age at diagnosis was 22 weeks (range 6‐30 weeks). Twelve women were diagnosed with advanced disease and died within 2 years after pregnancy, most within 6 months. In total, eight out of 10 live births ended in a preterm delivery because of preeclampsia, maternal deterioration, or therapy planning. Two out of six women who initiated chemotherapy during pregnancy delivered at term. Two neonates prenatally exposed to chemotherapy were growth restricted and one of them developed a systemic infection with brain abscess after preterm delivery for preeclampsia 2 weeks after chemotherapy. No malformations were reported.

Conclusions

The prognosis of gastric cancer during pregnancy is poor, mainly due to advanced disease at diagnosis, emphasizing the need for early diagnosis. Antenatal chemotherapy can be considered to reach fetal maturity, taking possible complications such as growth restriction, preterm delivery, and hematopoietic suppression at birth into account.

Management of cancer during pregnancy and current evidence of obstetric, neonatal and pediatric outcome: a review article

The diagnosis of cancer during pregnancy imposes a medical-ethical dilemma in weighing the risks of both mother and child. Increasing awareness of the feasibility of chemotherapy during pregnancy results in more pregnant patients receiving treatment for cancer. Information on obstetric and pediatric outcome of these high-risk pregnancies is greatly needed to guide physicians in patient counseling. In this review we present reported evidence for the incidence, diagnostic options, therapeutic management, obstetric risks, and neonatal outcome when cancer treatment is initiated during pregnancy. Decision-making when a cancer is diagnosed in a pregnant patient should be multidisciplinary, always taking the patient's perspective into account. Cancer treatment during pregnancy is associated with low birth weight and preterm delivery, therefore frequent obstetric follow-up during oncological treatment in a specialized center is mandatory. Short-term clinical, cardiac, and cognitive outcome of children pre-natally exposed to cancer treatment is overall reassuring. Long-term follow-up of children is warranted to define the possible effect of pre-natal cancer treatment on general health, fertility outcome, and the risk of secondary cancers.

Anticancer drugs during pregnancy

Although cancer diagnoses during pregnancy are rare, they have been increasing with the rise in maternal age and are now a topic of international concern. In some cases, the administration of chemotherapy is unavoidable, though there is a relative paucity of evidence regarding the administration of anticancer drugs during pregnancy. As more cases have gradually accumulated and further research has been conducted, we are beginning to elucidate the appropriate timing for the administration of chemotherapy, the regimens that can be administered with relative safety, various drug options and the effects of these drugs on both the mother and fetus. However, new challenges have arisen, such as the effects of novel anticancer drugs and the desire to bear children during chemotherapy. In this review, we outline the effects of administering cytotoxic anticancer drugs and molecular targeted drugs to pregnant women on both the mother and fetus, as well as the issues regarding patients who desire to bear children while being treated with anticancer drugs.

Simultaneous determination of abacavir and zidovudine from rat tissues using HPLC with ultraviolet detection

A simple high-performance liquid chromatography (HPLC) method has been developed and validated for the simultaneous determination of abacavir and zidovudine (AZT) in rat plasma, amniotic fluid, fetal, and placental tissues. Extraction of abacavir, AZT, and the internal standard, azidouridine (AZDU) in amniotic fluid was carried out by protein precipitation. Extraction from plasma, fetal and placental homogenates was achieved by using a salting out technique. Chromatographic separation was performed using a C8 column (150 mm x 4.6 mm, 5 microm). The mobile phase consisted of 12% acetonitrile in 25 mM sodium phosphate buffer (adjusted to pH 7 with sodium hydroxide) for the fetus, placenta, plasma and amniotic fluid samples at a flow rate of 0.8 mL/min. The method was validated over the range from 0.05 to 50 microg/mL for both abacavir and AZT in the four biological matrices. The absolute recovery of abacavir ranged from 79 to 94%, while AZT recoveries ranged from 79 to 90% in the different biological matrices. The internal standard recovery ranged from 90 to 92%. Acceptable intra- and inter-day assay precision (<10% R.S.D.) and accuracy (<10% error) were observed over 0.05-50 microg/mL for all four matrices.

Effects of maternally administered drugs on the fetal and neonatal kidney

The number of pregnant women and women of childbearing age who are receiving drugs is increasing. A variety of drugs are prescribed for either complications of pregnancy or maternal diseases that existed prior to the pregnancy. Such drugs cross the placental barrier, enter the fetal circulation and potentially alter fetal development, particularly the development of the kidneys. Increased incidences of intrauterine growth retardation and adverse renal effects have been reported. The fetus and the newborn infant may thus experience renal failure, varying from transient oligohydramnios to severe neonatal renal insufficiency leading to death. Such adverse effects may particularly occur when fetuses are exposed to NSAIDs, ACE inhibitors and specific angiotensin II receptor type 1 antagonists. In addition to functional adverse effects, in utero exposure to drugs may affect renal structure itself and produce renal congenital abnormalities, including cystic dysplasia, tubular dysgenesis, ischaemic damage and a reduced nephron number. Experimental studies raise the question of potential long-term adverse effects, including renal dysfunction and arterial hypertension in adulthood. Although neonatal data for many drugs are reassuring, such findings stress the importance of long-term follow-up of infants exposed in utero to certain drugs that have been administered to the mother.

Determination of didanosine in maternal plasma, amniotic fluid, fetal and placental tissues by high-performance liquid chromatography-tandem mass spectrometry

A rapid and efficient high-performance liquid chromatography (HPLC)-tandem mass spectrometry method for the determination of didanosine concentrations in maternal rat plasma, amniotic fluid, placental and fetal tissue samples has been developed and validated. Tissue samples were homogenized in optima water and centrifuged. The supernatant was subjected to solid-phase extraction (SPE) prior to analysis. Plasma and amniotic fluid samples were extracted without pretreatment. An Agilent 1100 Series HPLC coupled with a Micromass Quattro II triple quadrupole mass spectrometer was used for all analyses. Chromatographic resolution was achieved on a Nova-Pak phenyl analytical column (2.0 x 150 mm, 4 microm particle size) equipped with a Phenomenex Security-guard phenyl guard cartridge (2.0 x 4.0 mm) using 60% methanol in 10 mm ammonium acetate buffer mobile phase for all matrices at a flow rate of 0.15 mL/min. The method yields retention times of 2.9 min for didanosine and 3.0 min for the internal standard, stavudine. Limits of detection were 1 ng/mL for all matrices. Recoveries were 70% or greater for both compounds in the different matrices. Within- and between-run precision (%RSD) and accuracy (%error) was less than 15% for all matrices.

Management of Colorectal Cancer in Pregnancy: A Multimodality Approach

Colorectal cancer (CRC) is one of the 3 most common types of cancer in women, but CRC during pregnancy is rare, with a reported incidence of approximately 0.002%. Synchronous colon cancer during pregnancy presents a diagnostic and therapeutic challenge for clinicians because there are no generally accepted guidelines regarding diagnosis or treatment. The diagnosis is challenging because the presenting signs/symptoms of CRC are often attributed to the usual complications of pregnancy, which could delay the diagnosis and allow the cancer to progress to an advanced stage. Carcinogenesis of colon cancer in pregnancy is not clear, but a few studies suggest that the increased levels of estrogen and progesterone related to pregnancy stimulate the growth of CRC with their receptors. The aim of treatment is to start therapy for the mother as early as possible and to simultaneously deliver the baby at the earliest time allowable. The management mandates a multidisciplinary approach involving experts in obstetrics, neonatology, gastrointestinal surgery, and medical oncology. The medical community should be able to diagnose colon cancer earlier in pregnancy in order to improve prognosis. The primary care physician or obstetrician should refer the pregnant patient with significant gastrointestinal symptoms to the gastroenterologist for evaluation. Likewise, the gastroenterologist should be prepared to perform sigmoidoscopy (preferably without endoscopic medications) for significant lower gastrointestinal symptoms such as persistent rectal bleeding. Herein, the author reviews the literature concerning the diagnosis and treatment of CRC in pregnancy and discusses the role of newer agents approved for the treatment of CRC.

Simultaneous determination of zidovudine and lamivudine from rat tissues by liquid chromatography/tandem mass spectrometry

A simple liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed and validated for the simultaneous determination of zidovudine (AZT) and lamivudine (3TC) in rat plasma, amniotic fluid, placental, and fetal tissues. Samples were processed by acetonitrile precipitation. Chromatography was performed using a C18 column (5 microm, 150 x 3.9 mm i.d). The mobile phase consisted of 30% methanol and 7.5 mM ammonium acetate (pH 6.5). The method was validated in the range of 0.05-25 microg/mL for both 3TC and AZT in the four biological matrices. Finally, the method was applied to a study involving fetal transport following co-administration of these compounds at a dose of 25 mg/kg each in a pregnant rat.

Colon cancer during pregnancy

The medical community should become mobilized to diagnose colon cancer earlier in pregnancy to improve prognosis. The primary care physician or obstetrician should refer the pregnant patient with significant gastrointestinal complaints to the gastroenterologist for evaluation. Likewise, the gastroenterologist should be prepared to perform sigmoidoscopy, preferably without endoscopic medications, for significant lower gastrointestinal symptoms such as persistent rectal bleeding. Sigmoidoscopy is particularly sensitive in identifying colon cancer in pregnant patients because their cancers are usually distal and within reach of the sigmoidoscope.

Pharmacokinetics of intravenous acyclovir, zidovudine, and acyclovir-zidovudine in pregnant rats

The pharmacokinetics and placental transfer of acyclovir and zidovudine monotherapies and acyclovir-zidovudine combination therapy were compared in the pregnant rat. Timed-pregnancy Sprague-Dawley rats were used for the study. Doses of 60 mg of each drug/kg of body weight in monotherapy and in combination therapy were given by intravenous bolus, and samples of maternal plasma, amniotic fluid, fetal tissue, and placental tissue were collected over a period of 8 h postdose. Concentrations of each drug in the various matrices were measured by high-performance liquid chromatography. All data were analyzed by using WinNonlin. A one-compartment model with first-order elimination was used to fit the AZT plasma data from the combination therapy rats, but the plasma data from the other groups were fit to a two-compartment model. Tissue data were analyzed by noncompartmental analysis to generate area-under-the-concentration-time-curve values. Implementation of the combination therapy altered the pharmacokinetics of each drug compared to its monotherapy pharmacokinetics. The combination of these two drugs may potentiate fetal and amniotic fluid exposures to each drug.

Determination of acyclovir in maternal plasma, amniotic fluid, fetal and placental tissues by high-performance liquid chromatography

Acyclovir [9-[(2-hydroxyethoxy)-methyl]-guanosine, Zovirax, ACV] is a synthetic purine nucleoside analog active against herpes simplex virus types 1 (HSV-1), 2 (HSV-2), and varicella zoster virus. Acyclovir has frequently been used in HSV-2 seropositive mothers to prevent prenatal transmission of herpes virus to their unborn children. A fast and reproducible HPLC method for the determination of the highly polar acyclvoir in maternal rat plasma, amniotic fluid, placental tissue, and fetal tissue has been developed and validated. Plasma and amniotic fluid samples were prepared by protein precipitation using 2 M perchloric acid and syringe filtering. Tissue samples were homogenized in distilled water, centrifuged, and extracted using a C(18) solid-phase extraction method prior to analysis. Baseline resolution was achieved for acyclovir and the internal standard gancyclovir, an anti-viral of similar structure to acyclovir, using an Agilent Eclipse XDB C(8) column (150 x 2.1 mm, 5 microm). The mobile phase used for the plasma and amniotic fluid was 10 mM acetate/citrate buffer-3.7 mM aqueous octanesulfonic acid (87.5:12.5, v/v) at a flow-rate of 0.2 ml/min. The mobile phase used for the tissue samples was 30 mM acetate/citrate buffer with 5 mM octanesulfonic acid-acetonitrile (99:1, v/v). Both aqueous mobile phase portions were pH adjusted to 3.08. All separations were done using an Agilent 1100 Series HPLC system with UV detection of 254 nm. The assay was validated for each matrix over a range of 0.25-100 microg/ml over 3 days using five replicates of three spiked concentrations. The relative standard deviation and percent error for each validation data set was <15% for middle and high quality control (QC) points and <20% for all low QC points. All calibration curves showed good linearity with an R(2)>0.99. The extraction efficiency for recovery of acyclovir from all matrices was >80%.

Determination of 3'-azido-2',3'-dideoxyuridine in maternal plasma, amniotic fluid, fetal and placental tissues by high-performance liquid chromatography

3'-Azido-2',3'-dideoxyuridine (AZDU, Azddu, CS-87) is a nucleoside analog of 3'-azido-3'-deoxythymidine (zidovudine, AZT) that has been shown to inhibit human immunodeficiency virus (HIV-1). AZDU is a potential candidate for treatment of pregnant mothers to prevent prenatal transmission of HIV/AIDS to their unborn children. A rapid and efficient high-performance liquid chromatography (HPLC) method for the determination of AZDU concentrations in rat maternal plasma, amniotic fluid, placental and fetal tissue samples has been developed and validated. Tissue samples were homogenized in distilled water, protein precipitated and extracted using a C-18 solid-phase extraction (SPE) method prior to analysis. Plasma and amniotic fluid samples were protein precipitated with 2 M perchloric acid prior to analysis. Baseline resolution was achieved using a 4.5% acetonitrile in 40 mM sodium acetate (pH 7) buffer mobile phase for amniotic fluid, placenta and fetus samples and with a 5.5% acetonitrile in buffer solution for plasma at flow-rates of 2.0 ml/min. The HPLC system consists of a Hypersil ODS column (150x4.6 mm) with a Nova-Pak C-18 guard column with detection at 263 nm. The method yields retention times of 6.2 and 12.2 min for AZDU and AZT in plasma and 8.3 and 17.6 min for AZDU and AZT in amniotic fluid, fetal and placental tissues. Limits of detection ranged from 0.01 to 0.075 microg/ml. Recoveries ranged from 81 to 96% for AZDU and from 82 to 96% for AZT in the different matrices. Intra-day (n=6) and inter-day (n=9) precision (% RSD) and accuracy (% Error) ranged from 1.48 to 6.25% and from 0.50 to 10.07%, respectively.

Nucleoside-mediated mitigation of 5-fluorouracil-induced toxicity in synchronized murine erythroleukemic cells

5-Fluorouracil (5-FU) is a chemotherapeutic agent known to retard embryonic growth and induce cleft palate and limb deformities. The predominant mechanism underlying its toxic action is thought to be inhibition of thymidylate synthetase (TS), and hence thymidine triphosphate (dTTP) synthesis, resulting in alteration of the balance of deoxynucleotide (dNTP) pools and disruption of DNA synthesis. Indeed, previously we demonstrated retarded cell-cycle progression concurrent with a 60% decrease in TS activity in rat whole embryos following maternal exposure to 40 mg/kg 5-FU on Gestational Day 14 and in the murine erythroleukemic cell (MELC) suspension culture following exposure to 5-25 microM 5-FU for 2 hr. In the study described herein, we used high-performance liquid chromatography (HPLC) to demonstrate in both of these model systems that 5-FU exposure results in similar patterns of dNTP perturbations: a prolonged decrease in dTTP and dGTP levels and an increase in dCTP and dATP. In addition, we used centrifugal elutriation to synchronize MELC in the phases of the cell cycle (G0/G1 and early S) most sensitive to 5-FU to investigate the ability of nucleoside supplementation to mitigate 5-FU-induced toxicity. Our data indicate that following a 2-hr exposure to 5-25 microM 5-FU, supplementation with 1-10 microM thymidine (TdR) for 24 hr partially reverses 5-FU-induced toxicity as evidenced by increased cellular proliferation and cell-cycle progression and amelioration of 5-FU-induced perturbations of protein synthesis and cellular membrane permeability compared to unsupplemented 5-FU-exposed cells. However, TdR concentrations >/=100 microM inhibited growth or were cytotoxic. In comparison, supplementation with 10 microM-10 mM of deoxycytidine (CdR) was not toxic, but effected a dose-dependent recovery from 5-FU-induced toxicity. At 1-100 microM, neither deoxyadenosine nor deoxyguanosine supplementation reduced 5-FU-induced toxicity; at higher concentrations, both purine nucleotides inhibited cell growth. Although these results support the hypothesis that 5-FU disrupts the MELC cell cycle by depleting dTTP (a perturbation that is reversible by TdR supplementation), they also indicate that CdR supplementation offers an additional recovery pathway.

Maternal-fetal pharmacokinetics of zidovudine in rats

The disposition of zidovudine (AZT) was investigated in near-term (day 20) pregnant rats after intravenous bolus administration of AZT at 50 mg/kg. A compartmental pharmacokinetic model was developed to describe AZT concentrations in maternal plasma (1), placenta (2), fetus (3), amniotic fluid (4), and the maternal tissue compartment (5). Model equations were fitted simultaneously to all concentration data by NONLIN least-squares regression. The model that best described the AZT concentration data (F test, AIC, sum of weighted squared residuals) incorporated bidirectional transfer between maternal plasma reversible placenta, placenta reversible fetus, placenta reversible amniotic fluid, and maternal plasma reversible tissue compartment. Transfer rate constants (1/h) were as follows: k12, 0.58 +/- 0.41; k21, 47.64 +/- 46.61; k23, 67.50 +/- 42.03; k32, 13.09 +/- 8.80; k24, 0.62 +/- 0.03; k42, 0.32 +/- 0.06; k15, 5.75 +/- 7.00; k51, 4.12 +/- 1.01; and k10, 1.51 +/- 0.80. AZT rapidly distributed into tissue and placenta compartments. However, AZT accumulated more slowly into amniotic fluid. Intercompartmental distributional clearances suggest that the mechanism of maternal-placental, placental-fetal, and fetal-amniotic fluid transfer of AZT was by passive diffusion. This maternal-fetal model for AZT may offer a useful approach for describing the placental transfer kinetics of other antiviral nucleosides as well.

Biological modeling of 5-fluorouracil developmental toxicity

A biologically-based dose-response (BBDR) model is a mathematical description of the biological events leading to expression of a toxic response. As an alternative to current approaches in non-cancer risk assessment, such models will reduce uncertainty in that they will provide a more comprehensive description of toxicity. We are involved in construction of a BBDR model for the developmental toxicity of 5-fluorouracil (5-FU) in the rat using multiple approaches. First, to identify critical events in the pathogenesis of 5-FU developmental toxicity, thymidylate synthetase (TS) inhibition and alterations in cell kinetics and growth were examined in embryos following maternal administration of 5-FU on day 14 of gestation. A dose-related decline in TS activity was observed within 1 h; however, maximal inhibition and recovery were similar at 10, 20 and 40 mg/kg. Dose-dependent cell cycle alterations were observed within 4 h after exposure and were maximal at 8 h. Hindlimb growth reduction was observed 24 h after exposure to 40 mg/kg, but not at lower doses. At term hindlimb defects were observed at doses above 30 mg/kg. An integrated dose-response model for hindlimb defects was derived from empirical relationships among these events. The resultant dose-response somewhat over-predicted the developmental toxicity of 5-FU, although results of a Monte Carlo simulation indicated that these data were not incompatible with model predictions. Overall, the results suggest that TS inhibition is a key component of the mechanism of 5-FU developmental toxicology, but the model does not capture all of the critical events in the induction of hindlimb defects. A preliminary mechanistic model for the inhibition of embryonic TS, DNA synthesis and cell cycle following maternal exposure to 5-FU, independently derived from literature data to further examine the potential role of this pathway in its developmental toxicity, predicted a dose-response for TS inhibition and DNA synthesis that closely reflected the observed patterns. These results further suggest that TS inhibition, resultant deficits in DNA synthesis and cell cycle perturbations represent a critical mechanistic pathway in the developmental toxicity of 5-FU.

Effects of 5-fluorouracil on embryonic rat palate in vitro: fusion in the absence of proliferation

5-Fluorouracil (5-FU) inhibits the enzyme thymidylate synthetase (TS) which results in inhibition of DNA synthesis. 5-FU is teratogenic in many species, inducing cleft palate, limb, and tail defects. In the present study, gestation day (GD) 14 embryonic rat craniofacial explants were exposed to 5-FU in organ culture with increasing concentrations and durations of exposure. Palates exposed to 5-FU were morphologically abnormal and craniofacial shape, size, and palatal fusion pattern were affected with the severity of effects dependent on concentration and duration of exposure. Cleft palate was induced in vitro as opposing palates overlapped in a narrowed oral cavity. Palates exposed to higher levels of 5-FU were growth inhibited, but fused even though proliferation ceased and few cells were available to participate in elevation and fusion. This was demonstrated as a biphasic concentration-response profile for palatal fusion in which 0.05 to 0.15 micrograms 5-FU/ml produced decreasing rates of palatal fusion, while exposure to 0.15 to 3.0 micrograms/ml resulted in progressively increasing rates of fusion. The effects of 5-FU were detected biochemically as a reduction in TS activity which was concentration and time dependent during the first 12 hours, with a return to control levels by 24 hours. During the first day, 5-FU did not alter protein levels, but DNA levels significantly decreased at the high concentration, 2.0 micrograms/ml. After 5 days in culture, both DNA and protein decreased with increasing 5-FU concentration and duration of exposure. Also by the end of the culture period, 3H-TdR incorporation had decreased in a concentration dependent manner. It is concluded that progressive inhibition of proliferation and growth in organ culture results in two different morphological outcomes: cleft palate resulting from a narrowed oral cavity and increased incidence of anterior palatal fusion under conditions of strong growth reduction. This study demonstrates that elevation and fusion can occur in the absence of growth and proliferation. Based on these observations, severe inhibition of growth or proliferation would not necessarily be sufficient to induce cleft palate.