Management of Trastuzumab Deruxtecan-related nausea and vomiting in real-world practice

Background

Nausea and vomiting are common side effects of Trastuzumab Deruxtecan (T-DXd), but guidelines for optimal management were not initially available. This retrospective single-center study aimed at evaluating the efficacy of two antiemetic regimens in patients receiving T-DXd.

Methods

Data from metastatic breast cancer patients receiving T-DXd were collected. Two groups were defined: patients treated with 5-HT3 receptor antagonists (RA) ± dexamethasone (5-HT3-group) and patients treated with a fixed oral combination of netupitant (NK1RA) and palonosetron ± dexamethasone (NK1 group). Physicians preferentially offered the NK1 regimen to patients at higher risk of nausea and vomiting based on internal recommendations. Only nausea and vomiting during cycles 1 and 2 were considered. Comparisons of nausea and vomiting by the antiemetic prophylaxis group were assessed using chi-square.

Results

A total of 53 patients were included in the analysis. At cycle 1, 72% and 28% of patients received the 5-HT3 and NK1 prophylaxis, respectively. Overall, 58% reported nausea, with no differences between groups (58% vs. 60%; p = 0.832), but with a trend for lower grade in the NK1 group (33.3% G1; 26.7% G2) compared to the 5-HT3 group (23.7% G1; 31.6% G2; 2.6% G3). Vomiting was reported by 21% and 0% of patients in the 5-HT3 and the NK1 group, respectively (p = 0.054). Among the 15 patients in the 5-HT3 group with nausea at cycle 1 who escalated to NK1 at cycle 2, nausea decreased from 100% to 53% (p = 0.022) and vomiting decreased from 47% to 13% (p = 0.046).

Conclusions

The NK1 regimen improved vomiting control at cycle 1 and, when introduced at cycle 2, significantly improved both nausea and vomiting. The biased NK1 selection for higher-risk patients may have dampened the differences between groups at cycle 1. These findings support enhanced control of T-DXd-related nausea and vomiting with NK1RA.

Oncotype DX results increase concordance in adjuvant chemotherapy recommendations for early-stage breast cancer

Adjuvant chemotherapy recommendations for ER+/HER2- early-stage breast cancers (eBC) involve integrating prognostic and predictive information which rely on physician judgment; this can lead to discordant recommendations. In this study we aim to evaluate whether Oncotype DX improves confidence and agreement among oncologists in adjuvant chemotherapy recommendations. We randomly select 30 patients with ER+/HER2- eBC and recurrence score (RS) available from an institutional database. We ask 16 breast oncologists with varying years of clinical practice in Italy and the US to provide recommendation for the addition of chemotherapy to endocrine therapy and their degree of confidence in the recommendation twice; first, based on clinicopathologic features only (pre-RS), and then with RS result (post-RS). Pre-RS, the average rate of chemotherapy recommendation is 50.8% and is higher among junior (62% vs 44%; p < 0.001), but similar by country. Oncologists are uncertain in 39% of cases and recommendations are discordant in 27% of cases (interobserver agreement K 0.47). Post-RS, 30% of physicians change recommendation, uncertainty in recommendation decreases to 5.6%, and discordance decreases to 7% (interobserver agreement K 0.85). Interpretation of clinicopathologic features alone to recommend adjuvant chemotherapy results in 1 out of 4 discordant recommendations and relatively high physician uncertainty. Oncotype DX results decrease discordancy to 1 out of 15, and reduce physician uncertainty. Genomic assay results reduce subjectivity in adjuvant chemotherapy recommendations for ER +/HER2- eBC.

Modulation of the Estrogen/erbB2 Receptors Cross-talk by CDK4/6 Inhibition Triggers Sustained Senescence in Estrogen Receptor- and ErbB2-positive Breast Cancer

PURPOSE

The interplay between estrogen receptor (ER) and erbB tyrosine-kinase receptors (RTK) impacts growth and progression of ER-positive (ER+)/HER2-positive (HER2+) breast cancer and generates mitogenic signals converging onto the Cyclin-D1/CDK4/6 complex. We probed this cross-talk combining endocrine-therapy (fulvestrant), dual HER2-blockade (trastuzumab and pertuzumab), and CDK4/6-inhibition (palbociclib; PFHPert).

EXPERIMENTAL DESIGN

Cytotoxic drug effects, interactions, and pharmacodynamics were studied after 72 hours of treatment and over 6 more days of culture after drug wash-out in three ER+/HER2+, two HER2low, and two ER-negative (ER-)/HER2+ breast cancer cell lines. We assessed gene-expression dynamic and association with Ki67 downregulation in 28 patients with ER+/HER2+ breast cancer treated with neoadjuvant PFHPert in NA-PHER2 trial (NCT02530424).

RESULTS

In vitro, palbociclib and/or fulvestrant induced a functional activation of RTKs signalling. PFHPert had additive or synergistic antiproliferative activity, interfered with resistance mechanisms linked to the RTKs/Akt/MTORC1 axis and induced sustained senescence. Unexpected synergism was found in HER2low cells. In patients, Ki67 downregulation at week 2 and surgery were significantly associated to upregulation of senescence-related genes (P = 7.7E-4 and P = 1.8E-4, respectively). Activation of MTORC1 pathway was associated with high Ki67 at surgery (P = 0.019).

CONCLUSIONS

Resistance associated with the combination of drugs targeting ER and HER2 can be bypassed by cotargeting Rb, enhancing transition from quiescence to sustained senescence. MTORC1 pathway activation is a potential mechanism of escape and RTKs functional activation may be an alternative pathway for survival also in ER+/HER2low tumor. PFHPert combination is an effective chemotherapy-free regimen for ER+/HER2+ breast cancer, and the mechanistic elucidation of sensitivity/resistance patterns may provide insights for further treatment refinement.

Personalized Risk-Benefit Ratio Adaptation of Breast Cancer Care at the Epicenter of COVID-19 Outbreak

Northern Italy has been one of the European regions reporting the highest number of COVID-19 cases and deaths. The pandemic spread has challenged the National Health System, requiring reallocation of most of the available health care resources to treat COVID-19-positive patients, generating a competition with other health care needs, including cancer. Patients with cancer are at higher risk of developing critical illness after COVID-19 infection. Thus, mitigation strategies should be adopted to reduce the likelihood of infection in all patients with cancer. At the same time, suboptimal care and treatments may result in worse cancer-related outcome. In this article, we attempt to estimate the individual risk-benefit balance to define personalized strategies for optimal breast cancer management, avoiding as much as possible a general untailored approach. We discuss and report the strategies our Breast Unit adopted from the beginning of the COVID-19 outbreak to ensure the continuum of the best possible cancer care for our patients while mitigating the risk of infection, despite limited health care resources. IMPLICATIONS FOR PRACTICE: Managing patients with breast cancer during the COVID-19 outbreak is challenging. The present work highlights the need to estimate the individual patient risk of infection, which depends on both epidemiological considerations and individual clinical characteristics. The management of patients with breast cancer should be adapted and personalized according to the balance between COVID-19-related risk and the expected benefit of treatments. This work also provides useful suggestions on the modality of patient triage, the conduct of clinical trials, the management of an oncologic team, and the approach to patients' and health workers' psychological distress.

Abstract P4-05-09: Molecular differences between high and low tumor mutational burden (TMB) across breast cancer (BC) subtypes

Background. High-TMB (HTMB) is an emerging promising agnostic biomarker for predicting benefit from immune-checkpoint inhibitors, independently of tumor type. At ASCO 2019, the TAPUR trial reported an interesting 21% ORR in heavily pretreated metastatic BC patients with very HTMB [vHTMB, ≥9 mutations/megabase (Muts/Mb)]. We aimed to define the differential gene expression and methylation landscape between low and high TMB in each BC subtype. Methods. In TCGA, we identified 848 patients with WES data available for TMB estimation. [ER+/HER2- (LumA by PAM50)) n = 364; ER+/HER2- (LumB by PAM50), n = 147; HER2+, n = 158; and TN, n = 179]. High TMB was defined according to two different cut-offs: ≥9 (vHTMB) and≥5 Muts/Mb (HTMB). The second arbitrary cut-off was used to define a larger group allowing to better characterize the different molecular landscapes associated with high and low TMB in each BC subtype. The HTMB group was compared with an equal number of tumors with low TMB. We assessed the differential RNA expression and methylation of single genes and pathways (defined using Gene Ontology - GO). “Common” genes and pathways were defined as recurrently associated with TMB (p&lt;0.05) in all subtypes and with a combined p value ≤0.00001. Results. The overall prevalence of vHTMB (≥9) was 4.5%, with no substantial differences across subtypes (4.4%, LumA; 4.8% LumB; 5.7% HER2+; 3.3% TN). The prevalence of HTMB (≥5) was 13.7%, but it was different across BC subtypes (p=8.0E-07) (8.2%, LumA; 12.9% LumB; 13.3% HER2+; 25.7% TN). We found more “common” genes down-regulated (n=70) than up-regulated (n=3) in HTMB group. Two of these three genes (HSPE1 and FEZF1.AS1) have been associated with poor prognosis in BC. When we considered the “common” pathways, only 3 were up-regulated in HTMB, all implicated in post-transcriptional repression of gene expression (gene silencing by miRNA and mRNA binding involved in post-transcriptional gene silencing). Conversely, 66 were significantly down-regulated (including transcription coregulator and coactivator activity, protein serine/threonine kinase activity and ubiquitin-protein transferase activity and binding). Some genes and pathways were associated with TMB only in a specific BC subtype (p ≤0.00001). For instance, 16 pathways were down-regulated in the HTMB group of TNBC. These inlcuded 12 pathways implicated in immune response. Conversely, in LumB, 11 pathways were up-regulated in HTMB group and implicated in immune response. Intrestingly, these pathways were all significantly down-regulated in the HTMB group of LumA and TN. In HTMB group, we found 7 and 4 “common” genes hypermetylated and hypomethylated, respectively. Four pathways were commonly hypermethylated (chromatin silencing at rDNA, telomere organization and positive regulation of G1/S transition of mitotic cell cycle) and five were hypomethilated (including mitotic sister chromatid segregation). Considering private alterations, in TNBC, 23 of 27 significant pathways were hypermetylated in HTMB group including double-strand break repair via nonhomologous end joining, epigenetic negative regulation of gene expression, and regulation of gene silencing by miRNA. Conclusions. Very-high TMB which is considered potentially druggable (≥9 Muts/Mb) is rare in BC, and equally frequent in all subtypes. Instead, HTMB (≥5 Muts/Mb) is more frequent in TNBC. BCs with HTMB had a different molecular landscape. Overall, several genes are recurrently down-regulated in HTMB group, and this is at least partly due to miRNA regulated post-trascriptional silencing, which might rapresents a new mechanism of immune escape. The positive association between TMB and immune genes in LumB, as well as the negative association in TN and LumA, suggest that immune editing and surveillance might be dependent on the molecular context.

Citation Format: Luca Licata, Barbara Galbardi, Balázs Győrffy, Thomas Karn, Lorenzo Sica, Alessia Rognone, Patrizia Zucchinelli, Daniela Aldrighetti, Stefania Zambelli, Luca Gianni, Giampaolo Bianchini. Molecular differences between high and low tumor mutational burden (TMB) across breast cancer (BC) subtypes [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-05-09.

Phospho-TCTP as a therapeutic target of Dihydroartemisinin for aggressive breast cancer cells

Upregulation of Translationally Controlled Tumor Protein (TCTP) is associated with poorly differentiated aggressive tumors, including breast cancer, but the underlying mechanism(s) are still debated. Here, we show that in breast cancer cell lines TCTP is primarily localized in the nucleus, mostly in the phosphorylated form.

The effects of Dihydroartemisinin (DHA), an anti-malaria agent that binds TCTP, were tested on breast cancer cells. DHA decreases cell proliferation and induces apoptotic cell death by targeting the phosphorylated form of TCTP. Remarkably, DHA enhances the anti-tumor effects of Doxorubicin in triple negative breast cancer cells resulting in an increased level of apoptosis. DHA also synergizes with Trastuzumab, used to treat HER2/neu positive breast cancers, to induce apoptosis of tumor cells.

Finally, we present new clinical data that nuclear phospho-TCTP overexpression in primary breast cancer tissue is associated with high histological grade, increase expression of Ki-67 and with ER-negative breast cancer subtypes. Notably, phospho-TCTP expression levels increase in trastuzumab-resistant breast tumors, suggesting a possible role of phospho-TCTP as a new prognostic marker.

In conclusion, the anti-tumor effect of DHA in vitro with conventional chemotherapeutics suggests a novel therapeutic strategy and identifies phospho-TCTP as a new promising target for advanced breast cancer.

Metastatic breast cancer treated with lapatinib with a prolonged benefit: a case report and a review of therapeutic options available

A 53-year-old woman was treated, after trastuzumab progression of HER2-positive metastatic disease, with capecitabine plus lapatinib and subsequently with lapatinib alone. To date, she has had persistent remission of disease since 2009.

Lapatinib and letrozole as first-line therapy for metastatic breast cancer: case report of bone metastasis 18 years later

We describe a 63-year-old woman with a diagnosis of breast cancer who relapsed with multiple bone lesions 18 years later. The biological characteristics were estrogen receptor positive, progesterone receptor positive, and HER2-positive cancer. Lapatinib and letrozole was the treatment of choice as a manageable and active first-line therapy for this patient.

Lapatinib and Letrozole as First-Line Therapy for Metastatic Breast Cancer: Case Report of Bone Metastasis 18 Years Later

We describe a 63-year-old woman with a diagnosis of breast cancer who relapsed with multiple bone lesions 18 years later. The biological characteristics were estrogen receptor positive, progesterone receptor positive, and HER2-positive cancer. Lapatinib and letrozole was the treatment of choice as a manageable and active first-line therapy for this patient.

Hepatic trans-arterial chemoembolization (TACE) in metastatic breast cancer

e12017

Background: Liver metastasis is one of the most frequent cause of death in breast cancer patients. Among loco-regional approaches available for solid tumor liver metastasis, TACE allows a prolonged intra-lesion exposure to chemotherapeutic agents. Initial studies have highlighted a benefit in terms of response rate (RR) and overall survival (OS) in breast cancer patients treated with TACE versus systemic chemotherapy. Methods: We have retrospectively evaluated 14 patients affected by breast cancer with liver metastasis. Patients had been selected for liver TACE according to the presence of liver metastasis as only site of disease, or in the event of further sites of metastasis that were stable or in response from the previous treatment. All the patients received one or more TACE with DC-Bead (Biocompatibles UK) 100-300 micron, loaded with a chemotherapeutic agent (doxorubicin, paclitaxel).The systemic therapy received by patients before TACE could be confirmed or changed according to disease response. Objective of the study was to evaluate the response rate to TACE and its correlation with the response to the very last treatment performed before it. As surrogate of efficacy, time to progression (TTP) and OS were analyzed. Results: Among the 14 treated patients (5 heavily pretreated, with more than three lines of chemotherapy) only one obtained a complete remission (RC). Four patients presented a partial remission (PR). Three of them were progressing from previous treatment and one had a PR. Stable disease (SD) after TACE was obtained in four cases: two of them had a progressive disease (PD) before TACE and two had a responsive disease. Five patients did not respond to TACE (one with PR from the previous line of therapy). Median TTP observed in responsive patients (CR + PR) was 4.5 times higher than non-responders (PD) (13.2 vs. 2.9 months). Patients with SD had a median TTP of 4.3 months. Median OS of responsive patients was 25.6 months vs. 17.5 and 19.2 months of patients with SD or PD respectively. Conclusions: In our experience, chemoembolization can represent a valid therapeutic option in breast cancer patients with liver metastasis, independently from the response to the previous systemic therapy, and may prolong survival.

Mannose-binding lectin alleles in sub-Saharan Africans and relation with susceptibility to infections

Mannose-binding lectin (MBL) plays an important role in the early stages of primary infections and during the decay of maternal antibodies in infants. Various studies have looked at the relation between serum MBL concentrations, MBL gene alterations and susceptibility to infections. We investigated the distribution of variant MBL alleles in 626 unrelated adults from sub-Saharan African countries and looked for a potential relation between these alleles and the incidence, prevalence and death rate of tuberculosis for sub-Saharan Africa. We also evaluated the relation between MBL genotypes and susceptibility to HIV-1 infection in 188 Gabonese adults. We found that (i) the prevalence of the common variant MBL alleles is correlated with the incidence of tuberculosis in sub-Saharan Africa (r=0.565), (ii) the mutant MBL G57E allele, in either the homozygous or compound heterozygous state, is associated with susceptibility to HIV-1 infection in the Gabonese population (P=0.019).Our data plus those in the literature suggest that individuals who are homozygous for the mutant MBL alleles display increased susceptibility to infections. Interestingly, we found that individuals who are heterozygous for MBL mutations are much less susceptible to infections than those who are homozygous for the wild-type MBL allele.

CYP2D6 polymorphism in a Gabonese population: contribution of the CYP2D6*2 and CYP2D6*17 alleles to the high prevalence of the intermediate metabolic phenotype

AIMS

To determine the molecular basis of the intermediate extensive metaboliser (EM) CYP2D6 phenotype in healthy Gabonese subjects.

METHODS

The CYP2D6 phenotype of 154 healthy Gabonese subjects was assessed by giving the subject a single dose of 30 mg dextromethorphan, and collecting their urine for the next 8 h. The CYP2D6 genotype was determined for 50 individuals of the EM phenotypic group by Southern blotting and various PCR-based procedures aimed at identifying different CYP2D6 alleles.

RESULTS

We found that in the studied Gabonese population, as compared with a French population, there is significantly higher frequency of intermediate EM phenotype having lower frequency of CYP2D6 PM alleles. To clarify this discrepancy phenotype-genotype relationship was studied. We found that the CYP2D6*17 and CYP2D6*2 alleles, prevalent in this black population, are characterised by their low capacity for dextromethorphan demethylation. Our data also show that the CYP2D6*1 allele is associated with the highest in vivo activity followed by the CYP2D6*2 allele and then the CYP2D6*17 allele.

CONCLUSIONS

The higher frequencies of the CYP2D6*2 and CYP2D6*17 alleles than the CYP2D6*1 allele account for the high frequency of the intermediate EM phenotype in this black population. The polymorphism of the CYP2D6 enzyme activity in African populations could have important implications for use of drugs that are substrates for CYP2D6 and have a narrow therapeutic window.

Asymptotic behavior of the boundary value problem of four-wave mixing in photorefractive media

Solutions to the four-wave mixing problem asymptotically approach a spatial steady state over large interaction distances. In this limit, intensities of the two-point boundary value problem may be found from solutions to quadratic equations. The different limiting behavior exhibited for positive and negative gain has important implications for applications in which linearity of system response is important. In the regime studied, positive gain leads quickly to maximally depleted pumps with increasing input signal. For negative gain, such depletion does not occur until higher signal levels are reached.

Genotyping of the polymorphic N-acetyltransferase (NAT2*) gene locus in two native African populations

The hepatic N-acetyltransferase enzyme encoded by the NAT2* gene locus is responsible for the human polymorphic acetylation of numerous arylamine or hydrazine-containing drugs and xenobiotics including AIDS-related therapeutic agents such as isoniazid and sulphonamides. The genetic basis underlying the human acetylation polymorphism has been extensively studied in several populations but native African populations were poorly documented. In the present study, 117 unrelated black Africans, namely Dogons from Mali and Gabonese, were investigated for NAT2* allelic variability and genotype distribution. Thirteen NAT2* alleles were unambiguously identified by combined use of allele-specific reamplifications and restriction endonuclease digestions. Our results confirm the African origin of G191->A substitution in the NAT2* coding region which was previously associated with slow acetylation in African-Americans. The finding of high allelic diversity in the studied populations is consistent with the hypothesis of a single African origin for NAT2*-associated polymorphism. Finally, no excess of the slow acetylator phenotype is predicted in these populations, implying no need for fitting NAT2* polymorphism-sensitive therapies to black Africans, compared to Caucasians.

Sheared-beam imaging: an evaluation of its optical compensation of thick atmospheric turbulence

Sheared-beam imaging (SBI) should compensate the effects of an idealized layer of turbulence located either in a transmitter/detector plane or in an object plane. This motivated the study of optical compensation of SBI in the presence of uniformly distributed turbulence over long horizontal paths in the cases of ideally smooth and ideally rough extended objects. The phase error along a one-dimensional wave front resulting from SBI observation is computed numerically in the long-path regime and is compared with that of an equivalent conventional system for the case of a large smooth object. It is found that for the conditions considered the phase errors of the SBI system are greater than those of a conventional system. In the case of an ideally rough object the extra information furnished by the SBI observations does not lead to data that can be inverted to compute an image by the conventional shearing-interferometric algorithm. The phase errors in imaging a point reflector, however, are perfectly compensated.

Compensation of spacer-thickness variations in a holographic Fabry-Perot filter

The fabrication of a solid, holographically recorded Fabry-Perot interferometer that uses plate glass for the spacer has recently been reported. The component produced sharp, circular Fabry-Perot fringes in spite of its use of a plate-glass spacer. We develop a general theoretical characterization of such a component that accounts for its low sensitivity to spacer-thickness variations. We use the Kogelnik theory of volume holograms to calculate the phase change on reflection from the mirrors. This phase change results from the position of the fringes formed throughout the two holographic media during the recording process. An expression for the wavelength location of the transmission peak versus spacer-thickness variation is derived that agrees with the current experimental information available.

Image speckle contrast reduction resulting from integrative synthetic aperture imaging

Reduction of image speckle noise with the use of an integrative synthetic aperture imaging technique is studied. It is found that the Fourier inversion of the mutual intensity estimate [Appl. Opt. 30, 206-213 (1991)] yields an image intensity that corresponds exactly to the illumination of the object with partially coherent light from source optics imaging a delta-function incoherent source. An expression for the signal-to-noise ratio at an image point is derived for a large rough object with delta-function correlated amplitude reflection. A synthetic aperture system receiver of sufficiently small diameter yields image speckle with a signal-to-noise ratio (SNR) equal to 1. When the receiver and the transmitter diameters are equal, the SNR is 2 for linearly polarized speckle. The SNR continues to increase with receiver size and is linear in the diameter for large receiver-to-transmitter diameter ratios.

Estimator and signal-to-noise ratio for an integrative synthetic aperture imaging technique

An imaging system is described which uses the following concepts: laser illumination of objects, nonredundant apertures, and phase closure. A sparse transmitter array is envisioned, each aperture of which emits at a different laser frequency such that any pair of beams gives rise to a unique beat signal. The light reflected by an object thus irradiated is sensed by a spatially integrating detector array. An estimator is given for the Fourier components of the object at spatial frequencies corresponding to the unique temporal beats sensed by the receiver array. The standard deviation of the estimator is computed taking both shot noise and laser speckle into account. It is found that the signal-to-noise ratio for both kinds of noise increases with the square root of the area of the detector array. This allows the signal-to-noise ratio of the system to be increased independent of the resolution.

A flow microcalorimetric method for enzyme activity measurements: application to dihydrofolate reductase

A flow microcalorimetric method was developed for the analysis of enzymatic activities in crude tissue homogenates. It can be applied whenever a heat exchange is involved in an enzymatic reaction. The consequent sensitivity obviously depends on the enthalpy variation observed. Dihydrofolate reductase was chosen as an example; this enzyme is the molecular target of methotrexate, a widely used anticancer agent. This calorimetric method, whose sensitivity limit is 1.48 X 10(-4) units of dihydrofolate reductase per milliliter of reactant medium, allows enzyme activity measurements in tissues with low dihydrofolate reductase levels. A few examples of measurements in animal tissues are given. These measurements are of some interest; indeed, increased activity and increased levels of this enzyme are two of the mechanisms which may explain resistance to methotrexate.

pH-Stat titration method for dihydrofolate reductase activity measurements: application to determination of substrate Michaelis constant and antifolate inhibition constant

A pH-Stat titration method was developed for measuring dihydrofolate reductase (DHFR) activity; this method permits detection of very low DHFR activities corresponding to 100 pmol of substrate reduced per minute. This value is about ten times lower than those observed using the classical spectrophotometric method. This sensitivity makes it possible to measure the DHFR in crude tissue extracts. With beef liver DHFR, Michaelis constants for the cofactor NADPH and the natural substrate determined by this method were 1.9 +/- 0.3 X 10(-5) and 8.5 +/- 0.5 X 10(-7) M, respectively. The inhibition constant of methotrexate, a competitive inhibitor of dihydrofolate, was 3.4 +/- 1.3 X 10(-11) M.

Three beam interferometer for the observation of kinetic cooling in air

A three beam interferometer, which has recently been used to measure the time constant associated with kinetic cooling, is described in detail. Design features necessitated by operation of the device over a 5-m path are discussed, as well as system response to vibrational noise and drift, and means used for their monitoring and control. The effective working length resolution of the system, dL/L, was 1 part in 2.6 x 10(9) with a He-Ne laser source operating at 0.63 microm.